Glossary

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A

α=β Ratio:

A measure of the curvature of the cell survival curve and a measure of the sensitivity of a tissue or tumor to dose fractionation. The dose at which the linear and quadratic components of cell killing are equal.

Abscopal Effect:

The radiation response in tissue at a distance from the irradiated site invoked by local irradiation.

Absorbed Dose:

The energy imparted to a unit mass of matter by ionizing radiation. The unit of absorbed dose is the rad or gray (Gy). One rad equals 100 ergs per gram and one Gy is one Joule/Kg.

Absorbed Dose Rate:

Absorbed dose divided by the time it takes to deliver that dose. High dose rates are usually more damaging to humans and animals than low-dose rates. This is because repair of damage is more efficient when the dose rate is low.

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.

Activity:

The expectation value of the number of nuclear transformations occurring in a given quantity of material per unit of time. The SI unit of activity is per second (s-1) and its special name is becquerel (Bq).

Activity Median Aerodynamic Diameter (AMAD):

The value of aerodynamic diameter such that 50% of the airborne activity in a specified aerosol is associated with particles greater than the AMAD. Used when deposition depends principally on inertial impaction and sedimentation, typically when the AMAD is greater than about 0.5 lm.

Adaptive Response:

A post-irradiation cellular response that typically serves to increase the resistance of the cell to a subsequent radiation exposure.

AEC:

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

ALARA:

Acronym for "As Low As Reasonably Achievable." A principle of radiation protection philosophy that requires that exposures to ionizing radiation be kept as low as reasonably achievable, economic and social factors being taken into account. The protection from radiation exposure is ALARA when the expenditure of further resources would be unwarranted by the reduction in exposure that would be achieved.

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 Irradiation:

Radiation with alpha particles.

Ambient Dose Equivalent:

The dose equivalent at a point in a radiation field that would be produced by the corresponding expanded and aligned field in the International Commission on Radiation Units and Measurements (ICRU) sphere at a depth of 10 mm on the radius vector opposing the direction of the aligned field. The unit of ambient dose equivalent is joule per kilogram (J kg^-1) and its special name is sievert (Sv).

Annual Intake (AI):

The amount of a specified radionuclide entering the human body by inhalation or ingestion within 1 year.

Anthropogenic:

Of human origin.

Apoptosis:

An active biochemical process of programmed cell death following radiation or other insults.

Attenuation:

The reduction of dose equivalent or other physical properties of a radiation field upon the passage of radiation through matter.

Averted Dose:

The dose prevented or avoided by the application of a protective measure or set of protective measures, i.e., the difference between the projected dose if the protective measure(s) had not been applied and the expected residual dose.

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B

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. The typically quoted average individual exposure from background radiation is 360 millirems or 3.6 millisieverts per year.

Baseline Rates:

The annual disease incidence observed in a population in the absence of exposure to the agent under study.

Becquerel (Bq):

The SI unit of activity equal to one disintegration per second. [37 billion (3.7x10¹⁰) becquerels (Bq) = 1 curie (Ci)].

BEIR:

Several committees of the National Academy of Sciences-National Research Council on Biological Effects of Ionizing Radiation and their reports. For example, BEIR VI defined the health effects of radon.

Benign:

Usually refers to nonmalignant tumors.

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. Thin sheets of metal or plastic may stop beta particles.

Beta Radiation:

Radiation consisting of beta particles.

Biodosimetry:

The use of biological changes to detect past radiation exposure. Chromosome aberrations have been used widely in biological dosimetry.

Biological Dosimetry:

Area of radiation dosimetry that uses biological damage produced by radiation to estimate radiation dose. Chromosomal damage in blood lymphocytes is often used in biological dosimetry for exposure of humans to gamma radiation.

Biological Half-Life:

The time required, in the absence of further input, for a biological system (such as a human or animal) or compartment to eliminate, by biological processes, half the amount of a substance (such as a radioactive material) that has entered it.

Brachytherapy:

Radiation treatment of a patient using sealed or unsealed sources of radiation placed within the patient’s body.

Bystander Effects:

A response in unirradiated cells that is triggered by signals received from irradiated neighboring cells.

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C

Cancer:

A malignant tumor of potentially unlimited growth, capable of invading other tissue and of metastasis.

Categories of Exposure:

The International Commission on Radiological Protection distinguishes between three categories of radiation exposure: occupational, public, and medical exposures of patients.

Cesium, (¹³⁷Cs):

This environmentally important fission product is a beta-gamma emitter and has a long half-life (26.6 years). Cesium has metabolic properties similar to potassium. As a result, it is rather uniformly distributed in the body. It clears from the body quickly, with a half-life of days and weeks, and therefore has a rather low effectiveness in increasing cancer incidence.

Collective Dose:

The sum of individual doses received in a given period of time by a specified population from exposure to a specified source of radiation.

Committed Effective Dose, Ε(τ):

The sum of the products of the committed organ or tissue equivalent doses and the appropriate tissue weighting factors (w_T), where τ is the integration time in years following the intake. The commitment period is taken to be 50 years for adults, and to age 70 years for children.

Committed Equivalent Dose Equivalent (CEDE), H_T(τ):

The time integral of the equivalent dose rate in a particular tissue or organ that will be received by an individual following intake of radioactive material into the body by a Reference Person, where τ is the integration time in years.

Confidence Limits:

An interval giving the lowest and highest estimate of a parameter that is statistically compatible with the data. For a 95% confidence interval, there is a 95% chance that the interval contains the parameter.

Controlled Area:

A defined area in which specific protection measures and safety provisions are, or could be, required for controlling normal exposures or preventing the spread of contamination during normal working conditions, and preventing or limiting the extent of potential exposures. A controlled area is often within a supervised area, but need not be.

Contamination:

The deposition of radioactive material on the surfaces of structures, areas, objects, or people. The material also may be airborne, external, or internal (inside components or people).

Critical Organ:

The part of the body most susceptible to radiation damage under specific exposure conditions.

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 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 the same portion of the body or to the whole body, over a period of time (see 10 CFR 20.1003). Often refers to an occupationally exposed worker.

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D

Daughter Products:

Isotopes that are formed by the radioactive decay of some other isotope.

Decay:

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

U.S. 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, from the U.S. Energy and Research and Development Agency and incorporates various aspects of non-nuclear federal energy programs and policy. DOE is funding the Low-Dose Radiation Research Program described on this website.

Derived Air Concentration (DAC):

This equals the annual limit on intake, ALI, (of a radionuclide) divided by the volume of air inhaled by a Reference Person in a working year (i.e., 2.2 103 m³). The unit of DAC is Bq m^-3.

Designated Area:

An area that is either controlled or supervised.

Deterministic Effect:

Injury in populations of cells, characterized by a threshold dose and an increase in the severity of the reaction as the dose is increased further. Also termed tissue reaction. In some cases, deterministic effects are modifiable by post-irradiation procedures including biological response modifiers.

Detriment:

The total harm to health experienced by an exposed group and its descendants as a result of the group’s exposure to a radiation source. Detriment is a multi-dimensional concept. Its principal components are the stochastic quantities: probability of attributable fatal cancer, weighted probability of attributable non-fatal cancer, weighted probability of severe heritable effects, and length of life lost if the harm occurs.

Detriment-Adjusted Risk:

The probability of the occurrence of a stochastic effect, modified to allow for the different components of the detriment to express the severity of the consequence(s).

Diagnostic Reference Level:

Used in medical imaging with ionizing radiation to indicate whether, in routine conditions, the patient dose or administered activity (amount of radioactive material) from a specified procedure is unusually high or low for that procedure.

Diploid:

A full set of genetic material consisting of paired chromosomes, one chromosome from each parental set.

Directional Dose Equivalent, H’(d, Ω):

The dose equivalent at a point in a radiation field that would be produced by the corresponding expanded field in the ICRU sphere at a depth, d, on a radius in a specified direction, Ω. The unit of directional dose equivalent is joule per kilogram (J kg-1) and its special name is sievert (Sv).

DNA:

Deoxyribonucleic acid&emdash;the genetic material of life. Nuclear material that contains genes and is responsible for the genetic code.

DNA Damage Signaling:

Interacting biochemical processes that recognize and respond to DNA damage in cells; for example, by causing the arrest of the reproductive cell cycle.

DNA Replication:

The use of existing DNA as a template for the synthesis of new DNA strands.

DNA Repair:

The cell's ability to repair DNA damage and restore the original base sequences. This process can restore DNA damage produced by normal physiological processes, ionizing radiation, or chemicals. There are many forms of DNA repair, and many genes responsible for and involved in DNA repair have been identified.

DNA Sequence:

The relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome.

Dose:

The absorbed dose, given in rads (or in SI units, grays [Gy]), that represents the energy in ergs or Joules absorbed from the radiation per unit mass of tissue. Furthermore, the biologically effective dose or dose equivalent, given in rems or sieverts, is a measure of the biological damage to living tissue from radiation exposure.

Dose and Dose-Rate Effectiveness Factor (DDREF):

A judged factor that generalizes the usually lower biological effectiveness (per unit of dose) of radiation exposures at low doses and low dose rates as compared with exposures at high doses and high dose rates.

Dose Coefficient:

Used as a synonym for dose per unit intake of a radioactive substance, but sometimes also used to describe other coeffcients linking quantities or concentrations of activity to doses or dose rates, such as the external dose rate at a specified distance above a surface with a deposit of a specified activity per unit area of a specified radionuclide.

Dose Equivalent (H):

A quantity used for radiation protection purposes that expresses on a common scale for all radiation types. The product of the absorbed dose (D) from ionizing radiation and the quality factor (Q). The unit for H is joule per kilogram (J kg^-1), with the special name sievert (Sv). The quality factors are specified by the International Commission on Radiation Units and Measurements (ICRU) for different types of radiation and organ exposures.

Dose Limits:

The value of the effective dose or the equivalent dose to individuals from planned exposure situations that shall not be exceeded.

Dose Rate:

The quantity of absorbed dose delivered per unit of time.

Dose of Record, H_p(10):

The effective dose of a worker assessed by the sum of the measured personal dose equivalent H_p(10) and the committed effective dose retrospectively determined for the Reference Person using results of individual monitoring of the worker and the International Commission on Radiological Protection (ICRP) reference biokinetic and dosimetric computational models. Dose of record may be assessed with site-specific parameters of exposure, such as the type of materials and AMAD, but the parameters of the Reference Person shall be fixed as defined by the ICRP. Dose of record is assigned to the worker for purposes of recording, reporting and retrospective demonstration of compliance with regulatory dose limits.

Dose Response:

Correlation between a quantified exposure (dose) and the proportion of a population demonstrating a specific effect (response).

Dose-Response Assessment:

The process of characterizing the relationship 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-Threshold Hypothesis:

A given dose above background, below which it is hypothesized that the risk of excess cancer and/or heritable disease is zero. (See also Threshold Dose for Tissue Reactions).

Dosimetry:

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

Doubling Dose (DD):

The dose of radiation (Gy) that is required to produce as many heritable mutations as those arising spontaneously in a generation.

DS02:

Dosimetry System 2002, a system for estimating gamma and neutron exposure under a large variety of situations and which allows the calculation of absorbed dose to specific organs for members of the Life Span Study. DS02 improved on the DS86 dose system.

DS86:

Dosimetry System 1986, a system for estimating gamma and neutron exposure under a large variety of situations and which then allowed the calculation of absorbed dose to specific organs for members of the Life Span Study.

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E

Effect:

A biological change caused by an exposure.

Effective Dose E:

The tissue-weighted sum of the equivalent doses in all specified tissues and organs of the body. The unit for the effective dose is the same as for absorbed dose, J kg-1, and its special name is sievert (Sv).

Effective Dose Equivalent (HE):

The sum over specified tissues of the products of the dose equivalent in a tissue and the weighting factor for that tissue.

Effective Half-Life:

The time required for a radio nuclide 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.

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 wavelength.

Electron:

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

Element:

One of the 103 known chemical substances that cannot be broken down further without changing its chemical properties.

U.S. Environmental Protection Agency (EPA):

Created in 1970, the EPA is responsible for working with state and local governments to set standards that help control and prevent pollution and minimize the potential health effects of solid and hazardous waste and toxic and radioactive substances.

Epidemiology:

The study of the distribution and dynamics of diseases and injuries in human populations. The two main types of epidemiological studies of chronic disease are cohort (follow-up) studies and case-control (retrospective) studies.

Equivalent Dose:

Absorbed dose averaged over an organ or tissue and weighted for the radiation quality for the type of radiation of concern.

Eukaryote:

Organisms with membrane-bound nucleus and chromosomes. Higher plants and animals are eukaryotes.

Excess Absolute Risk:

The rate of disease incidence or mortality in an exposed population minus the corresponding disease rate in an unexposed population. The excess absolute risk is often expressed as the additive excess rate per Gy or per Sv.

Excess Lifetime Cancer Risk (ELCR):

Potential carcinogenic effects characterized by estimating the probability of cancer incidence in a population of individuals for a specific lifetime. Projected from 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.

Excess Relative Risk:

The rate of disease in an exposed population divided by the rate of disease in an unexposed population, minus 1.0. This is often expressed as the excess relative risk per Gy or per Sv

Exclusion:

The deliberate exclusion of a particular category of exposure from the scope of an instrument of regulatory control.

Exemption:

The determination by a regulatory body that a source or practice activity involving radiation need not be subject to some or all aspects of regulatory control.

Exposure:

Contact of an organism with a chemical, radiological, or physical agent.

Exposed Individuals:

The International Commission on Radiological Protection distinguishes between three categories of exposed individuals: workers (informed individuals), the public (general individuals), and patients, including their comforters and caregivers.

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.

Exposure Level:

The amount or concentration of a chemical or field strength of a radiation field.

External Radiation Dose:

The dose from sources of radiation located outside the body, most often from gamma rays. Beta rays can contribute to dose in the skin and other relatively superficial tissues.

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F

Fallout:

Radioactive debris from a nuclear detonation or other source, usually deposited from airborne particulates.

Fissile Material:

Material consisting of atoms whose nuclei can be split when irradiated with low energy (ideally, zero energy) neutrons. The fissile isotopes uranium-235 and plutonium-239 are used in nuclear weapons.

Fission:

The splitting (breaking apart, or fissioning) of the nucleus of a heavy atom such as uranium-235 or plutonium-239. The fission is usually caused by the absorption of a neutron.

Fission Product:

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

Flow Cytometry:

The analysis of biological material by detection of properties of cells or sub-cellular fractions using a combination of fluorescence and a laser beam. This makes it possible to sort cells or sub cellular fractions for further analysis.

Frank-Effect Level (FEL):

Exposure level that 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.

FSU:

Functional subunits of tissues; for example, nephrons in kidney, alveoli in lung.

Fusion:

Combining of two nuclei to form a heavier one. Fusing the isotopes of light elements such as hydrogen or lithium results in a large release of energy.

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G

Gamete:

Mature male or female reproductive cells with a haploid set of chromosomes.

Gamma Multi-Hit Model:

A generalization of the one-hit, dose-response model that provides a better description of dose-response data.

Gamma Radiation:

High-energy, short wavelength electromagnetic radiation emitted from the nucleus of an atom. Gamma rays are very penetrating and are shielded by dense materials such as lead. Gamma rays are similar to x-rays.

Genetic Effects of Radiation:

Effects arising from damage to genes in the germ cells of the mother or father, which are thus passed on to their children. The genetic effects of radiation will therefore not be seen in an irradiated individual but may occur in his or her offspring or in future generations.

Genetic Informatics:

Development of methods to search databases quickly, analyze DNA sequence information, and predict protein sequence and structure from DNA sequence data.

Genome:

All the genetic material in the chromosomes of a defined organism.

Genomics:

The study of an organism's entire genome. The field includes efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts.

Genomic Instability:

Radiation-induced changes that occur several cell generations after the radiation exposure. Occurs at a higher frequency than would be predicted from radiation-induced mutations in single or groups of genes.

Genotype:

The genetic constitution of an organism, as distinguished from its physical appearance (its phenotype).

Gray (Gy):

The new international system (SI) unit of absorbed radiation dose expressed in terms of energy per unit mass of tissue. 1 Gy = 1 Joule/Kg and also equals 100 rad.

Growth Factors:

Molecules that act to control cell reproduction and proliferation/differentiation of a population of cells.

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H

Half-Life:

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

Health Physics:

The science concerned with the recognition, evaluation, and control of health hazards that may arise from accidents or applications that result in exposure to ionizing radiation.

High-to-Low Dose Extrapolation:

The process of predicting human risks from low radiation exposures using either human or animal data on risks derived from high levels of exposure.

Homeostasis:

An ability of the body to maintain stability.

Hormesis:

The theory that small doses of radiation can induce beneficial biological processes and are healthful.

Human Equivalent Dose:

A dose that, when administered to humans, produces an effect equal to the same effect produced by a dose in animals.

Human Exposure Evaluation:

Describes 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 damaged or will damage the health of individuals.

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I

ICRP:

International Commission on Radiological Protection: The international body charged with providing an overview of radiation standards and regulations and information to help standardize these regulations.

ICRU:

International Commission on Radiation Units and Measurements: The international organization charged with standardizing radiation units and measures.

Incidence:

The rate of occurrence of a disease in a population within a specified period of time, often expressed as the number of cases of a disease arising per 100,000 individuals per year (or per 100,000 person-years).

Induced Genomic Instability:

The induction of an altered cellular state characterized by a persistent increase over many generations in the spontaneous rate of mutation or other genome-related changes.

Induced Radioactivity:

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

Intake, I:

Activity that enters the body through the respiratory tract or the gastrointestinal tract or the skin.

• Acute intake: A single intake by inhalation or ingestion, taken to occur instantaneously.
• Chronic intake: An intake over a specified period of time.

Internal Radiation Dose:

The dose to organs of the body from radioactive materials deposited and retained inside the body. It may consist of any combination of alpha, beta, and gamma radiation.

Inverse Dose Rate Effect:

An effect in which, for a given exposure, the probability of effect increases as the dose rate is decreased.

In Vitro:

Studies carried out in cell or culture systems outside the whole organism.

In Vivo:

Studies carried out in cell or culture systems inside the whole organism.

Iodine, ¹³¹I:

This short-lived radionuclide (8.1 days) becomes concentrated in the thyroid gland and can produce large radiation doses to this gland. It is used in medicine to diagnose and treat thyroid disease. The deposition, retention and radiation dose from this isotope can be modified by taking potassium iodide pills. This radio-nuclide is released from nuclear weapons and radiation accidents involving nuclear power plants. It is the isotope that was responsible for an increase in the incidence of thyroid cancer in children exposed to fallout after the Chernobyl accident. However, there is no evidence that ¹³¹I has increased the incidence of thyroid cancer in adults exposed by this accident.

Ion:

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

Ionization:

The process of adding to or removing one or more electrons from atoms or molecules, thereby creating ions and free radicals. High temperatures, metabolic processes, electrical discharges, and radiation 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.

Irradiate:

To expose or cause exposure to radiation.

Isotope:

Atoms of the same element that have an equal 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.

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J

Joule:

Measure of energy. Deposition of one Joule/Kg is equal to 1 Gy or 100 rads.

Justification:

The process of determining whether either (1) a planned activity involving radiation is, overall, beneficial, i.e. whether the benefits to individuals and to society from introducing or continuing the activity outweigh the harm (including radiation detriment) resulting from the activity; or (2) a proposed remedial action in an emergency or existing exposure situation is likely, overall, to be beneficial, i.e., whether the benefits to individuals and to society (including the reduction in radiation detriment) from introducing or continuing the remedial action outweigh its cost and any harm or damage it causes.

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K

Kerma:

The sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged particles per unit of mass of a specific material. SI unit of kerma is Joule/Kg and is the same as Gy.

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L

Lethal Dose Fifty:

A calculated dose of radiation or a chemical substance that is expected to kill 50% of the exposed individuals within 30 days. For single whole-body acute radiation exposure, the LD 50/30 is in the range from 400 to 500 rem (4 to 5 sieverts).

Life Span Study (LSS):

The long-term cohort study of health effects in the Japanese atomic bomb survivors in Hiroshima and Nagasaki.

Lifetime Exposure:

Total calculated exposure to radiation or a chemical that a human would receive in a lifetime (usually assumed to be 70 years).

Lifetime Risk Estimates:

Several types of lifetime risk estimates can be used to calculate the risk, over a lifetime, that an individual will develop, or die from, a specific disease caused by an exposure: (1) the excess lifetime risk (ELR), which is the difference between the proportion of people who develop or die from the disease in an exposed population and the corresponding proportion in a similar population without the exposure; (2) the risk of exposure-induced death (REID), which is defined as the difference in a cause-specific death rate for exposed and unexposed populations of a given sex and a given age at exposure, as an additional cause of death introduced into a population; (3) loss of life expectancy (LLE), which describes the decrease in life expectancy due to the exposure of interest; and (4) lifetime attributable risk (LAR), which is an approximation of the REID and describes excess deaths (or disease cases) over a follow-up period with population background rates determined by the experience of unexposed individuals. The LAR was used in this report to estimate lifetime risks.

Linear Dose Response:

A statistical model that expresses the risk of an effect (e.g., disease or abnormality) as being proportional to dose.

Linear Energy Transfer (LET):

The amount of energy deposited per unit of distance that the radiation travels in tissue. Alpha particles are examples of high LET radiation.

Linear-Quadratic Dose Response:

A statistical model that expresses the risk of an effect (e.g., disease, death, or abnormality) as the sum of two components, one proportional to dose (linear term) and the other one proportional to the square of dose (quadratic term).

LNTH:

The linear no-threshold model stating that any amount of radiation dose, no matter how small, results in increased radiation risk. For every unit of dose, there is an increase in risk.

Locus (Loci):

The position on a chromosome of a gene or other chromosome marker.

Lowest-Observed-Adverse-Effect-Level (LOAEL):

In an experiment, the lowest dose that produced an observable adverse effect.

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M

Malignant:

A cancer that tends to become progressively worse and to result in death if not treated; having the properties of anaplasia, invasiveness, and metastasis.

Marker:

An identifiable physical location on a chromosome (e.g., restriction enzyme cutting site, gene) whose inheritance can be monitored. Markers can be expressed DNA regions (genes) or DNA segments with no known coding function but with a determinable pattern of inheritance. See restriction fragment length polymorphism.

Medical Exposure:

Exposure incurred by patients as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly, while voluntarily helping in the support and comfort of patients; and by volunteers in a program of biomedical research involving their exposure.

Megabase (Mb):

Unit of length for DNA fragments, equal to 1 million nucleotides and roughly equal to 1 cM.

Meiosis:

The process of two consecutive cell divisions in the diploid progenitors of sex cells. Meiosis results in four rather than two daughter cells, each with a haploid set of chromosomes.

Messenger RNA (mRNA):

RNA that serves as a template for protein synthesis.

Metaphase:

A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plate of the cell. The stage of the cell cycle that is used to evaluate chromosome aberrations and mark gene location.

Metastasis:

The spread of cancer from one organ or part to another not directly connected with it.

Microbeam:

A machine capable of delivering defined radiation doses or particle numbers to known cellular locations. Microbeams can deliver a known number of alpha particles to known cellular organelles. Used to measure bystander effects.

Microcurie:

One curie divided by one million. To convert microcuries to curies, divide by one million. To convert microcuries to Becquerels, multiply by 37,000.

Micronuclei:

Chromosome fragments or lagging chromosomes that are not incorporated into the nucleus at cell division. Used in biological dosimetry and detection of genomic instability.

Minimum Detectable Level (MDL):

The threshold of detection for a biological response, substance, or device in question.

Mitosis:

The process of nuclear division in cells that produces daughter cells genetically identical to each other and to the parent cell.

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.

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.

Multifactorial Diseases:

Diseases that are attributable to multiple genetic and environmental factors.

Multistage Model:

A carcinogenesis dose-response model in which cancer is assumed to originate as a "malignant" cell initiated by a series of somatic-like mutations occurring in a finite number of 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.

Multistage Tumorigenesis:

The stepwise acquisition of cellular properties that can lead to the development of a tumor from a single (target) cell.

Mutagen/Mutagenicity:

An agent that causes a permanent genetic change in a cell in addition to that occurring during normal genetic recombination. Mutagenicity is the capacity of a chemical or physical agent to cause such permanent genetic alterations.

Mutation:

Any heritable change in DNA sequence. Can be induced by changes at the chromosome, gene, or DNA level.

Mutation Component (MC):

A quantity that provides a measure of the relative change in disease frequency per unit relative change in mutation rate, i.e., a measure of responsiveness; MC values differ for different classes of heritable disease.

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N

NAS/NRC (National Academy of Sciences/National Research Council):

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars involved in scientific and engineering research. As part of NAS, the National Research Council is designed to associate the broad community of science and technology with the needs of government. The NRC is the operating agency for NAS.

NCRP (National Council on Radiation Protection and Measurements):

A nonprofit corporation chartered by Congress to provide information that protects the public against radiation and provides recommendations on radiation measurements, quantities, and units.

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 that, among other activities, tests and certifies respiratory protective devices and air-sampling detector tubes, recommends occupational exposure limits for various substances, and assists in occupational safety and health investigations and research.

Nominal Risk Coefficient:

Sex-averaged and age-at-exposure-averaged lifetime risk estimates for a representative population.

Non-Cancer Diseases:

Somatic diseases other than cancer, e.g., cardiovascular disease and cataracts.

Nonstochastic Effects:

The severity of radiation-induced effects increases in affected individuals as the dose increases. A threshold usually exists for nonstochastic effects.

No Observable Adverse Effect Level (NOAEL):

From long-term toxicological studies, the levels that indicate a safe, lifetime exposure level for a given chemical. Used to establish tolerance levels for human diets. Also written, NOEL.

Naturally Occurring Radioactive Material (NORM):

Radioactive material containing no significant amounts of radionuclides other than naturally occurring radionuclides. Material in which the activity concentrations of the naturally occurring radionuclides have been changed by some process are included in NORM.

U.S. Nuclear Regulatory Commission (NRC):

NRC is an independent agency created from the U.S. Atomic Energy Commission in 1975 to regulate civilian uses of nuclear material. Specifically, NRC is responsible for ensuring that activities associated with the operation of nuclear power and fuel-cycle plants and the use of radioactive materials in medical, industrial, and research applications are carried out with adequate protection of public health and safety, the environment, and national security.

Nucleic Acid:

A large molecule composed of nucleotide subunits.

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.

Nucleotide:

A subunit of DNA or RNA consisting of a nitrogenous base (adenine, guanine, thymine, or cytosine in DNA; adenine, guanine, uracil, or cytosine in RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA and ribose in RNA). Thousands of nucleotides are linked to form a DNA or RNA molecule. See DNA, base pair, RNA.

Nucleus:

(1) 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. (2) The cellular organelle that contains the chromosomes and genetic material.

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O

Occupational Exposure:

This refers to all exposure incurred by workers in the course of their work, with the exception of (1) excluded exposures and exposures from exempt activities involving radiation or exempt sources; (2) any medical exposure; and (3) the normal local natural background radiation.

Oncogene:

Genes that encode the potential for cancer and, when activated, can induce cancer.

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.

Operational Quantities:

Quantities used in practical applications for monitoring and investigating situations involving external exposure. They are defined for measurements and assessment of doses in the body. In internal dosimetry, no operational dose quantities have been defined that directly provide an assessment of equivalent or effective dose. Different methods are applied to assess the equivalent or effective dose due to radionuclides in the human body. They are mostly based on various activity measurements and the application of biokinetic models (computational models).

Optimization of Protection (and safety):

The process of determining what level of protection and safety makes exposures, and the probability and magnitude of potential exposures, as low as reasonably achievable, economic and societal factors being taken into account.

Organ-Weighting Factor (WT):

Factor indicating the ratio of stochastic-effects risk attributable to a given organ's or tissue's irradiation to the total risk when the whole body is uniformly irradiated.

Organelle:

Any complex biological structure that forms a component of cells and performs a characteristic function. Examples of organelles are centrioles, the endoplasmic reticulum, kinetosomes, lysosomes, proteosomes, mitochondria, and ribosomes.

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.

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P

Particle:

A tiny mass of material. Airborne particles that exist in the atmosphere as a solid or liquid can be natural, caused by stirring of soil dusts, or anthropogenic means. They vary in size from coarse (diameter >3 mm) to fine (<3 mm) . Sometimes "inhalable" or "respirable" is used to describe particles (<2 mm) that 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.

PCC:

Premature chromosome condensation, a method of studying chromosomes that are in the interphase stage of the cell cycle. PCC is used in many chromosome-repair studies.

PCR:

Polymerase Chain Reaction, a method used to amplify the amount of DNA from a given region of a gene or chromosome to rapidly produce a highly specific amplification of the desired sequence. PCR also can be used to detect the existence of the defined sequence in a DNA sample. This method has enabled major advances in molecular biology by providing adequate amounts of known DNA for study.

Person-Gray (Gy):

The unit of population exposure obtained by summing dose equivalent values for all people in the exposed population. Thus, one person-Gy can result from one person being exposed to 1 Gy or to 100,000 people each exposed to 10 Gy.

Person-Year:

The sum of the number of years each person in a 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.

Personal Dose Equivalent, H_p(d):

An operational quantity: the dose equivalent in soft tissue (commonly interpreted as the "ICRU sphere") at an appropriate depth, d, below a specified point on the human body. The unit of personal dose equivalent is joule per kilogram (J kg^-1) and its special name is sievert (Sv). The specified point is usually given by the position where the individual’s dosimeter is worn.

Photon:

The indivisible unit or quantum of electromagnetic radiation. The photons' energy determines the radiation's nature from radio waves at the lowest energy levels through infrared, visible, and ultraviolet light to x-rays or gamma-rays, which have energy high enough to ionize atoms.

Photon Radiation:

Forms of electromagnetic radiation such as x-rays, gamma-rays, and sunlight.

Picocurie:

One trillionth (10-12) of a curie.

Planned Exposure Situations:

Everyday situations involving the planned operation of sources including decommissioning, disposal of radioactive waste and rehabilitation of the previously occupied land. Practices in operation are planned exposure situations.

Plutonium(²³⁹Pu):

This alpha-emitting radioactive element is produced in nuclear reactors and can also be used as a fuel for reactors. The primary concern for this fissionable radioactive element is that it is used to produce nuclear weapons. In most of its chemical forms, it is biologically inert. Only a very small fraction of any ingested ²³⁹Pu is taken into the body. However, if it is inhaled as an aerosol of small particles, it can become imbedded in the lung and remain for long periods of time, resulting in chronic irradiation of this organ. A fraction of the total deposited material can move from the lung and be deposited on bone surfaces and in the liver. Therefore, in experimental animal studies, there is an increase in lung, bone, and liver cancer following high levels of deposition of this radioisotope.

Polymerase, DNA or RNA:

Enzymes that catalyze the synthesis of nucleic acids on pre-existing nucleic acid templates, assembling RNA from ribonucleotides or DNA from deoxyribonucleotides.

Pooled Analysis:

An analysis of epidemiological data from several studies based on original data from those studies that are analyzed in parallel.

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 to an electron.

Potential Exposure:

Exposure that is not expected to be delivered with certainty but that may result from an accident at a source or an event or sequence of events of a probabilistic nature, including equipment failures and operating errors.

Potential Recoverability Correction Factor (PRCF):

A set of factors that take account of knowledge that different classes of germ line mutation will show different degrees of recoverability in live-born offspring, i.e., through differing capacities to allow completion of embryonic/fetal development.

Primer:

Short preexisting polynucleotide chain to which DNA polymerase can add new deoxyribonucleotides.

Principles of Protection:

A set of principles that apply equally to all controllable exposure situations: the principle of justification, the principle of optimization of protection, and the principle of application of limits on maximum doses in planned situations.

Probe:

Single-stranded DNA or RNA molecules of specific base sequence, labeled either radioactively or immunologically, which are used to detect the complementary base sequence by hybridization.

Prokaryote:

Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments. Bacteria are prokaryotes. Compare to eukaryote.

Progenitor Cell:

Undifferentiated cell capable of limited proliferation.

Projected Dose:

The dose that would be expected to be incurred if no protective measure(s) were to be taken.

Promoter:

(1) DNA site to which RNA polymerase will bind and initiate transcription. (2) Agent that is not carcinogenic by itself but is capable of amplifying a true carcinogen's effect by increasing the probability of late-stage cellular changes needed to complete the carcinogenic process. Promoters usually require protracted application to be effective in increasing cancer incidence.

Prospective Study:

An inquiry in which groups of individuals are selected, according to their exposure to certain factors and followed over time to determine differences in disease rates in relation to their factor exposure. Also called cohort study.

Protection Quantities:

Dose quantities that the ICRP has developed for radiological protection that allow quantification of the extent of exposure of the human body to ionizing radiation from both whole and partial body external irradiation and from intakes of radionuclides.

Proton:

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

Protraction:

The spreading out of a radiation dose over time by continuous or periodic delivery at a lower dose rate.

Public Exposure:

Exposure incurred by members of the public from radiation sources, excluding any occupational or medical exposure and the normal local natural background radiation.

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Q

Quadratic Dose Response Model:

A model that predicts that a biological effect continually increases out of proportion to an increase in dose. Effects at low doses would thus be relatively small.

Quality Factor (Q):

The factor characterizing the biological effectiveness of a radiation, based on the ionization density along the tracks of charged particles in tissue derived from the linear portions of the dose-response relationships for different radiation types.

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R

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. This has been replaced in the SI system with the gray, which is equal to 100 rads.

Radiation (Ionizing):

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

Radiation Detriment:

A concept used to quantify the harmful health effects of radiation exposure in different parts of the body. It is defined by the International Commission on Radiological Protection as a function of several factors, including incidence of radiation-related cancer or heritable effects, lethality of these conditions, quality of life, and years of life lost owing to these conditions.

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[Gy]) 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 around 1000 rad (10[Gy]) or more, death may occur within 2 to 4 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.

Radiation Units:

Units listed for easy conversion.
Units Conversion Factors
Becquerel (SI) 1 disintegration/s = 2.7 X 10^-11 Ci
Curie 3.7 X 10¹⁰ disintegrations/s = 3.7 X 10¹⁰ Bq
Gray (SI) 1 J/kg = 100 rad
Rad 100 ergs/g = 0.01 Gy
Rem 0.01 Sievert
Sievert (SI) 100 Rem

Radiation Weighting Factor, W_R:

A dimensionless factor by which the organ-or-tissue absorbed dose is multiplied to reflect the higher biological effectiveness of high-LET radiations compared with low-LET radiations. It is used to derive the equivalent dose from the absorbed dose averaged over a tissue or organ.

Radioactive (Decay):

Property of undergoing spontaneous nuclear transformation in which nuclear particles or electromagnetic energy are emitted.

Radioactive Material:

Material designated in national law or by a regulatory body as being subject to regulatory control because of its radioactivity, often taking account of both activity and activity concentration.

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.

Radiogenic:

Caused by radiation.

Radioisotope:

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

Radiological Attack:

The use of radioactive or nuclear materials for malicious purposes, such as blackmail, murder, sabotage, or terrorism.

Radionuclides:

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

Radiosensitivity:

Relative susceptibility of cells, tissues, organs, and organisms to the injurious action of radiation; radiosensitivity and its antonym, radioresistance, are used in a comparative sense rather than an absolute one.

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.

Random Error:

Errors that vary in a non-reproducible way. These errors can be treated statistically by use of the laws of probability.

Reference Animals and Plants:

A hypothetical entity with the assumed basic characteristics of a specific type of animal or plant, as described to the generality of the taxonomic level of Family, with defined anatomical, physiological, and life-history properties, that can be used for the purposes of relating exposure to dose, and dose to effects, for that type of living organism.

Reference Dose:

Toxicity value for evaluating noncarcinogenic (systemic) effects of daily exposure to contaminant levels without appreciable deleterious effects during a lifetime.

Reference Level:

In emergency or existing controllable exposure situations, this represents the level of dose or risk, above which it is judged to be inappropriate to plan to allow exposures to occur, and below which optimization of protection should be implemented. The chosen value for a reference level will depend upon the prevailing circumstances of the exposure under consideration.

Reference Male and Reference Female (Reference Individual):

An idealized male or female with characteristics defined by the International Commission on Radiological Protection (ICRP) for the purpose of radiological protection, and with the anatomical and physiological characteristics defined in the report of the ICRP Task Group on Reference Man (Publication 89, ICRP 2002).

Reference Person:

Voxel phantoms for the human body (male and female voxel phantoms based on medical imaging data) with the anatomical and physiological characteristics defined in the report of the ICRP Task Group on Reference Man (Publication 89, ICRP 2002).

Reference Phantom:

Voxel phantoms for the human body (male and female voxel phantoms based on medical imaging data) with the anatomical and physiological characteristics defined in the report of the International Commission on Radiological Protection Task Group on Reference Man (Publication 89, ICRP 2002).

Reference Value:

The value of a parameter recommended by the International Commission on Radiological Protection for use in a biokinetic model in the absence of more specific information, i.e., the exact value used to calculate the dose coefficients presented in the report. Reference values may be specified to a greater degree of precision than that which would be chosen to reflect the uncertainty with which an experimental value is known, to avoid the accumulation of rounding errors in a calculation.

Relative Biological Effectiveness (RBE):

The ratio of a dose of a low-LET reference radiation to a dose of the radiation considered that gives an identical biological effect. RBE values vary with the dose, dose rate, and biological endpoint considered. In radiological protection, the RBE for stochastic effects at low doses (RBEM) is of particular interest.

Relative Life Lost:

The ratio of the proportion of observed years of life lost among people dying of a disease in an exposed population and the corresponding proportion in a similar population without the exposure.

Relative Survival:

The ratio of the proportion of cancer patients who survive for a specified number of years (e.g., 5 years) following diagnosis to the corresponding proportion in a comparable set of cancer-free individuals.

Rem:

A unit of equivalent absorbed dose of radiation, taking into account the relative biological effectiveness of the particular radiation. The dose in rems is the dose in rads multiplied by the quality factor Q derived from the RBE. Rem has been replaced in the SI system with sievert. One sievert equals 100 rem.

Repair Processes:

Metabolic processes within a cell that can repair radiation damage before it is expressed as a biological effect, such as cell killing.

Repository:

A permanent resting place for radioactive wastes that will finally decay to natural radioactivity levels.

Representative Person:

An individual receiving a dose that is representative of the more highly exposed individuals in the population (see Publication 101, ICRP 2006a). This term is the equivalent of, and replaces, "average member of the critical group" described in previous ICRP Recommendations.

Residual Dose:

The dose expected to be incurred after protective measure(s) have been fully implemented (or a decision has been taken not to implement any protective measures).

Ribonucleic Acid (RNA):

A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

Ribosomes:

Small cellular components composed of specialized ribosomal RNA and protein; site of protein synthesis.

Risk:

The product of severity (consequence) impact and likelihood (probability) impact. Specifically for carcinogenic effects, risk is estimated as the incremental probability of an individual's developing cancer over a lifetime as a result of exposure to a potential carcinogen. 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 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 quantifying the probabilities and expected consequences of identified risks.

Risk Constraint:

A prospective and source-related restriction on the individual risk (in the sense of probability of detriment due to a potential exposure) from a source, which provides a basic level of protection for the individuals most at risk from a source and serves as an upper bound on the individual risk in optimization of protection for that source. This risk is a function of the probability of an unintended event causing a dose, and the probability of detriment due to that dose. Risk constraints correspond to dose constraints but refer to potential exposures.

Risk-Specific Dose:

The dose associated with a specified risk level.

Roentgen:

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

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S

Sensitivity Analysis:

This aims to quantify how the results from a model depend upon the different variables included in it.

Sievert:

The SI unit of equivalent absorbed dose equal to 1 Joule/Kg or 100 rems.

SI Units:

The international system of units as defined by the general conference on weights and measures in 1960. These units are generally based on meter/kilogram/second units, with special quantities for radiation including the becquerel, gray, sievert.

Somatic Cell:

Any cell in the body except gametes and their precursors.

Somatic Effects of Radiation:

Effects of radiation limited to the somatic cells of the exposed individual, as distinguished from genetic effects. Generally refers to the induction of cancer.

Source:

An entity for which radiological protection can be optimized as an integral whole, such as the x-ray equipment in a hospital, or the releases of radioactive materials from an installation. Sources of radiation, such as radiation generators and sealed radioactive materials, and, more generally, the cause of exposure to radiation or to radionuclides.

Source Region, S_i:

An anatomical region within the reference phantom body that contains the radionuclide following its intake. The region may be an organ, a tissue, the contents of the gastrointestinal tract or urinary bladder, or the surfaces of tissues as in the skeleton, the alimentary tract, and the respiratory tract.

Specific Absorbed Fraction:

The fraction of energy of that emitted as a specified radiation type in a source region, S, that is absorbed in 1 kg of a target tissue, T.

Specific Activity:

A measure of the amount of radioactivity in a unit weight (generally one gram) of material.

Specific Energy:

The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose).

Standard Deviation:

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

Standard Geometric Deviation:

Measure of value dispersion about a geometric mean; the frequency-distribution portion that is one standard geometric deviation to either side of the geometric mean; accounts for 68% of total samples.

Standard Mortality Ratio (SMR):

The ratio of the disease or mortality rate in a certain specific population compared to that in a standard population.

Standard Normal Deviation:

Measure of value dispersion about a mean; positive square root of the average of squares of individual deviations from the mean.

Statistical Power:

The probability that an epidemiological study will detect a given level of elevated risk with a specified degree of confidence.

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 present in sufficient numbers at control areas to permit a valid statistical comparison with the areas being tested.

Stochastic Effects of Radiation:

Malignant disease and heritable effects for which the probability of an effect occurring, but not its severity, is regarded as a function of dose without threshold.

Strontium(⁹⁰Sr):

This is one of the most environmentally dangerous fission products produced by nuclear weapons and reactors. It has a long half-life (27.7 years), is readily taken into the body, and is deposited and retained in the bone matrix. It, as well as its daughter, Yttrium (⁹⁰Y), are beta emitters and result in radiation of the bone and bone marrow. An increase in bone cancer has been shown in experimental animals given large doses of ⁹⁰Sr.

Suppressor Gene:

A gene that can suppress the action of another gene, such as an oncogene.

Systematic Error:

Errors that are reproducible and tend to bias a result in one direction. Their causes can be assigned, at least in principle, and they can have constant and variable components. Generally these errors cannot be treated statistically.

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T

Target region, Ti:

Anatomical region within the body (reference phantom) in which radiation is absorbed. The region may be an organ or a specified tissue as in the gastrointestinal tract, urinary bladder, skeleton, and respiratory tract.

Target Theory (Hit Theory):

A theory explaining some biological effects of radiation on the basis that ionization, which occurs in a discrete volume (the target) within a cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response.

Threshold Dose for Tissue Reactions:

Dose estimated to result in only 1% incidence of tissue reactions.

Tissue Weighting Factor, W_T:

The factor by which the equivalent dose in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body (ICRP 1991b).

Track Structure:

Spatial patterns of energy deposition in matter along the track from the passage of ionizing radiation.

Transport of Risk (also called Transfer of Risk):

Taking a risk coefficient estimated for one population and applying it to another population with different characteristics.

Threshold Dose:

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

Threshold Hypothesis:

The assumption that no radiation injury occurs below a specified dose.

Threshold Limit Value (TLV):

Refers to airborne concentrations of substances. Represents conditions under which nearly all workers are believed to be protected while repeatedly exposed for an 8-hour 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].

Total Effective Dose Equivalent (TEDE):

The sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).

Toxicity:

Degree of danger a substance poses to animal or plant life.

Toxicity Assessment:

Characterization of the toxicological properties and effects of radiation or 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.

Transuranic Elements:

All elements beyond uranium on the periodic table. Transuranic elements are anthropogenic.

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.

Tritium:

The heaviest isotope of hydrogen and a source of beta radiation. Using tritium gas as a booster enhances the power of modern nuclear weapons. Tritium has a half-life of 12 years.

Tumor:

Any abnormal mass of cells resulting from excessive cellular multiplication or lack of differentiation.

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U

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.

Unit of Activity:

Units of Activity (Ci):	Units of Activity (Bq):
1 Centi Ci = 0.01 Ci	1 Kilo Bq (kBq) = 1,000 Bq
Milli 0.0001 Ci	Mega (MBq) 1,000,000 Bq
Micro 0.00001 Ci	Giga (GBq) 1,000,000,000 Bq
Nano 0.00000001 Ci	Tera (TBq) 1,000,000,000,000 Bq
Pico 0.000000000001 Ci	Peta (PBq) 1,000,000,000,000,000 Bq

Unit Risk:

The unit risk factors (URFs) provide estimates of 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.

UNSCEAR (United Nations Scientific Committee on the Effect of Atomic Radiation):

International body that publishes periodic reports on sources and effects of ionizing radiation.

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V

Virus:

A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus.

Voxel Phantom:

Computational anthropomorphic phantom based on medical tomographic images where the anatomy is described by small three-dimensional volume elements (voxels) specifying the density and the atomic composition of the various organs and tissues of the human body.

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W

Weighting Factor (WT):

A factor that represents the proportion of the total stochastic (cancer plus genetic) risk resulting from irradiation of tissue (T) to the total risk, when the whole body is irradiated uniformly.

Whole Body Dose Equivalent (Hwb):

The dose equivalent associated with uniform irradiation of the whole body.

Working Level (WL):

A unit of air concentration of potential alpha energy released from radon and its daughters. One working level is the concentration of radon daughters that has a potential energy release of 1.3 X 105 MeV per liter of air or SI units of 2.08 X 10-5 Jh/m³.

Working Level Month (WLM):

Exposure of one working level from radon and its daughters for 170 hours. (3.5 X 10-3 Jh/m³).

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X

X-Ray:

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.

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Y

Yeast Artificial Chromosome (YAC):

A vector used to clone DNA fragments (up to 400 Kb); it is constructed from the telomeric, centromeric, and replication origin sequences needed for replication in yeast cells.

Yield:

The energy released by a nuclear explosion.

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