The Risk Assessment Information System

Glossary of Environmental Restoration Terms

These definitions are from the United States Department of Energy (DOE) Oak Ridge Operations Office (ORO) Environmental Restoration/Waste Management Risk Assessment Program staff and affiliates and the following sources:

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Abatement: The reduction in degree or intensity of pollution.

Absorbed Dose: The energy imparted to a unit mass of matter by ionizing radiation. The unit of absorbed dose is the rad or gray. One rad equals 100 ergs per gram. The amount of a substance absorbed into the body, usually per unit of time. The most common unit of dose is mg per kg body weight per day (mg/kg-day).

Absorption: The penetration of one substance into or through another. Specifically, the penetration of a substance into the body from the skin, lungs, or digestive tract.

ACRS: Advisory Committee on Reactor Safeguards.

Acceptable Daily Intake: An estimate of the daily exposure dose that is likely to be without deleterious effect even if continued exposure occurs over a lifetime.

Accuracy: The degree of agreement between a measured value and the true value; usually expressed as +/- percent of full scale.

ACGIH: American Conference of Governmental Industrial Hygienists; an organization of professional personnel in governmental agencies or educational institutions engaged in occupational safety and health programs. ACGIH develops and publishes recommended occupational exposure limits (see TLV) for hundreds of chemical substances and physical agents.

Action Levels: Regulatory levels recommended by EPA for enforcement by Food and Drug Administration and United States Department of Agriculture when pesticide residues occur in food or feed commodities for reasons other than the direct application of the pesticide. As opposed to "tolerances" which are established for residues occurring as a direct result of proper usage, action levels are set for inadvertent residues resulting from previous legal use or accidental contamination. In the Superfund program, the existence of a contaminant concentration in the environment high enough to warrant action or trigger a response under SARA and the National Oil and Hazardous Substances Contingency Plan. The term is also used in other regulatory programs.

Activated Carbon: A highly adsorbent form of carbon used to remove odors and toxic substances from liquid or gaseous emissions. In waste treatment it is used to remove dissolved organic matter from waste water. It is also used in motor vehicle evaporative control systems.

Activated Sludge: Product that results when primary effluent is mixed with bacteria-laden sludge and then agitated and aerated to promote biological treatment, speeding the breakdown of organic matter in raw sewage undergoing secondary waste treatment.

Activator: A chemical added to a pesticide to increase its activity.

Acute: Diseases or responses with short and generally severe course (often due to high pollutant concentrations).

Acute Exposure: A single exposure to a toxic substance which results in severe biological harm or death. Acute exposures are usually characterized as lasting no longer than a day, as compared to longer, continuing exposure over a period of time.

Acute Toxicity: Any poisonous effect produced within a short period of time following exposure, usually up to 24-96 hours, resulting in biological harm and often death.

Added Risk: The difference between the cancer incidence under the exposure condition and the background incidence in the absence of exposure.

Administrative Order On Consent: A legal agreement signed by EPA and an individual, business, or other entity through which the violator agrees to pay for correction of violations, take the required corrective or cleanup actions, or refrain from an activity. It describes the actions to be taken, may be subject to a comment period, applies to civil actions, and can be enforced in court.

Administrative Order: A legal document signed by EPA directing an individual, business, or other entity to take corrective action or refrain from an activity. It describes the violations and actions to be taken, and can be enforced in court. Such orders may be issued, for example, as a result of an administrative complaint whereby the respondent is ordered to pay a penalty for violations of a statute.

Administrative Procedures Act: A law that spells out procedures and requirements related to the promulgation of regulations.

Administrative Record: All documents which EPA considered or relied on in selecting the response action at a Superfund site, culminating in the record of decision for remedial action or, an action memorandum for removal actions.

Adsorption: retention of a substance by soil particles.

AEC: Atomic Energy Commission, 1947-1974. Broken up in 1974 into the Energy Research and Development Administration (ERDA) and the Nuclear Regulatory Commission (NRC). ERDA later became the Department of Energy (DOE).

Aerobic Treatment: Process by which microbes decompose complex organic compounds in the presence of oxygen and use the liberated energy for reproduction and growth. (Such processes include extended aeration, trickling filtration, and rotating biological contactors.)

Aerobic: Life or processes that require, or are not destroyed by, the presence of oxygen.

Air Emissions: The release or discharge of a pollutant (from a stationary source) into the ambient air. For anthropogenic sources this may involve release (1) by means of a stack or (2) as a fugitive dust, mist or vapor as a result inherent to the manufacturing or formulating process. Pollutants may also be discharged from mobile sources, from area sources such as roads and fields, and from non-manufacturing, stationary sources.

Air Pollution: The presence in the outdoor atmosphere of any dust, fumes, mist, smoke, other particulate matter, vapor, gas, odorous substances, or a combination thereof, in sufficient quantities and of such characteristics and duration as to be, or likely to be, injurious to health or welfare, animal or plant life, or property, or as to interfere with the enjoyment of life or property.

Air Quality Control Region: Federally designated area that is required to meet and maintain federal ambient air quality standards. May include nearby locations in the same state or nearby states that share common air pollution problems.

Air Quality Criteria: The levels of pollution and lengths of exposure above which adverse effects may occur on health and welfare.

Air Quality Standards: The level of pollutants prescribed by law or regulation that cannot be exceeded during a specified time in a defined area.

Airborne Particulates: Total suspended particulate matter found in the atmosphere as solid particles or liquid droplets. Chemical composition of particulates varies widely, depending on location and time of year. Airborne particulates include: windblown dust, emissions from industrial processes, smoke from the burning of wood and coal, and motor vehicle or non-road engine exhausts.

ALARA: Acronym for "As Low As Reasonably Achievable," means making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical, consistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in relation to state of technology, the economics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and in relation to utilization of nuclear energy and licensed materials in the public interest. (see 10 CFR 20.1003).

Alpha Decay: The emission of a nucleus of a helium atom from the nucleus of an element, generally of a heavy element, in the process of its radioactive decay.

Alpha Particle: The nuclei of a helium atom (with two neutrons and two protons each) that are discharged by radioactive decay of many heavy elements, such as uranium-238 and plutonium-239.

Alpha Radiation: Radiation consisting of helium nuclei (atomic wt. 4, atomic number 2) that are discharged by radioactive disintegration of some heavy elements, including uranium-238, radium-226, and plutonium-239.

Alternative Technology: Approach that aims to use resources efficiently or to substitute resources in order to do minimum damage to the environment. This approach permits a large degree of personal user control over the technology.

Anaerobic: A life or process that occurs in, or is not destroyed by, the absence of oxygen.

Antagonism: Interference or inhibition of the effect of one chemical by the action of another.

Anthropogenic: Of human origin.

Annual Limit on Intake (ALI): The derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. ALI is the smaller value of intake of a given radionuclide in a year by the reference man (mannequin used to determine dose) that would result in a committed effective dose equivalent of 5 rems(0.05 sievert) or a committed dose equivalent of 50 rems (0.5 sievert) to any individual organ or tissue. (see 10 CFR 20.1003.)

Applicable or Relevant and Appropriate Requirements (ARARs): Any state or federal statute that pertains to protection of human life and the environment in addressing specific conditions or use of a particular cleanup technology at a Superfund site.

Aquifer: An underground bed or layer of earth, gravel, or porous stone that contains water.

Artesian (Flowing) Aquifer: Aquifer in which water is held under pressure by confining layers, forcing water to rise in wells above the top of the aquifer.

Asbestos: A mineral fiber that can pollute air or water and cause cancer or asbestosis when inhaled. EPA has banned or severely restricted its use in manufacturing and construction.

Asbestos Abatement: Procedures to control fiber release from asbestos-containing materials in a building or to remove them entirely, including removal, encapsulation, repair, enclosure, encasement, and operations and maintenance programs.

Asbestos-Containing Waste Materials (ACWM): Mill tailings or any waste that contains commercial asbestos and is generated by a source covered by the Clean Air Act Asbestos NESHAPS.

Asbestosis: A disease associated with inhalation of asbestos fibers. The disease makes breathing progressively more difficult and can be fatal.

Atom: The smallest particle of an element that cannot be divided or broken up by chemical means. It consists of a central core of protons and neutrons, called the nucleus. Electrons revolve in orbits in the region surrounding the nucleus.

Atomic Energy: Energy released in nuclear reactions. Of particular interest is the energy released when a neutron initiates the breaking up or fissioning of an atom's nucleus into smaller pieces (fission), or when two nuclei are joined together under millions of degrees of heat (fusion). It is more correctly called nuclear energy.

Atomic Number (symbolized Z): The number of protons in a nucleus. It determines the chemical properties of an element.

Atomic Weight: The nominal atomic weight of an isotope is given by the sum of the number of neutrons and protons in each nucleus. The exact atomic weight differs fractionally from that whole number, because neutrons are slightly heavier than protons and the mass of the nucleus is also affected by the binding energy.

Attenuation: The process by which a compound is reduced in concentration over time, through absorption, adsorption, degradation, dilution, and/or transformation.

Attributable Risk: The rate of a disease in exposed individuals that can be attributed to the exposure. This measure is derived by subtracting the rate (usually incidence or mortality) of the disease among nonexposed persons from the corresponding rate among exposed individuals.


Background Level: In air pollution, the level of pollutants present in ambient air from natural sources. More generally, the level of pollution present in any environmental medium attributable to natural or ubiquitous sources.

Background Radiation: Radiation from cosmic sources, naturally occurring radioactive materials, including radon (except as a decay product of source or special nuclear material) and global fallout as it exists in the environment from the testing of nuclear explosive devices. It does not include radiation from source, byproduct, or special nuclear materials regulated by the Nuclear Regulatory Commission. The typically quoted average individual exposure from background radiation is 360 millirems per year.

Becquerel: A unit of radioactivity equal to one disintegration per second. [37 billion (3.7x1010) becquerels = 1 curie (Ci)].

Benign: Not malignant; remaining localized.

Best Available Control Measures (BACM): A term used to refer to the most effective measures (according to EPA guidance) for controlling small or dispersed particulates from sources such as roadway dust, soot and ash from woodstoves and open burning of brush, timber, grasslands, or trash.

Best Available Control Technology: An emission limitation (including a visible emission standard) based on the maximum degree of reduction for each pollutant subject to regulation under the [Clean Air] Act which would be emitted from any proposed major stationary source or major modification which the Administrator, on a case-by-case basis, taking into account energy, environmental, and economic impacts and other costs, determines is achievable for such source or modification through application of production processes or available methods, systems, and techniques, including fuel cleaning or treatment or innovative fuel combustion techniques for control of such pollutant.

Best Demonstrated Available Technology (BDAT): As identified by EPA, the most effective commercially available means of treating specific types of hazardous waste. The BDATs may change with advances in treatment technologies.

Best Management Practice (BMP): Methods that have been determined to be the most effective, practical means of preventing or reducing pollution from nonpoint sources.

Beta Decay: The emission of electrons or positrons (particles identical to electrons, but with a positive electrical charge) from the nucleus of an element in the process of radioactive decay of the element.

Beta Particle: A charged particle emitted from a nucleus during radioactive decay, with a mass equal to 1/1837 that of a proton. A negatively charged beta particle is identical to an electron. A positively charged beta particle is called a positron. Large amounts of beta radiation may cause skin burns, and beta emitters are harmful if they enter the body. Beta particles may be stopped by thin sheets of metal or plastic.

Beta Radiation: Radiation consisting of electrons or positrons emitted in many radioactive disintegrations, at speeds approaching the speed of light.

Bias: Any difference between the true value and that actually obtained due to all causes other than sampling variability.

Binding Energy: The energy that is required to separate the nucleons in a nucleus into separate, free particles.

Bioaccessibility: The fraction of the total amount of a substance that is potentially available for absorption. Bioaccessibility is therefore used to help predict bioavailability.

Bioaccumulation: The process whereby certain toxic substances collect in living tissues, thus posing a substantial hazard to human health or the environment.

Bioactivity: Bioactivity is the specific effect upon exposure to a substance. It includes tissue uptake and the consequent physiological response (e.g. antioxidant, anti-inflammatory, etc). It also includes information on how the bioactive compounds are transported and reach the target tissue, how they interact with biomolecules, metabolism and biotransformation characteristics, as well as the biomarkers' generation and the consequent physiological responses.Thus, bioavailability includes the terms bioaccessibility and bioactivity.

Bioconcentration: Bioconcentration is when the concentration of a substance builds up in the tissues and is absorbed faster than it is removed. It occurs directly from the water (or other environmental media) rather than from eating contaminated food which would be bioaccumulation.

Bioavailability: In the environment, only a portion of the total quantity of chemical present is potentially available for uptake by organisms. This concept is referred to as the biological availability (or bioavailability) of a chemical. Thus, bioavailability includes the term bioaccessibility.

Biological Half-life: The time required for a biological system (such as a human or animal) to eliminate, by natural processes, half the amount of a substance (such as a radioactive material) that has been absorbed into that system.

Biological Magnification: Also called biomagnification, refers to the process whereby certain substances such as pesticides or heavy metals move up the food chain, work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances become concentrated in tissues or internal organs as they move up the chain.

Bioremediation: Use of living organisms to clean up oil spills or remove other pollutants from soil, water, or wastewater; use of organisms such as non-harmful insects to remove agricultural pests or counteract diseases of trees, plants, and garden soil.

Biota: The sum total of the living organisms of any designated area.

Body Burden: The total amount of a specific substance (for example, lead) in an organism, including the amount stored, the amount that is mobile, and the amount absorbed.

Bone Seeker: A radioisotope that tends to accumulate in the bones when it is introduced into the body. An example is strontium-90, which behaves chemically like calcium.

Breeder Reactor: A reactor that is designed to produce more fissile material than it consumes; also sometimes called "fast reactor" since most breeder reactors use fast neutrons for sustaining the nuclear chain reaction.

BTU: British thermal unit. The amount of energy gained by a pound of water when its temperature is increased by one degree Fahrenheit.

Burial Ground (Graveyard): A disposal site for radioactive waste materials that uses earth or water as a shield.

Burn-up: The amount of energy that has been generated from a unit of nuclear fuel; usually measured in megawatt-days per metric ton of initial heavy metal.

By-product Material: There are basically two types of by-product materials. The first are produced by a nuclear reactor and the second are produced by the uranium and thorium mining process. A more precise definition reads: "(1) Any radioactive material (except special nuclear material) yielded in, or made radioactive by, exposure incident to the process of producing or utilizing special nuclear material, and (2) The tailings or wastes produced by the extraction or concentration of uranium or thorium from ore processed primarily for its source material content, including discrete surface wastes resulting from uranium solution extraction processes. Underground ore bodies depleted by these solution extraction operations do not constitute "by-product material" within this definition (10 CFR 20.1003)."


Calorie: A unit of heat or energy sufficient to raise the temperature of 1 gram of water by 1 degree Celsius. In dietetics, the kilocalorie is the unit usually used, frequently called a "calorie," omitting the prefix.

Cancer: An abnormal, potentially unlimited, disorderly new tissue growth.

Cap: A layer of clay, or other impermeable material installed over the top of a closed landfill to prevent entry of rainwater and minimize leachate.

Carcinogen: A substance or agent that produces or incites cancerous growth.

Carcinogenesis: Development of carcinoma; or, in more recent usage, producing any kind of malignancy.

Carcinogenic: Cancer causing.

Carcinogenic Potency: The gradient of the dose-response curve for a carcinogen.

Carcinoma: Malignant new growth made up of epithelial cells tending to infiltrate the surrounding tissues and give rise to metastasis.

CAS Registration Number: An organization from Columbus, Ohio, which indexes information published in Chemical Abstracts by the American Chemical Society and provides index guides by which information about particular substances may be located in the Abstracts when needed. CAS numbers identify specific chemicals.

Cask: A thick-walled container (usually lead) used to transport radioactive material. Also called a coffin.

Cells: In solid waste disposal, holes where waste is dumped, compacted, and covered with layers of dirt on a daily basis.

CERCLA (Comprehensive Environmental Restoration and Compensation Liability Act): Cleanup Program focuses on human health and environmental concerns related to human health. The cleanup program is primarily carried out by EPA, working with States, on sites designated for cleanup on the NPL. Cleanup Program emphasizes local source control, prevention of further spread from sources. Cleanup Program is prohibited from "restoring" natural resources, although cleanup may prevent further injuries to natural resources.

CFR: Code of Federal Regulations. A collection of the regulations that have been promulgated under U.S. law.

CH (contact handled) waste: TRU waste that requires little or no shielding and emits a maximum external radiation dose rate of 0.2 rem/hr (200 mrem/hr).

Chronic: Having a persistent, recurring or long-term nature. As distinguished from acute.

Chronic Effect: An adverse effect on a human or animal in which symptoms recur frequently or develop slowly over a long period of time.

Chronic Exposure: Multiple exposures occurring over an extended period of time, or a significant fraction of the animal's or the individual's life-time.

Chronic Toxicity: The capacity of a substance to cause long-term poisonous human health effects.

Cladding: The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents corrosion of the fuel by the coolant and the release of fission products into the coolant. Aluminum, stainless steel, and zirconium alloys are common cladding materials.

Cleanup: Actions taken to deal with a release or threat of release of a hazardous substance that could affect humans and/or the environment. The term "cleanup" is sometimes used interchangeably with the terms remedial action, removal action, response action, or corrective action.

Closure: The procedure a landfill operator must follow when a landfill reaches its legal capacity for solid waste: ceasing acceptance of solid waste and placing a cap on the landfill site.

Cohort Study: An epidemiologic study that observes subjects in differently exposed groups and compares the incidence of symptoms. Although ordinarily prospective in nature, such a study is sometimes carried out retrospectively, using historical data.

Collective Dose: The sum of the individual doses received on a given period of time by a specified population from exposure to a specified source of radiation.

Commercial Waste: All solid waste emanating from business establishments such as stores, markets, office buildings, restaurants, shopping centers, and theaters.

Commercial Waste Management Facility: A treatment, storage, disposal, or transfer facility which accepts waste from a variety of sources, as compared to a private facility which normally manages a limited waste stream generated by its own operations.

Committed Dose Equivalent: This is the dose to some specific organ or tissue that is received from an intake of radioactive material by an individual during the 50-year period following the intake (see 10 CFR 20.1003).

Committed Effective Dose Equivalent: The committed dose equivalent for a given organ multiplied by a weighting factor (see 10 CFR 20.1003).

Comparative Risk: An expression of the risks associated with two (or more) actions leading to the same goal; may be expressed quantitatively (a ratio of 1.5) or qualitatively (one risk greater than another risk). Any comparison among the risks of two or more hazards with respect to a common scale.

Compliance Monitoring: Collection and evaluation of data, including self monitoring reports, and verification to show whether pollutant concentrations and loads contained in permitted discharges are in compliance with the limits and conditions specified in the permit.

Composite Sample: A series of water samples taken over a given period of time and weighted by flow rate. Or a soil sample that consists of soil taken from various depths or various locations.

Concentration Ratio: The ratio of the concentration of a compound or radionuclide in an organism or its tissues to the concentration in the surrounding media under equilibrium, or steady-state conditions.

Confidence Interval: A range of values (a1 < a < a2) determined from a sample of definite rules so chosen that, in repeated random samples from the hypothesized population, an arbitrarily fixed proportion of that range will include the true value, x, of an estimated parameter. The limits, a1 and a2, are called confidence limits; the relative frequency with which these limits include a is called the confidence coefficient; and the complementary probability is called the confidence level. As with significance levels, confidence levels are commonly chosen as 0.05 or 0.01, the corresponding confidence coefficients being 0.95 or 0.99. Confidence intervals should not be interpreted as implying that the parameter itself has a range of values; it has only one value, a. On the other hand, the confidence limits (a1, a2) being derived from a sample, are random variables, the values of which on a particular sample either do or do not include the true value a of the parameter. However, in repeated samples, a certain proportion of these intervals will include a provided that the actual population satisfied the initial hypothesis.

Confined Aquifer: An aquifer in which groundwater is confined under pressure which is significantly greater than atmospheric pressure.

Confounding Factors: Variables that may introduce differences between cases and controls which do not reflect differences in the variables of primary interest.

Consent Decree: A legal document, approved by a judge, that formalizes an agreement reached between EPA and potentially responsible parties (PRPs) through which PRPs will conduct all or part of a cleanup action at a Superfund site; cease or correct actions or processes that are polluting the environment; or otherwise comply with EPA initiated regulatory enforcement actions to resolve the contamination at the Superfund site involved. The consent decree describes the actions PRPs will take and may be subject to a public comment period.

Contamination: Contact with an admixture of an unnatural agent, with the implication that the amount is measurable. The deposition of unwanted radioactive material on the surfaces of structures, areas, objects, or people. It may also be airborne, external, or internal (inside components or people).

Contingency Plan: A document setting out an organized, planned, and coordinated course of action to be followed in case of a fire, explosion, or other accident that releases toxic chemicals, hazardous waste, or radioactive materials that threaten human health or the environment.

Continuous Sample: A flow of water from a particular place in a plant to the location where samples are collected for testing; may be used to obtain grab or composite samples.

Controlled Liquid Waste: Waste that meets the definition of a liquid waste and is in a container or piping system; a waste stream that can be shut off without a release to the environment.

Cooperative Agreement: An assistance agreement whereby EPA transfers money, property, services or anything of value to a state for the accomplishment of CERCLA-authorized activities or tasks.

Corrective Action: EPA can require treatment, storage and disposal facilities (TSDF) handling hazardous waste to undertake corrective actions to clean up spills resulting from failure to follow hazardous waste management procedures or other mistakes. The process includes cleanup procedures designed to guide TSDFs toward in spills.

Cost-benefit Analysis: A formal quantitative procedure comparing costs and benefits of a proposed project or act under a set of preestablished rules. To determine a rank ordering of projects to maximize rate of return when available funds are unlimited, the quotient of benefits divided by costs is the appropriate form; to maximize absolute return given limited resources, benefits-costs is the appropriate form.

Cost-Effective Alternative: An alternative control or corrective method identified after analysis as being the best available in terms of reliability, performance, and cost. Although costs are one important consideration, regulatory and compliance analysis does not require EPA to choose the least expensive alternative. For example, when selecting or approving a method for cleaning up a Superfund site the Agency balances costs with the long-term effectiveness of the methods proposed and the potential danger posed by the site.

Cost Recovery: A legal process by which potentially responsible parties who contributed to contamination at a Superfund site can be required to reimburse the Trust Fund for money spent during any cleanup actions by the federal government.

Cost Sharing: A publicly financed program through which society, as a beneficiary of environmental protection, shares part of the cost of pollution control with those who must actually install the controls. In Superfund, the government may pay part of the cost of a cleanup action with those responsible for the pollution paying the major share.

Cradle-to-Grave or Manifest System: A procedure in which hazardous materials are identified and followed as they are produced, treated, transported, and disposed of by a series of permanent, linkable, descriptive documents (e.g., manifests). Commonly referred to as the cradle-to-grave system.

Criteria: Descriptive factors taken into account by EPA in setting standards for various pollutants. These factors are used to determine limits on allowable concentration levels, and to limit the number of violations per year. When issued by EPA, the criteria provide guidance to the states on how to establish their standards.

Criteria Pollutants: The 1970 amendments to the Clean Air Act required EPA to set National Ambient Air Quality Standards for certain pollutants known to be hazardous to human health. EPA has identified and set standards to protect human health and welfare for six pollutants: ozone, carbon monoxide, total suspended particulates, sulfur dioxide, lead, and nitrogen oxide. The term, "criteria pollutants" derives from the requirement that EPA must describe the characteristics and potential health and welfare effects of these pollutants. It is on the basis of these criteria that standards are set or revised.

Critical Effect: The first adverse effect, or its known precursor, that occurs as the dose rate increases.

Critical Mass: The amount of a fissile substance that will allow a self-sustaining chain reaction. The amount depends both on the properties of the fissile element and on the shape of the mass.

Critical Organ: That part of the body that is most susceptible to radiation damage under the specific conditions under consideration.

Criticality: A term used in reactor physics to describe the state when the number of neutrons released by fission is exactly balanced by the neutrons being absorbed (by the fuel and poisons) and escaping the reactor core. A reactor is said to be "critical" when it achieves a self-sustaining nuclear chain reaction, as when the reactor is operating.

Cumulative Dose: The total dose resulting from repeated exposures of ionizing radiation to an occupationally exposed worker to the same portion of the body, or to the whole body, over a period of time (see 10 CFR 20.1003).

Curie (Ci): The basic unit used to describe the intensity of radioactivity in a sample of material. The curie is equal to 37 billion (3.7*1010) disintegrations per second, which is approximately the activity of 1 gram of radium. A curie is also a quantity of any radionuclide that decays at a rate of 37 billion disintegrations per second. It is named for Marie and Pierre Curie, who discovered radium in 1898.


Daughter Products: Isotopes that are formed by the radioactive decay of some other isotope. In the case of radium-226, for example, there are 10 successive daughter products, ending in the stable isotope, lead-206.

Decay: The decrease in the amount of any radioactive material with the passage of time due to the spontaneous emission from the atomic nuclei of either alpha or beta particles, often accompanied by gamma radiation. Every decay process has a definite half-life.

Decay Correction: The amount by which the calculated radioactivity (for example, of a release of radioisotopes) must be reduced after a period of time, to allow for its radioactive decay during that time.

Decay Products: Degraded radioactive materials, often referred to as "daughters" or "progeny", radon decay products of most concern from a public health standpoint are polonium-214 and polonium-218.

Decommissioning: Decontamination and dismantlement of retired, contaminated facilities and removal and/or disposal of the resulting wastes.

Decontamination: Removal of harmful substances such as noxious chemicals, harmful bacteria or other organisms, or radioactive material from exposed individuals, rooms and furnishings in buildings, or the exterior environment.

De Minimis Risk: From the legal maxim "de minimis non curat lex" or "the law is not concerned with trifles."

Deep-Well Injection: Deposition of raw or treated, filtered hazardous waste by pumping it into deep wells, where it is contained in the pores of permeable subsurface rock.

Department of Energy (DOE): This Federal agency's mission is to achieve efficiency in energy use, diversity in energy sources, a more productive and competitive economy, improved environmental quality, and a secure national defense. DOE was created on October 1, 1977 out of the Energy and Research and Development Agency as well as various aspects of non-nuclear federal energy policy and programs. The DOE complex, which is located over 22 States with sites that range in size from small to very large, produced and tested nuclear weapons.

Depleted Uranium: Uranium having a percentage of uranium-235 smaller than the 0.7 percent found in natural uranium. It is obtained from spent (used) fuel elements or as byproduct tails, or residues, from uranium isotope separation.

Dermal Exposure: Contact between a chemical and the skin.

Dermal Toxicity: The ability of a pesticide or toxic chemical to poison people or animals by contact with the skin.

Deterministic Effect: The health effects, the severity of which varies with the dose and for which a threshold is believed to exist. Radiation-induced cataract formation is an example of a deterministic effect (also called a non-stochastic effect) (see 10 CFR 20.1003).

Dewater: 1. Remove or separate a portion of the water in a sludge or slurry to dry the sludge so it can be handled and disposed. 2. Remove or drain the water from a tank or trench.

Direct Discharger: A municipal or industrial facility which introduces pollution through a defined conveyance or system such as outlet pipes; a point source.

Direct Filtration: A method of treating water which consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. Sedimentation is not used.

Direct Runoff: Water that flows over the ground surface or through the ground directly into streams, rivers, and lakes.

Discharge: Flow of surface water in a stream or canal or the outflow of groundwater from a flowing artesian well, ditch, or spring. Can also apply to discharge of liquid effluent from a facility or of chemical emissions into the air through designated venting mechanisms.

Dose: The absorbed dose, given in rads (or the international system of units, grays), that represents the energy absorbed from the radiation in a gram of any material. Furthermore, the biological dose or dose equivalent, given in rem or sieverts, is a measure of the biological damage to living tissue from the radiation exposure.

Dose-effect: The relationship between dose (usually an estimate of dose) and the gradation of the effect in a population, that is a biological change measured on a graded scale of severity, although at other times one may only be able to describe a qualitative effect that occurs within some range of exposure levels.

Dose Equivalent: The product of the absorbed dose from ionizing radiation and such factors as account for differences in biological effectiveness due to the type of radiation and its distribution in the body as specified by the International Commission on Radiological Units and Measurements (ICRU).

Dose-response: A correlation between a quantified exposure (dose) and the proportion of a population that demonstrates a specific effect (response).

Dose-response Assessment: The process of characterizing the relation between the dose of an agent administered or received and the incidence of an adverse health effect in exposed populations and estimating the incidence of the effect as a function of human exposure to the agent.

Dose-Response Relationship: The quantitative relationship between the amount of exposure to a substance and the extent of toxic injury or disease produced.

Dosimetry: The theory and application of the principles and techniques involved in the measurement and recording of ionizing radiation doses.

Downgradient: The direction that groundwater flows; similar to "downstream" for surface water.

Drawdown: 1. The drop in the water table or level of water in the ground when water is being pumped from a well. 2. The amount of water used from a tank or reservoir. 3. The drop in the water level of a tank or reservoir.

DWPF: Defense Waste Processing Facility, the name of the vitrification plant for high-level radioactive wastes at the Savannah River Site.


Ecological Fallacy: The inference that a correlation between variables derived from data grouped in social or other aggregates (ecological units) will hold between persons (individual units).

Ecological Impact: The total effect of an environmental change, natural or man-made, on the community of living things.

Ecological Indicator: A characteristic of the environment that, when measured, quantifies magnitude of stress, habitat characteristics, degree of exposure to a stressor, or ecological response to exposure. The term is a collective term for response, exposure, habitat, and stressor indicators.

Ecological Risk Assessment: The application of a formal framework, analytical process, or model to estimate the effects of human action(s) on a natural resource and to interpret the significance of those effects in light of the uncertainties identified in each component of the assessment process. Such analysis includes initial hazard identification, exposure and dose response assessments, and risk characterization.

Ecology: The science dealing with the relationship of all living things with each other and with their environment.

Ecosystem: The interacting system of a biological community and its nonliving surroundings.

Ecosystem Structure: Attributes related to instantaneous physical state of an ecosystem; examples include species population density, species richness or evenness, and standing crop biomass.

Ecotone: A habitat created by the juxtaposition of distinctly different habitats; an edge habitat; or an ecological zone or boundary where two or more ecosystems meet.

Effect: A biological change caused by an exposure.

Effective Half-life: The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination.

Efficacy: A measure of the probability and intensity of beneficial effects.

Effluent: Waste material discharged into the environment, treated or untreated. Generally refers to surface water pollution.

ELCR (excess lifetime cancer risk): Potential carcinogenic effects that are characterized by estimating the probability of cancer incidence in a population of individuals for a specific lifetime from projected intakes (and exposures) and chemical-specific dose-response data (i.e., slope factors). By multiplying the intake by the slope factor, the ELCR result is a probability.

Electron: An elementary particle carrying 1 unit of negative electric charge. Its mass is 1/1837 that of a proton.

Electromagnetic Radiation: A traveling wave motion resulting from changing electric or magnetic fields. Familiar electromagnetic radiation ranges from x-rays (and gamma rays) of short wavelength, through the ultraviolet, visible, and infrared regions, to radar and radio waves of relatively long wave length.

Element: One of the 103 known chemical substances that cannot be broken down further without changing its chemical properties. Some examples include hydrogen, nitrogen, gold, lead, and uranium.

EMBAM (Environmental Management Benefit Assessment Matrix): a management tool to evaluate and categorize EM activities based on the risks associated with the activities of each project.

Emission: Like effluent but used in regard to air pollution.

Emission Rate: The amount of pollutant emitted per unit of time.

Endangered Species: Animals, birds, fish, plants, or other living organisms threatened with extinction by man-made or natural changes in their environment. Requirements for declaring a species endangered are contained in the Endangered Species Act.

Endangerment Assessment: A study to determine the nature and extent of contamination at a site on the National Priorities List and the risks posed to public health or the environment. EPA or the state conducts the study when a legal action is to be taken to direct potentially responsible parties to clean up a site or pay for it. An endangerment assessment supplements a remedial investigation.

Enforceable Requirements: Conditions or limitations in permits issued under the Clean Water Act, Section 402 or 404 that, if violated, could result in the issuance of a compliance order or initiation of a civil or criminal action under federal or applicable state laws. If a permit has not been issued, the term includes any requirement which, in the Regional Administrator's (RA) judgment, would be included in the permit when issued. Where no permit applies, the term includes any requirement which the RA determines is necessary for the best practical waste treatment technology to meet applicable criteria.

Enforcement: EPA, state, or local legal actions to obtain compliance with environmental laws, rules, regulations, or agreements and/or obtain penalties or criminal sanctions for violations. Enforcement procedures may vary, depending on the requirements of different environmental laws and related implementing regulations. Under CERCLA, for example, EPA will seek to require potentially responsible parties to clean up a Superfund site, or pay for the cleanup, whereas under the Clean Air Act the agency may invoke sanctions against cities failing to meet ambient air quality standards that could prevent certain types of construction or federal funding. In other situations, if investigations by EPA and state agencies uncover willful violations, criminal trials and penalties are sought.

Enforcement Decision Document (EDD): A document that provides an explanation to the public of EPA's selection of the cleanup alternative at enforcement sites on the National Priorities List. Similar to a Record of Decision.

Enrichment: The addition of nutrients (e.g., nitrogen, phosphorus, carbon compounds) from sewage effluent or agricultural runoff to surface water, greatly increases the growth potential for algae and other aquatic plants.

Entomb: A method of decommissioning in which radioactive contaminants are encased in a structurally long-lived material, such as concrete. The entombment structure is appropriately maintained, and continued surveillance is carried out until the radioactivity decays to a level permitting decommissioning and ultimate unrestricted release of the property.

Environment: Water, air, land, and all plants and man and other animals living therein, and the interrelationships which exist among them.

Environmental Assessment: An environmental analysis prepared pursuant to the National Environmental Policy Act to determine whether a federal action would significantly affect the environment and thus require a more detailed environmental impact statement.

Environmental Audit: An independent assessment of the current status of a party's compliance with applicable environmental requirements or of a party's environmental compliance policies, practices, and controls.

Environmental Equity: Equal protection from environmental hazards of individuals, groups or communities regardless of race, ethnicity, or economic status.

Environmental Exposure: Human exposure to pollutants originating from facility emissions. Threshold levels are not necessarily surpassed, but low level chronic pollutant exposure is one of the most common forms of environmental exposure.

Environmental Impact Appraisal: An environmental review supporting a negative declaration, i.e., the action is not a major Federal action significantly affecting the environment. It describes a proposed EPA action, its expected environmental impact, and the basis for the conclusion that no significant impact is anticipated.

Environmental Impact Statement: A document required of Federal agencies by the National Environmental Policy Act for major projects or legislative proposals. They provide information for decision makers on the positive and negative effects of the undertaking, and list alternatives to the proposed action, including taking no action. For example, an environmental impact assessment report, prepared by an applicant for an NPDES permit to discharge as a new source, identifies and evaluates the environmental impacts of the applicant's proposed source and feasible alternatives.

Environmental Pathway: All routes of transport by which a toxicant can travel from its release site to human populations including air, food chain, and water. The connected set of environmental media through which a potentially harmful substance travels from source to receptor.

Environmental Protection Agency (EPA): Created in 1970, the EPA is responsible for working with state and local governments to control and prevent pollution in areas of solid and hazardous waste, pesticides, water, air, drinking water, and toxic and radioactive substances.

Epidemiology: The study of the distribution and dynamics of diseases and injuries in human populations. Specifically, the investigation of the possible causes of a disease and its transmission.

Estuary: Regions of interaction between rivers and near-shore ocean waters, where tidal action and river flow mix fresh and salt water. Such areas include bays, mouths of rivers, salt marshes, and lagoons. These brackish water ecosystems shelter and feed marine life, birds, and wildlife.

Evaporation Ponds: Areas where sewage sludge is dumped and dried.

Exposure: contact of an organism with a chemical, radiological, or physical agent. Exposure is quantified as the amount of the agent available at the exchange boundaries of the organism (e.g.; skin, lungs, gut) and available for absorption.

Exposure Assessment: The process of measuring or estimating the intensity, frequency, and duration of human exposures to an agent currently present in the environment or of estimating hypothetical exposures that might arise from the release of new chemicals into the environment.

Exposure Concentration: The concentration of a chemical or other pollutant representing a health threat in a given environment.

Exposure Indicator: A characteristic of the environment measured to provide evidence of the occurrence or magnitude of a response indicator's exposure to a chemical or biological stress.

Exposure Level: The amount (concentration) of a chemical at the absorptive surfaces of an organism.

External Radiation Dose: The dose from sources of radiation located outside the body. This is most often from gamma rays, though beta rays can contribute to dose in the skin and other relatively superficial tissues.

Extra Risk: The added risk to that portion of the population that is not included in measurement of background tumor rate.


False Negative Results: Results which show no effect when one is there.

False Positive Results: Results which show an effect when one is not there.

Feasibility Study:

  • Analysis of the practicability of a proposal; e.g., a description and analysis of potential cleanup alternatives for a site such as one on the National Priorities List. The feasibility study usually recommends selection of a cost-effective alternative. It usually starts as soon as the remedial investigation is underway; together, they are commonly referred to as the "RI/FS".
  • A small-scale investigation of a problem to ascertain whether a proposed research approach is likely to provide useful data.

Federal Facilities Compliance Act (FFCA or FFCAct): An amendment to RCRA, the FFCA waives immunity for DOE and other Federal Agencies, allowing States and the EPA to impose penalties for non-compliance and requires DOE to develop plans for treating the hazardous components of radioactive wastes subject to RCRA requirements.

Fertile material: A material, which is not itself fissile(fissionable by thermal neutrons), that can be converted into a fissile material by irradiation in a reactor. There are two basic fertile materials, uranium-238 and thorium-232. When these fertile materials capture neutrons, they are converted into fissile plutonium-239 and uranium-233, respectively.

Fissile Material: A material consisting of atoms whose nuclei can be split when irradiated with low energy (ideally, zero energy) neutrons.

Fissile Uranium: These isotopes can be split by low to zero energy neutrons in a self-sustaining chain-reaction to release enormous amounts of energy. Natural uranium consists of only 0.711 % U-235, 99.28305 % U-238, and 0.0054 % U-234. U-235 is so fissionable with slow neutrons that a self-sustaining fission chain-reaction can be made to occur in a reactor constructed from natural uranium and a graphite moderator. Standard nuclear reactors use uranium that has been enriched to 3 or 4 % U-235.

Fissile Waste: A subcategory of the other radioactive waste types (LLW, TRU, HLW) that contains U233, U235, Pu238,Pu239, and Pu241 of sufficient percentage to release energy by fission after absorbing neutrons.

Fission: The splitting of the nucleus of an element into fragments. Heavy elements such as uranium or plutonium release energy when fissioned.

Fission Product: Any atom created by the fission of a heavy element. Fission products are usually radioactive.

Flocculation: Process by which clumps of solids in water or sewage aggregate through biological or chemical action so they can be separated from water or sewage.

Fly-ash: Small solid ash particles from the noncombustible portion of fuel that are small enough to escape with the exhaust gases.

Food Chain: Dependence of a series of organisms, one upon the other, for food. The chain begins with plants and ends with the largest carnivores.

Fossil Fuel: Natural gas, petroleum, coal, and any form of solid, liquid, or gaseous fuel derived from such materials for the purpose of creating useful heat.

Frank-effect Level (FEL): Exposure level which produces unmistakable adverse effects, such as irreversible functional impairment or mortality, at a statistically or biologically significant increase in frequency or severity between an exposed population and its appropriate control.

Fuel Cycle: The complete series of steps involved in supplying a fuel. Most often refers to the fissionable fuel for a nuclear reactor and includes management of spent fuel.

Fuel Reprocessing: The processing of reactor fuel to separate the unused fissionable material from waste material.

Fusion: The combining of two nuclei to form a heavier one. Fusion of the isotopes of light elements such as hydrogen or lithium gives a large release of energy.


Gamma Multihit Model: A generalization of the one-hit dose-response model which provides a better description of dose-response data.

Gamma Radiation: High-energy, short wavelength, electromagnetic radiation emitted from the nucleus. Gamma radiation frequently accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead or depleted uranium. Gamma rays are similar to x-rays.

Gaussian Distribution Model: A commonly used assumption about the distribution of values for a parameter, also called the normal distribution. For example, a Gaussian air dispersion model is one in which the pollutant is assumed to spread in air according to such a distribution and described by two parameters, the mean and standard deviation of the normal distribution.

Gaseous Diffusion : A method of isotopic separation based on the fact that gas atoms of molecules with different masses will diffuse through a porous barrier (or membrane) at different rates, according to their weight. This method is used to separate the lighter uranium-235 element from the heavier uranium-238 element. It requires large gaseous diffusion plants (GDPs) and enormous amounts of electric power.

Geographic Information System (GIS): A computer system designed for storing, manipulating, analyzing, and displaying data in a geographic context. See SADA.

Grab Sample: A single sample collected at a particular time and place that represents the composition of the water only at that time and place.

Gray (Gy): The new international system (SI) unit of radiation dose expressed in terms of absorbed energy per unit mass of tissue. The gray is the unit of absorbed dose and replaces the rad. 1 gray = 1 Joule/kilogram and also equals 100 rad.

Greenhouse Effect: The warming of the Earth's atmosphere attributed to a build-up of carbon dioxide or other gases; some scientists think that this build-up allows the sun's rays to heat the Earth, while infra-red radiation makes the atmosphere opaque to a counterbalancing loss of heat.

Gross Alpha/Beta Particle Activity: The total radioactivity due to alpha or beta particle emission as inferred from measurements on a dry sample.

Groundwater: The supply of fresh water under the Earth's surface that forms a natural reservoir.

Groundwater Discharge: Groundwater entering near coastal waters which has been contaminated by landfill leachate, deep well injection of hazardous wastes, septic tanks, etc.


Habitat: The place where a population (e.g., human, animal, plant, microorganism) lives and its surroundings, both living and non-living.

Habitat Indicator: A physical attribute of the environment measured to characterize conditions necessary to support an organism, population, or community in the absence of pollutants, e.g., salinity of esturine waters or substrate type in streams or lakes.

Half-life: The time in which half the atoms of a radioactive substance will have disintegrated, leaving half the original amount. Half of the residue will disintegrate in another equal period of time.

Hazard: A condition or physical situation with a potential for an undesirable consequence, such as harm to life or limb.

Hazard Assessment: An analysis and evaluation of the physical, chemical and biological properties of the hazard.

Hazard Evaluation: A component of risk evaluation that involves gathering and evaluating data on the types of health injury or disease that may be produced by a chemical and on the conditions of exposure under which such health effects are produced.

Hazard Identification: The process of determining whether exposure to an agent can cause an increase in the incidence of a health condition.

Hazard Ranking System (HRS): The principle screening tool used by EPA to evaluate risks to public health and the environment associated with abandoned or uncontrolled hazardous waste sites. The HRS calculates a score based on the potential of hazardous substances spreading from the site through the air, surface water, or groundwater, and on other factors such as density and proximity of human population. This score is the primary factor in deciding if the site should be on the National Priorities List and, if so, what ranking it should have compared to other sites on the list.

Hazardous Air Pollutants: Air pollutants which are not covered by ambient air quality standards but which, as defined in the Clean Air Act, may reasonably be expected to cause or contribute to irreversible illness or death. Such pollutants include asbestos, beryllium, mercury, benzene, coke oven emissions, radionuclides, and vinyl chloride.

Hazardous and Solid Waste Amendments (HSWA): This 1984 Act amended RCRA and required phasing out land disposal of untreated hazardous waste by more stringent hazardous waste management standards (broken down into thirds with a time table for each third). Some of the other mandates of this law include increased enforcement authority for EPA and a program requiring corrective action.

Hazardous Chemical: An EPA designation for any hazardous material requiring an MSDS under OSHA's Hazard Communication Standard. Such substances are capable of producing fires and explosions or adverse health effects like cancer and dermatitis. Hazardous chemicals are distinct from hazardous waste.

Hazardous Substance: 1. Any material that poses a threat to human health and- /or the environment. Typical hazardous substances are toxic, corrosive, ignitable, explosive, or chemically reactive. 2. Any substance designated by EPA to be reported if a designated quantity of the substance is spilled in the waters of the United States or if otherwise released into the environment.

Hazardous Waste (HAZ): HAZ is waste regulated under the Resource Conservation and Recovery Act (RCRA). RCRA regulates solid waste, hazardous waste, and Underground Storage Tanks (USTs) holding petroleum or certain chemicals. Waste that is ignitable, corrosive, reactive, toxic, or contains certain amounts of toxic chemicals is considered hazardous according to the RCRA definition. In Oak Ridge the term Hazardous Waste also included wastes regulated under the Toxic Substances Control Act (TSCA). These are wastes that are contaminated with polychlorobiphenyls (PCB's) or asbestos. When the term Hazardous Waste is used, it implies that the material can be certified NOT to be contaminated with radioactive material, otherwise the term Mixed Waste is used.

Hazards Analysis: Procedures used to (1) identify potential sources of release of hazardous materials from fixed facilities or transportation accidents; (2) determine the vulnerability of a geographical area to a release of hazardous materials; and (3) compare hazards to determine which present greater or lesser risks to a community.

Hazards Identification: Providing information on which facilities have extremely hazardous substances, what those chemicals are, how much there is at each facility, how the chemicals are stored, and whether they are used at high temperatures.

Health Advisory Level: A non-regulatory health-based reference level of chemical traces (usually in ppm) in drinking water at which there are no adverse health risks when ingested over various periods of time. Such levels are established for one day, 10 days, long term and life-time exposure periods. They contain a large margin of safety.

Health Assessment: An evaluation of available data on existing or potential risks to human health posed by a Superfund site. The Agency for Toxic Substances and Disease Registry (ATSDR) of the Department of Health and Human Services (DHHS) is required to perform such an assessment at every site on the National Priorities List.

Health and Safety Study: Any study of any effect of a chemical substance or mixture on health or the environment or on both, including underlying data and epidemiological studies, studies of occupational exposure to a chemical substance or mixture, toxicological, clinical, and ecological studies of a chemical substance or mixture, and any test performed pursuant to the Toxic Substances Control Act (TSCA).

Health Effect: A deviation in the normal function of the human body.

Health Effect Assessment: The component of risk assessment which determines the probability of a health effect given a particular level or range of exposure to a hazard.

Health Hazard (types of): 1. Acute toxicity: The older term used to describe immediate toxicity. Its former use was associated with toxic effects that were severe (e.g., mortality) in contrast to the term "subacute toxicity" that was associated with toxic effects that were less severe. The term "acute toxicity" is often confused with that of acute exposure. 2. Allergic reaction: Adverse reaction to a chemical resulting from previous sensitization to that chemical or to a structurally similar one. 3. Chronic toxicity: The older term used to describe delayed toxicity. However, the term "chronic toxicity" also refers to effects that persist over a long period of time whether or not they occur immediately or are delayed. The term "chronic toxicity" is often confused with that of chronic exposure. 4. Idiosyncratic reaction: A genetically determined abnormal reactivity to a chemical. 5. Immediate versus delayed toxicity: Immediate effects occur or develop rapidly after a single administration of a substance, while delayed effects are those that occur after the lapse of some time. These effects have also been referred to as acute and chronic, respectively. 6. Reversible versus irreversible toxicity: Reversible toxic effects are those that can be repaired, usually by a specific tissue's ability to regenerate or mend itself after chemical exposure, while irreversible toxic effects are those that cannot be repaired. 7. Local versus systemic toxicity: Local effects refer to those that occur at the site of first contact between the biological system and the toxicant; systemic effects are those that are elicited after absorption and distribution of the toxicant from its entry point to a distant site.

Health Physics: The science concerned with the recognition, evaluation, and control of health hazards which may arise from the use and application of ionizing radiation.

Heavy Metals: Metallic elements like mercury, chromium, cadmium, arsenic, and lead, with high molecular weights. They can damage living things at low concentrations and tend to accumulate in the food chain.

HEU: (highly enriched uranium): An important fissile material that has been used for nuclear weapons, usually defined as uranium whose proportion of uranium-235, the fissile isotope of uranium, has been increased to over 90%.

HI (hazard index): The sum of hazard quotients for substances that affect the same target organ or organ system. Because different pollutants (air toxics) can cause similar adverse health effects, combining hazard quotients associated with different substances is often appropriate. EPA has drafted revisions to the national guidelines on mixtures that support combining the effects of different substances in specific and limited ways. Ideally, hazard quotients should be combined for pollutants that cause adverse effects by the same toxic mechanism. Because detailed information on toxic mechanisms is not available for most substances, EPA aggregates the effects when they affect the same target organ regardless of the mechanism. The HI is only an approximation of the aggregate effect on the target organ (e.g., the lungs) because some of the substances might cause irritation by different (i.e., non-additive) mechanisms. As with the hazard quotient, aggregate exposures below an HI of 1.0 derived using target organ specific hazard quotients likely will not result in adverse non-cancer health effects over a lifetime of exposure and would ordinarily be considered acceptable. An HI equal to or greater than 1.0, however, does not necessarily suggest a likelihood of adverse effects. Because of the inherent conservatism of the reference dose (RfD) and reference concentration (RfC) methodology, the acceptability of exceedances must be evaluated on a case-by-case basis, considering such factors as the confidence level of the assessment, the size of the uncertainty factors used, the slope of the dose-response curve, the magnitude of the exceedance, and the number or types of people exposed at various levels above the RfD or RfC. Furthermore, the HI cannot be translated to a probability that adverse effects will occur, and it is not likely to be proportional to risk.

HQ (hazard quotient): The ratio of the potential exposure to the substance and the level at which no adverse effects are expected. A hazard quotient less than or equal to one indicates that adverse noncancer effects are not likely to occur, and thus can be considered to have negligible hazard. HQs greater than one are not statistical probabilities of harm occurring. Instead, they are a simple statement of whether (and by how much) an exposure concentration exceeds the reference dose (RfD) or reference concentration (RfC). Moreover, the level of concern does not increase linearly or to the same extent as HQs increase above one for different chemicals because RfCs do not generally have equal accuracy or precision and are generally not based on the same severity of effect. Thus, we can only say that with exposures increasingly greater than the RfD or RfC, (i.e., HQs increasingly greater than 1), the potential for adverse effects increases, but we do not know by how much. An HQ of 100 does not mean that the hazard is 10 times greater than an HQ of 10. Also an HQ of 10 for one substance may not have the same meaning (in terms of hazard) as another substance resulting in the same HQ.

HLW (high level waste): Generally the highly radioactive material resulting from reprocessing of spent nuclear fuel. This includes mainly the liquid wastes remaining from the recovery of uranium and plutonium in a fuel reprocessing plant. This HLW may also be in the form of sludge, calcine, or other products into which such liquid wastes are converted to facilitate their handling and storage. Such waste contains fission products and traces of TRUs that result in the release of considerable decay energy. For this reason, heavy shielding is required to absorb penetrating radiation, and cooling systems are needed to dissipate decay heat from HLW. Currently, no HLW exists on the Oak Ridge Reservation.

High Risk Community: A community located within the vicinity of numerous sites or facilities or other potential sources of environmental exposure/health hazards which may result in high levels of exposure to contaminants or pollutants. In determining risk or potential risk, factors such as total weight of toxic contaminants, toxicity, routes of exposure, and other factors may be used.

High-to-Low Dose Extrapolation: The process of prediction of low exposure risk to humans from the measured high exposure-high risk data involving rodents.

Holding Pond: A pond or reservoir, usually made of earth, built to store polluted runoff.

Homeostasis: A tendency to stability in the normal body states of the organism.

Hormesis: The notion that small doses of radiation can be healthful.

Hot: A colloquial term meaning highly radioactive.

Hot Spot: The region in a radiation/ contamination area in which the level of radiation/contamination is significantly greater than in neighboring regions in the area.

Human Equivalent Concentration: Exposure concentration for humans that has been adjusted for dosimetric differences between experimental animal species and humans to be equivalent to the exposure concentration associated with observed effects in the experimental animal species. If occupational human exposures are used for extrapolation, the human equivalent concentration represents the equivalent human exposure concentration adjusted to a continuous basis.

Human Equivalent Dose: A dose which, when administered to humans, produces an effect equal to that produced by a dose in animals.

Human Exposure Evaluation: Describing the nature and size of the population exposed to a substance and the magnitude and duration of their exposure. The evaluation could concern past, current, or anticipated exposures.

Human Health Risk: The likelihood that a given exposure or series of exposures may have or will damage the health of individuals.

Hydrogeologic Cycle: The natural process recycling water from the atmosphere down to (and through) the earth and back to the atmosphere again.

Hydrogeology: The geology of groundwater, with particular emphasis on the chemistry and movement of water.

Hydrologic Cycle: Movement or exchange of water between the atmosphere and the earth.

Hydrology: The science dealing with the properties, distribution, and circulation of water.


ICRP: International Commission on Radiological Protection.

Incidence: The number of new cases of a disease in a population over a period of time.

Indicator Organisms: A species, whose presence or absence may be characteristic of environmental conditions in a particular area of habitat; however, species composition and relative abundance of individual components of the population or community are usually considered to be a more reliable index of water quality.

Individual Risk: The risk to an individual rather than to a population.

Induced Radioactivity: Radioactivity produced in any material as a result of nuclear reactions, especially by absorption of neutrons.

Infiltration: The penetration of water through the ground surface into sub-surface soil or the penetration of water from the soil into sewer or other pipes through defective joints, connections, or manhole walls. T he technique of applying large volumes of waste water to land to penetrate the surface and percolate through the underlying soil.

Infiltration Gallery: A subsurface groundwater collection system, typically shallow in depth, constructed with open-jointed or perforated pipes that discharge collected water into a water-tight chamber from which the water is pumped to treatment facilities and into the distribution system. Usually located close to streams or ponds.

Infiltration Rate: The quantity of water than can enter the soil in a specified time interval.

Influent: Water, wastewater, or other liquid flowing into a reservoir, basin, or treatment plant.

Injection Well: A well into which fluids are injected for purposes such as waste disposal, improving the recovery of crude oil, or solution mining.

Injection Zone: A geological formation receiving fluids through a well.

Internal Radiation Dose: The dose to organs of the body from radioactive materials inside the body. It may consist of any combination of alpha, beta, and gamma radiation.

Inversion: An atmospheric condition caused by a layer of warm air preventing the rise of relatively cool air trapped beneath it. This holds down pollutants that might otherwise be dispersed, and can cause an air pollution episode.

In Vitro: Outside the living organism. Literally, in glass.

In Vivo: Within the living organism.

Ion: (1) An atom that has too many or too few electrons, causing it to have an electrical charge, and therefore, be chemically active. (2) An electron that is not associated (in orbit) with a nucleus.

Ionization: The process of adding one or more electrons to, or removing one or more electrons from, atoms or molecules, thereby creating ions. High temperatures, electrical discharges, or nuclear radiations can cause ionization.

Ionize: To split off one or more electrons from an atom, thus leaving it with a positive electric charge. The electrons usually attach to other atoms or molecules giving them a negative charge.

Ionizing Radiation: Any radiation capable of displacing electrons from atoms or molecules, thereby producing ions. Some examples are alpha, beta, gamma, x-rays, neutrons, and ultraviolet light. High doses of ionizing radiation may produce severe skin or tissue damage.

Irradiation: Exposure to radiation.

Isotope: Atoms of the same element that have the same number of protons (and hence the same chemical properties), but a different number of neutrons, and therefore, different atomic weights. Although chemical properties are the same, radioactive and nuclear (radioactive decay) properties may be quite different for each isotope of an element.

Isotopic Enrichment: A process by which the relative abundance of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one particular isotope and depleted in its other isotopic forms.



Karst: A geologic formation of irregular limestone deposits with sinks, underground streams, and caverns.

Kiloton (KT): In the context of nuclear weapons, this term, which means 1,000 tons, is always used as a measure of explosive power. It is equal to the explosive power of 1,000 tons of TNT.


Land Disposal Restrictions (LDR): These restrictions were mandated by the 1984 HSWA amendments to RCRA. They prohibit the disposal of hazardous wastes into or on the land unless the waste meets treatability standards of lower toxicity.

Land Farming (of waste): A disposal process in which hazardous waste deposited on or in the soil is degraded naturally by microbes.

Landfills: Sanitary landfills are disposal sites for non-hazardous solid wastes spread in layers, compacted to the smallest practical volume, and covered by material applied at the end of each operating day. Secure chemical landfills are disposal sites for hazardous waste, selected and designed to minimize the chance of release of hazardous substances into the environment.

Latency Period: The period of time from exposure to an agent to the onset of a health effect.

LC50: Lethal concentration fifty. A calculated concentration [in air] which when administered by the respiratory route is expected to kill 50% of a population of experimental animals during an exposure of four hours. Ambient concentration is expressed in milligrams per liter. A calculated concentration in water which is expected to kill 50% of a population of aquatic organisms after a specified time of exposure.

LD50: Lethal dose fifty. A calculated dose of a chemical substance which is expected to kill 50% of a population of experimental animals exposed through a route other than respiration. Dose is expressed in milligrams per kilogram of body weight.

Leachate: Liquid that has percolated through solid waste and has extracted dissolved or suspended materials from it.

Leaching: The process by which nutrient chemicals or contaminants are dissolved and carried away by water, or are moved into a lower layer of soil.

Lethal dose: (Lethal dose 50/30) The dose of radiation expected to cause death to an exposed population within 30 days to 50 percent (LD 50/30) of those exposed. Typically, the LD 50/30 is in the range from 400 to 450 rem (4 to 5 sieverts) received over a very short period of time.

LEU (low enriched uranium): Most nuclear reactors run on LEU, which is usually 3%-5% uranium-235. LEU cannot be used in nuclear weapons.

Level of Concern: The concentration in air of an extremely hazardous substance above which there may be serious immediate health effects to anyone exposed to it for short periods.

Lifecycle Analysis: Studying the environmental impacts of a product/service from "cradle to grave."

Lifetime Exposure: Total amount of exposure to a substance that a human would receive in a lifetime (usually assumed to be 70 years).

Liquid Waste: Any waste material that is determined to contain "free liquids" as defined by Method 9095 (Paint and Filter Liquid Test), as described in "Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods," Environmental Protection Agency (EPA), Pub. No. SW-846, November 1986. Liquid waste may then be classified as controlled, uncontrolled, or sanitary.

Listed Waste: Wastes listed as hazardous under RCRA but which have not been subjected to the Toxic Characteristics Listing Process because the dangers they present are considered self-evident.

LLW (low level waste): is radioactive waste that is more easily described by what it is not. This is even true of the Policies Acts and Regulations that define LLW as radioactive material that is not TRU, HLW, spent nuclear fuel, or by product material from uranium, plutonium, or thorium mines (Atomic Energy Act section 11(e)2 by-product material). LLW can consist of short-lived and long-lived isotopes.

Logit Model: A dose-response model which, like the probit model, leads to an S-shaped dose-response curve, symmetrical about the 50% response point. The logit model leads to lower "very safe doses" than the probit model even when both models are equally descriptive of the data in the observable range.

Log-probit Model: A dose-response model which assumes that each animal has its own threshold dose, below which no response occurs and above which a tumor [or other effect] is produced by exposure to a chemical.

Lowest-Observed-Adverse-Effect-Level (LOAEL): The lowest dose in an experiment which produced an observable adverse effect.


Malignant: Tending to become progressively worse and to result in death if not treated; having the properties of anaplasia, invasiveness, and metastasis.

Mass Number (symbolized A): The number of nucleons (neutrons and protons) in the nucleus of an atom. Also known as the atomic weight of an atom.

Material Safety Data Sheet (MSDS): A compilation of information required under the OSHA Communication Standard on the identity of hazardous chemicals, health, and physical hazards, exposure limits, and precautions. Section 311 of SARA requires facilities to submit MSDSs under certain circumstances.

Maximum Contaminant Level (MCL): The maximum permissible level of a contaminant in water delivered to any user of a public system. MCLs are enforceable standards.

Maximum Contaminant Level Goal (MCLG): Under the Safe Drinking Water Act, a non-enforceable concentration of a drinking water contaminant, set at the level at which no known or anticipated adverse effects on human health occur and which allows an adequate safety margin. The MCLG is usually the starting point for determining the regulated Maximum Contaminant Level.

Maximum Tolerated Dose: The maximum doses that an animal species can tolerate for a major portion of its lifetime without significant impairment or toxic effect other than carcinogenicity.

Media: Specific environments-air, water, soil-which are the subject of regulatory concern and activities.

Melanoma: A pigmented, highly malignant form of cancer of the skin.

MEM: A management tool to evaluate and categorize EM activities based on the risks associated with the activities of each project.

Metastasis: The transfer of disease from one organ or part to another not directly connected with it.

Metric Ton: 1,000 kilograms or about 2,204 pounds. The usual U.S. ton measurement, called a short ton, is 2,000 pounds.

Micron: One one-millionth of a meter.

Mill Tailings: A slurry of about 40 percent solids (including radioactive particles and chemically hazardous metals) and 60 percent liquid, primarily water.

Mixed Waste: Any radioactive waste that meets the requirements of LLW that contains a RCRA hazardous component. Also, mixed waste is waste that contains both hazardous waste and radioactive material (source, special nuclear, or by-product material as regulated by the Atomic Energy Act of 1954 [42 U.S.C. 2011 et seq.]). Mixed waste is classified by DOE according to the type of radioactive waste that it contains as either mixed low-level waste (MLLW), or mixed transuranic waste (MTRU). DOE's high-level waste (HLW) is assumed to be mixed waste because it contains hazardous components or exhibits the characteristic of corrosivity. If the radionuclides are those that have an atomic number greater than 92, have a half life longer than 20 years, and are present in concentrations greater than 100 nanocuries/gram the waste is termed Mixed TRU waste. The waste in Oak Ridge is predominantly Mixed Low Level Waste.

Mobility: The ability of a chemical element or a pollutant to move into and through the environment (e.g., the mobilization of an element from a water column to sediment).

Molecule: A group of atoms held together by chemical forces. A molecule is the smallest unit of a compound that can exist by itself and retain all of its chemical properties.

Monitoring: Periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and/or pollutant levels in various media or in humans, plants, and animals.

Monitoring Well: A well used to obtain water quality samples or measure groundwater levels. Well drilled at a hazardous waste management facility or Superfund site to collect groundwater samples for the purpose of physical, chemical, or biological analysis to determine the amounts, types, and distribution of contaminants in the groundwater beneath the site.

Morbidity: A departure from a state of physical or mental well-being, resulting from disease or injury. Frequently used only if the affected individual is aware of the condition. Awareness itself connotes a degree of measurable impact. Frequently, but not always, there is a further restriction that some action has been taken such as restriction of activity, loss of work, seeking of medical advice, etc.

Mortality: Death; the death rate; ratio of number of deaths to a given population.

Mortality Rate: The number of deaths that occur in a given population during a given time interval; usually deaths per 103 or 105 people per year. Can be age, sex, race, and cause specific.

MOX: A fuel composed of a mixture of plutonium dioxide and uranium dioxide.

MSHA: The Department of Labor (DOL) Mine Safety and Health Administration (MSHA) is the agency directly responsible for the nation's mine safety and health programs.

Multistage Model: A carcinogenesis dose-response model where it is assumed that cancer originates as a "malignant" cell, which is initiated by a series of somatic-like mutations occurring in finite steps. It is also assumed that each mutational stage can be depicted as a Poisson process in which the transition rate is approximately linear in dose rate.

Mutagen/Mutagenicity: An agent that causes a permanent genetic change in a cell other than that which occurs during normal genetic recombination. Mutagenicity is the capacity of a chemical or physical agent to cause such permanent alternation.


National Ambient Air Quality Standards (NAAQS): Standards established by EPA that apply for outside air throughout the country. (See: criteria pollutants, state implementation plans, emissions trading.)

National Emissions Standards For Hazardous Air Pollutants (NESHAPS): Emissions standards set by EPA for an air pollutant not covered by NAAQS that may cause an increase in fatalities or in serious, irreversible, or incapacitating illness. Primary standards are designed to protect human health, secondary standards to protect public welfare (e.g., building facades, visibility, crops, and domestic animals).

National Estuary Program: A program established under the Clean Water Act Amendments of 1987 to develop and implement conservation and management plans for protecting estuaries and restoring and maintaining their chemical, physical, and biological integrity, as well as controlling point and nonpoint pollution sources.

National Interim Primary Drinking Water Regulations: Commonly referred to as NIPDWRs.

National Municipal Plan: A policy created in 1984 by EPA and the states to bring all publicly owned treatment works (POTWs) into compliance with Clean Water Act requirements.

National Oil and Hazardous Substances Contingency Plan (NOHSCP/NCP): The federal regulation that guides determination of the sites to be corrected under both the Superfund program and the program to prevent or control spills into surface waters or elsewhere.

National Pollutant Discharge Elimination System (NPDES): A provision of the Clean Water Act which prohibits discharge of pollutants into waters of the United States unless a special permit is issued by EPA, a state, or, where delegated, a tribal government on an Indian reservation.

National Priorities List (NPL): EPA's list of the most serious uncontrolled or abandoned hazardous waste sites identified for possible long-term remedial action under Superfund. The list is based primarily on the score a site receives from the Hazard Ranking System. EPA is required to update the NPL at least once a year. A site must be on the NPL to receive money from the Trust Fund for remedial action.

National Response Center: The federal operations center that receives notifications of all releases of oil and hazardous substances into the environment; open 24 hours a day, is operated by the U.S. Coast Guard, which evaluates all reports and notifies the appropriate agency.

National Response Team (NRT): Representatives of 13 federal agencies that, as a team, coordinate federal responses to nationally significant incidents of pollution, oil spill, a major chemical release, or a Superfund response action and provide advice and technical assistance to the responding agency(ies) before and during a response action.

National Secondary Drinking Water Regulations: Commonly referred to as NSDWRs.

Natural Uranium: Uranium as found in nature. It contains 0.7 percent uranium-235, 99.3 percent uranium-238, and a trace of uranium-234.

Navigable Waters: Traditionally, waters sufficiently deep and wide for navigation by all, or specified vessels; such waters in the United States come under federal jurisdiction and are protected by certain provisions of the Clean Water Act.

NARM Wastes (Naturally-Occurring and Accelerator-Produced Radioactive Materials): are orphan wastes not consistently regulated under any current federal standard. NARM includes such materials as radium-226 and thorium-230 produced outside the nuclear fuel-cycle, and radionuclides produced by particle accelerators. NARM wastes are generated by both federal and non-federal facilities.

Neoplasm: An aberrant new growth of abnormal cells or tissue in which the growth is uncontrollable and progressive.

Neutron: An elementary particle slightly heavier than a proton, with no electric charge.

NIOSH: National Institute for Occupational Safety and Health of the Public Health Service, U.S. Department of Health and Human Services (DHHS). A federal agency which, among other activities, tests and certifies respiratory protective devices and air sampling detector tubes, recommends occupational exposure limits for various substances and assists OSHA and MSHA in occupational safety and health investigations and research.

No Further Remedial Action Planned: Determination made by EPA following a preliminary assessment that a site does not pose a significant risk and so requires no further activity under CERCLA.

No Observable Adverse Effect Level (NOAEL): From long-term toxicological studies of agriculture chemical active ingredients, levels at which indicate a safe, lifetime exposure level for a given chemical. Used to establish tolerance for human diets. Also written, NOEL.

Nonpoint Source: A contributing factor to water pollution that cannot be traced to a specific spot; like agricultural fertilizer runoff, sediment from construction.

Non-potable: Water that is unsafe or unpalatable to drink because in contains objectionable pollution, contamination, minerals, or infective agents.

Naturally Occurring Radioactive Materials (NORM): NORM is a subset of NARM and refers to materials not covered under the Atomic Energy Act whose radioactivity has been enhanced (radionuclide concentrations are either increased or redistributed where they are more likely to cause exposure to man) usually by mineral extraction or processing activities. Examples are exploration and production wastes from the oil and natural gas industry and phosphate slag piles from the phosphate mining industry. This term is not used to describe or discuss the natural radioactivity of rocks and soils, or background radiation, but instead refers to materials whose radioactivity is technologically enhanced by controllable practices.

NPL: National Priority List.

NPT: The nuclear Non-Proliferation Treaty.

Nucleon: Common name for a constituent particle of the atomic nucleus. At present, applied to protons and neutrons, but may include any other particles found to exist in the nucleus.

Nuclear Regulatory Commission (NRC): NRC is an independent regulatory agency created out of the Atomic Energy Commission in 1975 to regulate the civilian uses of nuclear material. Specifically, the NRC is responsible for ensuring that activities associated with the operation of nuclear power plants and fuel cycle plants, and medical, industrial, and research applications, are carried out with adequate protection of the public health and safety, the environment, and national security. At full complement, the NRC has five Commissioners nominated by the President and confirmed by the Senate; the President designates one of the Commissioners as Chairman. NRC regulates all commercial Atomic Energy Act materials. Except in a few cases, NRC does not regulate DOE. NRC does not regulate NARM.

Nucleus: The nucleus of an atom is the central core that comprises almost all the weight of the atom. All atomic nuclei (except H-1, which has a single proton) contain both protons and neutrons.

NRDA (Natural Resource Damage Assessment): Restores natural resources by returning ecosystems to full function and compensating the public for injuries to its resources. This is part of the natural resource management role of Department of Natural Resource trustees.


Oncogenic: A substance that causes tumors, whether benign or malignant.

One-hit Model: The basic dose-response model based on the concept that a tumor can be induced by a single receptor that has been exposed to a single quantum or effective dose unit of a chemical.

Operable Unit: Term for each of a number of separate activities undertaken as part of a Superfund site cleanup. A typical operable unit would be removal of drums and tanks from the surface of a site.

Operation And Maintenance: Activities conducted after a Superfund site action is completed to ensure that the action is effective. Actions taken after construction to assure that facilities constructed to treat waste water will be properly operated and maintained to achieve normative efficiency levels and prescribed effluent limitations in an optimum manner. On-going asbestos management plan in a school or other public building, including regular inspections, various methods of maintaining asbestos in place, and removal when necessary.

Oral Toxicity: Ability of a pesticide to cause injury when ingested.

OSHA: Occupational Safety and Heath Administration of the U.S. Department of Labor. Federal agency with safety and health regulatory and enforcement authorities for most U.S. industry and business.

Outfall: The place where effluent is discharged into receiving waters.


PAH: Polycyclic aromatic hydrocarbon.

Particle: A tiny mass of material. Airborne particles, materials that exist in the atmosphere as a solid or liquid, can be natural, caused by stirring of soil dusts, or anthropogenic. They vary in size from coarse (diameter > 3 Ám) to fine (< 3Ám) . Sometimes inhalable or respirable is used to describe those particles (< 2 Ám) which can be inhaled through the nose and enter the lungs.

Particulates: Fine liquid or solid particles such as dust, smoke, mist, fumes, or smog, found in the air or emissions.

Partition Coefficient: Measure of the sorption phenomenon, whereby a pesticide is divided between the soil and water phase; also referred to as adsorption partition coefficient.

Perched Water: Zone of unpressurized water held above the water table by impermeable rock or sediment.

Percolation: Downward flow or filtering of water through pores or spaces in rock or soil.

Performance Data (for incinerators): Information collected, during a trial burn, on concentrations of designated organic compounds and pollutants found in incinerator emissions. Data analysis must show that the incinerator meets performance standards under operating conditions specified in the RCRA permit.

Performance Standards: (1) Regulatory requirements limiting the concentrations of designated organic compounds, particulate matter, and hydrogen chloride in emissions from incinerators. (2) Operating standards established by EPA for various permitted pollution control systems, asbestos inspections, and various program operations and maintenance requirements.

Person-rem: Used as a unit of population dose; the average dose per individual expressed in rems times the population affected.

Person-year: The sum of the number of years each person in the study population is at risk; a metric used to aggregate the total population at risk assuming that 10 people at risk for one year is equivalent to 1 person at risk for 10 years.

pH: A measure of the acidity or alkalinity of a material, liquid or solid (pH is represented on a scale of 0 to 14 with 7 representing a neutral state, 0 representing the most acid, and 14 the most alkaline).

Photon: The indivisible unit, or quantum, of electromagnetic radiation. The energy of the photons determines the nature of the radiation, from radio waves at the lowest energy levels, up through infra-red, visible, and ultra-violet light, to X-or gamma-rays, which have energy high enough to ionize atoms.

Picocurie: One trillionth (10-12) of a curie.

Plume: A visible or measurable discharge of a contaminant from a given point of origin. Can be visible or thermal in water as it extends downstream from the pollution source, or visible in air as, for example, a plume of smoke. The area of radiation leaking from a damaged reactor. Area downwind within which a release could be dangerous for those exposed to leaking fumes.

Plutonium: A highly toxic, heavy, radioactive metallic element. There are 15 isotopes of plutonium, of which only five are produced in significant quantities: plutonium-238, -239, -240, -241, and -242. Plutonium-239 is the most important plutonium isotope as it is fissile and is used in nuclear weapons and some reactors. One the other hand, plutonium-240 is unsuitable for use in nuclear weapons and reactor fuel. Thus, in a reactor whose main purpose is plutonium production, the rate at which plutonium-240 is formed controls the length of time fuel is allowed to remain under irradiation. Plutonium is categorized according to plutonium-240 content, as follows: super-grade has 2-3% Pu-240; weapons-grade has less than 7% Pu-240; fuel-grade has 7-18 (or sometimes given as 7-19) % Pu-240; and reactor-grade has 18 or greater (or 19 or greater) % Pu-240. (Note: Despite what the name implies, "reactor-grade" plutonium has been used successfully to make a nuclear bomb.)

Point Source: A single isolated stationary source of pollution.

Pollutant: Any material entering the environment that has undesired effects.

Pollution: The presence of matter or energy whose nature, location or quantity produces undesired environmental effects.

Population at Risk: A limited population that may be unique for a specific dose-effect relationship; the uniqueness may be with respect to susceptibility to the effect or with respect to the dose or exposure itself.

Population Dose (population exposure): The summation of individual radiation doses received by all those exposed to the source or event being considered.

Positron: An elementary particle with a positive electric charge, but in other respects identical with an electron.

Preliminary Assessment: The process of collecting and reviewing available information about a known or suspected waste site or release.

Primary Drinking Water Regulation: Applies to public water systems and specifies a contaminant level, which, in the judgment of the EPA Administrator, will not adversely affect human health.

Probability: The chance that a particular event will occur given the population of all possible events. See definition for risk.

Probable Error: The magnitude of error which is estimated to have been made in determination of results.

Probit Analysis: A statistical transformation which will make the cumulative normal distribution linear. In analysis of dose-response, when the data on response rate as a function of dose are given as probits, the linear regression line of these data yields the best estimate of the dose-response curve. The probit unit is y = 5 + Z(p) , where p = the prevalence of response at each dose level and Z(p) = the corresponding value of the standard cumulative normal distribution.

Process Wastes: Any designated toxic pollutant or combination of pollutants, whether in wastewater or otherwise present, which is inherent to or unavoidable resulting from any manufacturing process, including that which comes into direct contact with or results from the production or use of any raw material, intermediate product, finished product, byproduct or waste product and is discharged into the navigable waters.

Proportionate Mortality Ratio (PMR): The fraction of all deaths from a given cause in the study population divided by the same fraction from a standard population. A tool for investigating cause-specific risks when only data on deaths are available. If data on the population at risk are also available, SMRs are preferred.

Prospective Study: An inquiry in which groups of individuals are selected in terms of whether they are or are not exposed to certain factors, and then followed over time to determine differences in the rate at which disease develops in relation to exposure to the factor. Also called cohort study.

Proton: An elementary particle with a positive electric charge and a mass that is given the value 1 on the scale of atomic weights.

Public Comment Period: The time allowed for the public to express its views and concerns regarding an action by EPA (e.g., a Federal Register Notice of proposed rule-making, a public notice of a draft permit, or a Notice of Intent to Deny).

Public Hearing: A formal meeting wherein EPA officials hear the public's views and concerns about an EPA action or proposal. EPA is required to consider such comments when evaluating its actions. Public hearings must be held upon request during the public comment period.

Public Notice: 1. Notification by EPA informing the public of Agency actions such as the issuance of a draft permit or scheduling of a hearing. EPA is required to ensure proper public notice, including publication in newspapers and broadcast over radio stations. 2. In the safe drinking water program, water suppliers are required to publish and broadcast notices when pollution problems are discovered.

Pyrophoric Uranium: A chemical that will ignite spontaneously in air at or below a temperature of 130 Fahrenheit (54.5 C). Uranium metal is pyrophoric especially when finely divided. This is a fire hazard as any pyrophoric substance can spontaneously self-ignite when exposed to normal atmospheric conditions. Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint.


Quality Assurance/Quality Control: A system of procedures, checks, audits, and corrective actions to ensure that all EPA research design and performance, environmental monitoring and sampling, and other technical and reporting activities are of the highest achievable quality.


Rad: A unit of absorbed dose of radiation defined as deposition of 100 ergs of energy per gram of tissue. It amounts to approximately one ionization per cubic micron.

Radiation: The emission of particles (i.e., alpha, beta, or gamma particles) or rays (i.e., alpha, beta, gamma, or x-rays) by the nucleus of an atom.

Radiation Shielding: Reduction of radiation by interposing a shield of absorbing material between any radioactive source and a person, work area, or radiation-sensitive device.

Radiation Sickness (syndrome): The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure (greater than 200 rads or 2 gray) of the whole body (or large part) to ionizing radiation. The earliest of these symptoms are nausea, fatigue, vomiting, and diarrhea, which may be followed by loss of hair (epilation), hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, where the radiation exposure has been approximately 1,000 rad (10 gray) or more, death may occur within two to four weeks. Those who survive 6 weeks after the receipt of a single large dose of radiation to the whole body may generally be expected to recover.

Radioactive (Decay): Property of undergoing spontaneous nuclear transformation in which nuclear particles or electromagnetic energy are emitted.

Radioactivity: The spontaneous discharge of radiation from atomic nuclei. This is usually in the form of beta or alpha radiation, together with gamma radiation. Beta or alpha emission results in transformation of the atom into a different element, changing the atomic number by +1 or -2 respectively.

Radioisotope: A radioactive isotope. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. More that 1300 natural and artificial radioisotopes have been identified.

Radionuclides: Radioactive elements. These may be subdivided into natural radionuclides such as radium or uranium which are normally present in the earth, or artificial radionuclides which are not normally present (or normally present in very small amounts) and are produced by nuclear fission.

Radium (Ra): A radioactive metallic element with atomic number 88. As found in nature, the most common isotope has a mass number of 226. It occurs in minute quantities associated with uranium in pitchblende, camotite, and other minerals.

Radon (Rn): A radioactive element that is one of the heaviest gases known. Its atomic number is 86. It is a daughter of radium.

RAUs: Risk analysis units.

RCRA: Resource Conservation and Recovery Act. RCRA gave EPA authority to control hazardous waste from " cradle-to-grave." This includes the minimization, generation, transportation, treatment, storage, and disposal of hazardous waste. RCRA also set forth a framework for the management of non-hazardous solid wastes. RCRA focuses only on active and future facilities and does not address abandoned or historical sites.

Reasonable Maximum Exposure (RME): The maximum exposure reasonably expected to occur in a population.

Reasonably Available Control Technology (RACT): Control technology that is both reasonably available, and both technologically and economically feasible. Usually applied to existing sources in nonattainment areas; in most cases is less stringent than new source performance standards.

Receiving Waters: bodies of water that receive runoff or wastewater discharges, such as rivers, streams, lakes, estuaries, and groundwater.

Recharge: The process by which water is added to a zone of saturation, usually by percolation from the soil surface, e.g., the recharge of an aquifer.

Recharge Area: A land area in which water reaches the zone of saturation from surface infiltration, e.g., where rainwater soaks through the earth to reach an aquifer.

Recharge Rate: The quantity of water per unit of time that replenishes or refills an aquifer.

Recommended Maximum Contaminant Level (RMCL): The maximum level of a contaminant in drinking water at which no known or anticipated adverse affect on human health would occur, and that includes an adequate margin of safety. Recommended levels are nonenforceable health goals.

Record of Decision (ROD): A public document that explains which cleanup alternative(s) will be used at National Priorities List sites where, under CERCLA, Trust Funds pay for the cleanup.

Reference Concentration (RfC): An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious noncancer effects during a lifetime.

Reference Dose: Toxicity value for evaluating noncarcinogenic (systemic) effects of daily exposure to contaminant levels without appreciable deleterious effects during a lifetime. See our toxicity values.

Regional Deposited Dose (RDD): The deposited dose of particles calculated for the region of interest as related to the observed effect. For respiratory effects of particles, the deposited dose is adjusted for ventilatory volumes and the surface area of the respiratory region effected (mg/ For extra respiratory effects of particles, the deposited dose in the total respiratory system is adjusted for ventilatory volumes and body weight (mg/min-kg).

Relative Biological Effectiveness (RBE): A factor that can be determined for different types of ionizing radiation, representing the relative amount of biological change caused by 1 rad. It depends upon the density of ionization along the tracks of the ionizing particles, being highest for the heavy particles: alpha rays and neutrons.

Relative Risk: The ratio of the rate of the disease (usually incidence or mortality) among those exposed to the rate among those not exposed.

Rem: A unit of equivalent absorbed dose of radiation, taking account of the relative biological effectiveness of the particular radiation. The dose in rems is the dose in rads multiplied by the RBE.

Remedial Action (RA): The actual construction or implementation phase of a Superfund site cleanup that follows remedial design.

Remedial Design: A phase of remedial action that follows the remedial investigation/ feasibility study and includes development of engineering drawings and specifications for a site cleanup.

Remedial Investigation (RI): An in-depth study designed to gather data needed to determine the nature and extent of contamination at a Superfund site; establish site cleanup criteria; identify preliminary alternatives for remedial action; and support technical and cost analyses of alternatives. The remedial investigation is usually done with the feasibility study. Together they are usually referred to as the "RI/FS".

Remedial Project Manager (RPM): The EPA or state official responsible for overseeing on-site remedial action.

Remedial Response: Long-term action that stops or substantially reduces a release or threat of a release of hazardous substances that is serious but not an immediate threat to public health.

Remediation: Cleanup or other methods used to remove or contain a toxic spill or hazardous materials from a Superfund site; for the Asbestos Hazard Emergency Response program, abatement methods including evaluation, repair, enclosure, encapsulation, or removal of greater than 3 linear feet or square feet of asbestos-containing materials from a building.

Remote Handling: Techniques used for the handling of radioactive materials behind the protection of walls which will absorb the radiation. It includes the use of robotics in radioactive areas.

Removal Action: Short-term immediate actions taken to address releases of hazardous substances that require expedited response.

Reportable Quantity (RQ): Quantity of a hazardous substance that triggers reports under CERCLA. If a substance exceeds its RQ, the release must be reported to the National Response Center, the State Emergency Response Commission, and community emergency coordinators for areas likely to be affected.

Repository: A permanent resting place for radioactive wastes, which will finally decay to natural levels of radioactivity.

Reprocessing: Chemical treatment of spent fuel from a nuclear reactor to separate unused uranium and plutonium from radioactive fission product wastes. This allows recycle of valuable fuel material and minimizes the volume of high-level waste materials.

Response Action: Generic term for actions taken in response to actual or potential health-threatening environmental events such as spills, sudden releases, and asbestos abatement/management problems; A CERCLA-authorized action involving either a short-term removal action or a long-term removal response. This may include but is not limited to: removing hazardous materials from a site to an EPA-approved hazardous waste facility for treatment, containment or treating the waste on-site, identifying and removing the sources of groundwater contamination and halting further migration of contaminants; 3. Any of the following actions taken in school buildings in response to AHERA to reduce the risk of exposure to asbestos: removal, encapsulation, enclosure, repair, and operations and maintenance.

RH (Remote Handled) Waste: TRU waste that 1) emits an external dose rate greater than 0.2 rem/hr (200 mrem/hr) and less than or equal to 1000 rem/hr. 2) has a relatively large quantity of beta and gamma emitting radionuclides with half lives typically 30 years or less. 3) requires shielding.

Risk: The product of: impact of severity (consequence) and impact of likelihood (probability). Specifically for carcinogenic effects, risk is estimated as the incremental probability of an individual developing cancer over a lifetime as a result of exposure to a potential carcinogen. Specifically for noncarcinogenic (systemic) effects, risk is not expressed as a probability but rather is evaluated by comparing an exposure level over a period of time to a reference dose derived for a similar exposure period.

Risk Analysis: A detailed examination including risk assessment, risk evaluation, and risk management alternatives, performed to understand the nature of unwanted, negative consequences to human life, health, property, or the environment; an analytical process to provide information regarding undesirable events; the process of quantification of the probabilities and expected consequences for identified risks.

Risk Assessment: The process of establishing information regarding acceptable levels of a risk and/or levels of risk for an individual, group, society, or the environment.

Risk Characterization: This last step in the risk assessment process characterizes the potential for adverse health effects and evaluates the uncertainty involved.

Risk Communication: The exchange of information about health or environmental risks among risk assessors and managers, the general public, news media, interest groups, etc.

Risk Estimate: A description of the probability that organisms exposed to a specific dose of a chemical or other pollutant will develop an adverse response (e.g., cancer).

Risk Estimation: The scientific determination of the characteristics of risks, usually in as quantitative a way as possible. These include the magnitude, spatial scale, duration and intensity of adverse consequences and their associated probabilities as well as a description of the cause and effect links.

Risk Evaluation: A component of risk assessment in which judgments are made about the significance and acceptability of risk.

Risk Factor: Characteristic (e.g., race, sex, age, obesity) or variable (e.g., smoking, occupational exposure level) associated with increased probability of a toxic effect.

Risk Identification: Recognizing that a hazard exists and trying to define its characteristics. Often risks exist and are even measured for some time before their adverse consequences are recognized. In other cases, risk identification is a deliberate procedure to review, and it is hoped, anticipate possible hazards.

Risk Management: The process of evaluating and selecting alternative regulatory and non-regulatory responses to risk. The selection process necessarily requires the consideration of legal, economic, and behavioral factors.

Risk Specific Dose: The dose associated with a specified risk level.

Roentgen: A unit of gamma radiation measured by the amount of ionization in air. In non-bony biological tissue 1 roentgen delivers a dose approximately equal to 1 rad.


Sanitary Waste: This waste stream consists of cardboard, paper, plastic, glass, metal, food waste, office waste, etc.

Sanitary Liquid waste: is non-radioactive and non-hazardous material that meets the definition of a liquid waste.

Sanitary Solid Waste: Non-radioactive and non-hazardous material including garbage, refuse, and other discarded solid sanitary waste materials including those materials resulting from industrial, commercial, and agricultural operations, and from community activities. Solid sanitary waste does not include solids or dissolved material in domestic sewage or other significant pollutants in water resources, such as silt, dissolved or suspended solids in industrial waste water effluents, dissolved materials in irrigation return flows or other common water pollutants.

SARA: The Superfund Amendments and Reauthorization Act of 1986 was enacted to revise and extend the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980. CERCLA authorizes Federal cleanup of uncontrolled hazardous waste sites and the response to releases of hazardous substances. SARA and CERCLA fund the program to carry out the EPA solid waste emergency and long-term removal/remediation activities.

Secondary Drinking Water Regulations: Non-enforceable regulations applying to public water systems and specifying the maximum contamination levels that, in the judgment of EPA, are required to protect the public welfare. These regulations apply to any contaminants that may adversely affect the odor or appearance of such water and consequently may cause people served by the system to discontinue its use.

Secure Maximum Contaminant Level: Maximum permissible level of a contaminant in water delivered to the free flowing outlet of the ultimate user, or of contamination resulting from corrosion of piping and plumbing caused by water quality.

Sediment: Eroded soil and rock material, and plant debris, transported and deposited by water.

Specific Activity: A measure of the radioactivity of a unit weight (generally one gram) of material.

Sievert: A unit of equivalent absorbed dose equal to 100 rems.

Site: An area or place (usually contaminated) within the jurisdiction of the EPA and/or a state.

Site Assessment Program: A means of evaluating hazardous waste sites through preliminary assessments and site inspections to develop a Hazard Ranking System score.

Site Inspection: The collection of information from a Superfund site to determine the extent and severity of hazards posed by the site. It follows and is more extensive than a preliminary assessment. The purpose is to gather information necessary to score the site, using the Hazard Ranking System, and to determine if it presents an immediate threat requiring prompt removal.

Slope Factor: Toxicity value for evaluating the probability of an individual developing cancer from exposure to contaminant levels over a lifetime. See our toxicity values.

Solid Waste: Any solid, semi-solid, liquid and containerized gaseous material generated as a result of routine operations and/or construction/demolition activities. This is the legal definition per the Resource Conservation and Recovery Act.

Somatic Effects of Radiation: Effects of radiation limited to the exposed individual, as distinguished from genetic effects, which may also affect subsequent unexposed generations.

Source: A place where pollutants are emitted, for example a chimney stack.

Source Material: Source Material is the Uranium or Thorium ores mined from the Earth. Source material is defined in 10 CFR 20.1003 as "(1) Uranium, or thorium or any combination of uranium and thorium in any physical or chemical form; or (2) Ores that contain, by weight, one-twentieth of 1 percent (0.05 percent), or more, of uranium, thorium, or any combination or uranium and thorium. Source material does not include special nuclear material."

Source Term: The release rate of hazardous agent from a facility or activity.

Special Nuclear Material (SNM): SNM is defined in 10 CFR 20.1003 as "(1) Plutonium, uranium-233, uranium enriched in the isotope 233 or in isotope 235, and any other material that the NRC, pursuant to the provisions of section 51 of the Atomic Energy Act, determines to be SNM, but does not include source material; (2) or any material artificially enriched by any of the foregoing but does not include source material." SNM is important in the fabrication of weapons grade materials and as such has strict licensing and handling controls.

Spent Fuel: Spent fuel consists of irradiated fuel discharged from a nuclear reactor. Three categories of spent fuel are: fuel from commercial light-water reactors (LWRs), fuel from non-LWR commercial reactors, and special fuels associated with government-sponsored research and demonstration programs, universities, and private industries.

Spontaneous Fission: The spontaneous splitting of the nucleus into two new nuclei, generally with the emission of one or more neutrons and the release of energy.

Standard Deviation: A measure of dispersion or variation, usually taken as the square root of the variance.

Standard Geometric Deviation: Measure of dispersion of values about a geometric mean; the portion of the frequency distribution that is one standard geometric deviation to either side of the geometric mean; accounts for 68% of the total samples.

Standard Normal Deviation: Measure of dispersion of values about a mean value; the positive square root of the average of the squares of the individual deviations from the mean.

Statistical Significance: The statistical significance determined by using appropriate standard techniques of statistical analysis with results interpreted at the stated confidence level and based on data relating species which are present in sufficient numbers at control areas to permit a valid statistical comparison with the areas being tested.

Steady State Exposure: Exposure to an environmental pollutant whose concentration remains constant for a period of time.

Stochastic Effects: Effects that occur by chance, generally occurring without a threshold level of dose, whose probability is proportional to the dose and whose severity is independent of the dose. In the context of radiation protection, the main stochastic effects are cancer and genetic effects.

Subchronic: Of intermediate duration, usually used to describe studies or levels of exposure between 5 and 90 days.

Superfund: The program operated under the legislative authority of CERCLA and SARA that funds and carries out EPA solid waste emergency and long-term removal and remedial activities. These activities include establishing the National Priorities List, investigating sites for inclusion on the list, determining their priority, and conducting and/or supervising the cleanup and other remedial actions.

Superfund Innovative Technology Evaluation: EPA program to promote development and use of innovative treatment technologies in Superfund site cleanups.

Surface Impoundment: Treatment, storage, or disposal of liquid hazardous wastes in ponds.

Surface Runoff: Precipitation, snow melt, or irrigation in excess of what can infiltrate the soil surface and be stored in small surface depressions; a major transporter of nonpoint source pollutants.

Surface Water: All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors directly influenced by surface water.

Synergetic: Working together; an agent that works synergistically with one or more other agents.

Synergism: An interaction between two substances that results in a greater effect than either of the substances could have had acting independently.

Synergistic Effects: Joint effects of two or more agents, such as drugs that increase each other's effectiveness when taken together.

Systemic Effects: Systemic effects are those that require absorption and distribution of the toxicant to a site distant from its entry point, at which point effects are produced. Most chemicals that produce systemic toxicity do not cause a similar degree of toxicity in all organs, but usually demonstrate major toxicity to one or two organs. These are referred to as the target organs of toxicity for that chemical. Systemic effects do not include cancer.


Target Organ: The biological organ(s) most adversely affected by exposure to a chemical, physical, or biological agent.

TCE: Trichloroethene (trichloroethylene) is an organic solvent and degreaser.

Technology-Based Limitations: Industry-specific effluent limitations applied to a discharge when it will not cause a violation of water quality standards at low stream flows. Usually applied to discharges into large rivers.

Technology-Based Standards: Effluent limitations applicable to direct and indirect sources which are developed on a category-by-category basis using statutory factors, not including water-quality effects.

Teratogenic: Substances that are suspected of causing malformations or serious deviations from the normal type, which can not be inherited in or on animal embryos or fetuses.

Thermonuclear Weapon: A nuclear weapon that gets a large part of its explosive power from fusion reactions.

Thorium (Th): A radioactive element with the atomic number 90 and, as found in natural ores, an atomic weight of approximately 232. There are 11 other known isotopes. Thorium is used in the process of making nuclear bombs. Thorium is also used in ceramics, glass and gas mantles.

Threshold: A pollutant concentration [or dose] below which no deleterious effect occurs.

Threshold Dose: The minimum application of a given substance required to produce an observable effect.

Threshold Level: Time-weighted average pollutant concentration values, exposure beyond which is likely to adversely affect human health.

Threshold Limit Value (TLV): Refers to airborne concentrations of substances and represents conditions under which it is believed that nearly all workers are protected while repeatedly exposed for an 8-hr day, 5 days a week (expressed as parts per million (ppm) for gases and vapors and as milligrams per cubic meter (mg/m3) for fumes, mists, and dusts).

TNT Equivalent: The weight of TNT which would release the same amount of energy as a particular nuclear explosion. One ton of TNT releases approximately 1.2 billion calories (that is, 5.1 kiloJoules per gram). Nuclear explosions are usually measured in kilotons (KT) or megatons (MT).

Tolerances: Permissible residue levels for pesticides in raw agricultural produce and processed foods. Whenever a pesticide is registered for use on a food or a feed crop, a tolerance (or exemption from the tolerance requirement) must be established. EPA establishes the tolerance levels, which are enforced by the Food and Drug Administration and the Department of Agriculture.

Ton: A metric ton is 1,000 kilograms. This is approximately 2,200 pounds, and very nearly equal to a British ton (2,240 pounds). The U.S. ton is 2,000 pounds.

Toxicant: A substance that kills or injures an organism through chemical or physical action or by altering the organism's environment; for example, cyanides, phenols, pesticides, or heavy metals; especially used for insect control.

Toxicity: The degree of danger posed by a substance to animal or plant life.

Toxicity Assessment: Characterization of the toxicological properties and effects of a chemical, with special emphasis on establishment of dose response characteristics.

Toxicity Profile: An examination, summary, and interpretation of a hazardous substance to determine levels of exposure and associated health effects. See our toxicity profiles.

Toxicology: The study of the adverse effects of chemicals and radionuclides on living organisms.

Toxic Substance: A chemical or mixture that may present an unreasonable risk of injury to health or the environment.

Toxic Wastes: Wastes that contain substances in sufficient quantity to impinge harmfully on biological systems.

Trace: A very small amount of a material. Usually used in reference to concentrations which are on the order of or less than 1-10 parts per million.

Trace Metals: Metals normally found in trace amounts due to their insolubility or to their relative lack of abundance in the crust of the earth.

Treatment: (1) Any method, technique, or process designed to remove solids and/or pollutants from solid waste, waste streams, effluents, and air emissions. (2) methods used to change the biological character or composition of any regulated medical waste so as to substantially reduce or eliminate its potential for causing disease.

Tritium: A radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons.

TRU (transuranic waste): Waste that contains more than 100 nCi/g of alpha emitting isotopes with atomic numbers greater than 92 and half-lives greater than 20 years. Such wastes result primarily from fuel reprocessing and from the fabrication of plutonium weapons and plutonium-bearing reactor fuel. Generally, little or no shielding is required ("contact-handled" TRU waste), but energetic gamma and neutron emissions from certain TRU nuclides and fission-product contaminants may require shielding or remote handling ("remote-handled" TRU waste).

Trust Fund (CERCLA): A fund set up under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) to help pay for cleanup of hazardous waste sites and for legal action to force those responsible for the sites to clean them up.

Tumor: Any abnormal mass of cells resulting from excessive cellular multiplication.


Uncertainty Analysis: A detailed examination of the systematic and random errors of a measurement or estimate; an analytical process to provide information regarding the uncertainty.

Uncertainty Factor: One of several, generally 10-fold factors, used in operationally deriving the Reference Dose (RfD) from experimental data. UFs are intended to account for (1) the variation in sensitivity among the members of the human population; (2) the uncertainty in extrapolating animal data to the case of humans; (3) the uncertainty in extrapolating from data obtained in a study that is of less-than-lifetime exposure; and (4) the uncertainty in using LOAEL data rather than NOAEL data.

Unconfined Aquifer: An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well.

Uncontrolled Liquid Waste: Waste that meets the definition of a liquid waste and is not bounded by container or piping, e.g., runoff, natural ponds, streams.

Underground Injection Control (UIC): The program under the Safe Drinking Water Act that regulates the use of wells to pump fluids into the ground.

Underground Sources of Drinking Water: Aquifers currently being used as a source of drinking water or those capable of supplying a public water system. They have a total dissolved solids content of 10,000 milligrams per liter or less, and are not "exempted aquifers."

Underground Storage Tank: A tank located at least partially underground and designed to hold gasoline or other petroleum products or chemicals.

Unit Risk: The unit risk factors (URFs) provide estimates of the risks due to a unit inventory of contaminant (i.e., risk/gram or risk/curie). URFs can be calculated for water, soil, air, and radiation. URFs can be used to calculate risk for quantities greater than unity only if the relationship is linear.

Unreasonable Risk: Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), "unreasonable adverse effects" means any unreasonable risk to man or the environment, taking into account the medical, economic, social, and environmental costs and benefits of any pesticide.

Uranium: A radioactive element with the atomic number 92 and, as found in natural ores, an atomic weight of approximately 238. The two principal natural isotopes are uranium-235 (0.7 percent of natural uranium), which is fissile, and uranium-238 (99.3 percent of natural uranium), which is fissionable by fast neutrons and is fertile. Natural uranium also includes a minute amount of uranium-234.

Uranium Chips Waste: Machine turnings which were cut from uranium parts that consist of <1% U235 (loose chips or wool bundle).


Variance: Government permission for a delay or exception in the application of a given law, ordinance, or regulation.

Viability Assessment: A Department of Energy decision making process to judge the prospects for geologic disposal of high-level radioactive wastes at Yucca Mountain based on (1) specific design work on the critical elements of the repository and waste package, (2) a total system performance assessment that will describe the probable behavior of the repository, (3) a plan and cost estimate for the work required to complete a license application, and (4) an estimate of the costs to construct and operate the repository (see 10 CFR Part 60).

Volatile Organic Compound (VOC): Any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity.


Waste Characterization: Identification of chemical and microbiological constituents of a waste material.

Waste Isolation Pilot Plant (WIPP): The WIPP, which is under development by the Department of Energy, is a potential geologic disposal facility for transuranic (TRU) radioactive waste generated as by-products from DOE's nuclear weapons production. The WIPP is located underground in excavated, natural salt formations, near Carlsbad, New Mexico. Before DOE can dispose of waste at the WIPP, it must demonstrate that the WIPP complies with EPA's radioactive waste disposal standards. DOE must submit a "compliance application" to EPA showing how the WIPP facility will meet the standards. The WIPP facility is scheduled to begin operation in the Spring of 1998, subject to EPA approval of DOE's compliance application. Also, DOE plans to submit a petition to EPA, to demonstrate that Mixed TRU (MTRU) disposal at the WIPP will not migrate beyond the WIPP unit boundary, and therefore the waste would not need to be treated to meet RCRA Land Disposal Restrictions (LDR). At present, DOE is required to treat all MTRU to meet the appropriate LDR's.

Water Pollution: The addition of sewage, industrial wastes, or other harmful or objectionable material to water in concentrations or in sufficient quantities to result in measurable degradation of water quality.

Water Quality Criteria: Levels of water quality expected to render a body of water suitable for its designated use. Criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes.

Water Quality Standards: State-adopted and EPA-approved ambient standards for water bodies. The standards prescribe the use of the water body and establish the water quality criteria that must be met to protect designated uses.

Water Quality-Based Limitations: Effluent limitations applied to dischargers when mere technology-based limitations would cause violations of water quality standards. Usually applied to discharges into small streams.

Water Quality-Based Permit: A permit with an effluent limit more stringent than one based on technology performance. Such limits may be necessary to protect the designated use of receiving waters (i.e., recreation, irrigation, industry or water supply).

Watershed: The land area that drains into a stream; the watershed for a major river may encompass a number of smaller watersheds that ultimately combine at a common delivery point.

Water Table: Top of an unconfined aquifer, below which the pore spaces are saturated with water.

Well Injection: The subsurface emplacement of fluids into a well.

Well Monitoring: Measurement, by on-site instruments or laboratory methods, of the quality of water in a well.

Well Plug: A watertight and gastight seal installed in a bore hole or well to prevent movement of fluids.


X-rays: Penetrating electromagnetic radiation (photon) having a wavelength that is much shorter than that of visible light. These rays are usually produced by excitation of the electron field around certain nuclei.

Xenobiote: Any biote displaced from its normal habitat; a chemical foreign to a biological system.


Yield: The energy released by a nuclear explosion.

Yucca Mountain: Located in Nevada, Yucca Mountain is being characterized as a potential geologic repository for High Level Waste, Spent Nuclear Fuel, and possibly for waste that is defined as Greater-than-Class-C (GTCC). A key element of permanent disposal is that it must be able to isolate highly radioactive waste for thousands of years because its radioactivity can harm people and the environment. According to the 1992 Energy Policy Act, EPA is to set generally applicable standards based upon public health and safety standards and be consistent with the findings and recommendations of the National Academy of Sciences for the protection of the public from releases from radioactive materials stored or disposed of in the repository at the Yucca Mountain site.


Zero Order Analysis: The simplest approach to quantification of a risk with a limited treatment of each risk component (e.g. source terms, transport, health effects, etc.).

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