**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

**Glossary**

*Absolute risk (AR).*

The rate of disease among a population.

*Absorbed dose (D).*

The mean energy imparted by ionizing radiation to a medium per unit mass. Units: gray (Gy), rad. 1 Gy = 1 J/kg = 100 rads. 1 rad = 0.01 J/kg = 100 ergs/g.

*Activity.*

The amount of radioactivity defined in terms of the mean number of decays per unit time. Units: becquerel (Bq), curie (Ci). 1 Bq = 1 s^{−1} = 2.7 × 10^{−11} Ci. 1 Ci = 3.7 × 10^{9} Bq.

*Additive effect.*

When two agents do not interact, the combined effect is equal to the sum of the effects of the two agents acting alone.

*Apoptosis.*

Programmed cell death. The cell death is characterized by a distinctive fragmentation of DNA that is regulated by cellular functions.

*Attributable risk (AR).*

The estimated rate of a disease (such as lung cancer) that could, in theory, be prevented if all exposures to a particular causative agent (such as radon) were eliminated.

*Background radiation.*

The radiation to which a member of the population is exposed from natural sources, such as terrestrial radiation due to naturally occurring radionuclides in the soil, cosmic radiation originating in outer space, and naturally occurring radionuclides in the human body.

*Baseline rate of cancer.*

The annual cancer incidence observed in a population in the absence of the specific agent being studied; the baseline rate includes cancers from a number of other causes, such as smoking, occupational exposures, and so forth.

*Becquerel (Bq).*

SI unit of activity (see *Units*). 1 Bq = 1 s^{−1} = 2.7 × 10^{−11} Ci.

*BEIR V.*

Report of the fifth National Research Council Committee on the Biological Effects of Ionizing Radiation; the report was published in 1990.

*Beta particle.*

A charged particle emitted from a nucleus during radioactive decay that, if negatively charged, is identical to an electron.

*Bias.*

Factors that influence the outcome of data collection, such as causing certain measurements to have a greater chance of being included than others.

*Cancer.*

A malignant tumor of potentially unlimited growth, capable of invading surrounding tissue or spreading to other parts of the body by metastasis.

*Carcinogen.*

An agent that can cause cancer. Ionizing radiation is a physical carcinogen; there are also chemical and biological carcinogens; biological carcinogens may be extrinsic (*e.g.*, viruses) or intrinsic (genetic defects).

*Carcinoma.*

A malignant tumor (cancer) of epithelial origin.

*Case-control study.*

An epidemiologic study in which people with disease and a similarly composed control group are compared in terms of exposures to a putative causative agent.

*Cell culture.*

The growing of cells *in vitro* (in a glass or plastic container, or in suspension) in such a manner that the cells are no longer organized into tissues.

*Cohort study.*

An epidemiologic study in which groups of people (the cohort) are identified with respect to the presence or absence of exposure to a disease-causing agent, and in which the outcomes of disease rates are compared; also called a follow-up study.

*Collective effective dose*.

Number of persons times average effective dose (see *Effective dose*). Unit: person-Sv (special name used with collective dose).

*Competing risks.*

Causes other than the agent under study that contribute to the mortality rate. The mortality rate from these other causes is not included in the risk of dying from the factor under study.

*Confidence interval (CI).*

An interval estimate of an unknown parameter, such as a risk. A 95% confidence interval, as an example, is constructed from a procedure

**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

that is theoretically successful in capturing the parameter of interest in 95% of its applications. Confidence limits are the end points of a confidence interval.

*Constant relative risk (CRR).*

A risk model that assumes the ratio of the risk at a specific dose and the risk in the absence of the dose remains constant after a certain time.

*Curie (Ci).*

Former special unit of activity (see *Units*). 1 Ci = 3.7 × 10^{10} Bq.

*DEF.*

The reduction in risk for low doses.

*DNA.*

Deoxyribonucleic acid; the genetic material of cells.

*Deletions.*

Type of mutation in which sections of DNA are removed; term can refer to the removal of a single base or many bases.

*Dose.*

Short name for absorbed dose (1 Gy = 1 J/kg) and also for equivalent dose, effective dose, and weighted dose (1 Sv = 1 J/kg). Definitions of low, medium, and high doses vary widely in the literature. For the purposes of this report, dose levels have been defined as follows:

Low dose: 0-100 mGy (mSv)

Medium dose: In excess of 100 mGy up to a maximum of 1 Gy

High dose: In excess of 1 Gy up to the very high total doses used in radiation therapy (on the order of 20–60 Gy)

*Dose-effect (dose-response) model.*

A mathematical formulation and description of the way the effect (or biological response) depends on dose.

*Dose rate.*

The absorbed dose delivered per unit time.

*Dose-rate effectiveness factor (DREF).*

The factor by which the effect caused by a specific type of radiation changes at low doses or low dose rates (protracted or fractionated delivery of dose) as compared to high doses delivered at high (or acute) dose rates.

*Dose and dose-rate effectiveness factor (DDREF)*.

A judged factor by which the radiation effect, per unit of dose, caused by a given high or moderate dose of radiation received at high dose rates is reduced when doses are low or are received at low dose rates.

*Dosimetric model.*

A method for estimating risk based on the use of physical models for doses to target cells and the use of results from epidemiologic studies of exposures to humans from other types of radiations.

*Ecological fallacy.*

The fact that two populations differ in many factors other than the one being evaluated and that one or more of these other factors may be the underlying reason for any difference noted in their morbidity or mortality experience.

*Ecologic study.*

A method of epidemiologic study in which rates of health effects outcome based on population rather than individual data are related to the measure of population radiation exposure.

*Effective dose.*

Sum over the absorbed doses to different organs from different radiation types multiplied by organ weighting factors and radiation weighting factors, as defined by the International Commission for Radiation Protection (ICRP). Unit: 1 Sv = 1 J/kg = 100 rem. Equal effective doses are meant to correspond—apart from age- and sex-dependent differences—to roughly the same overall risk. For a uniform whole-body exposure by a specified radiation type the effective dose equals the absorbed dose times the radiation weighting factor.

*Electron volt* (eV)*.*

A special unit of energy: 1 eV = 1.6 × 10^{−19} J = 1.6 × 10^{−12} erg; 1 eV is equivalent to the energy gained by an electron in passing through a potential difference of 1 V; 1 keV=1000 eV; 1 MeV = 1,000,000 eV.

*Empirical model.*

A model that is derived from measurements in populations, as opposed to a theoretical model.

*Epidemiology.*

The study of the determinants of the frequency of disease in humans. The two main types of epidemiologic studies of chronic disease are cohort (or follow-up) studies and case-control studies.

*Equivalent dose.*

Absorbed dose multiplied by the *quality factor*, *Q*, which represents, for the purposes of radiation protection and control, the effectiveness of the radiation relative to sparsely ionizing radiation (see *Quality factor*). Units: 1 Sv = 1 J/kg = 100 rem. 1 rem = 0.01 Sv.

*Etiology.*

The science or description of cause(s) of disease.

*Excess absolute risk (EAR).*

The rate of disease in an exposed population minus the rate of disease in an unexposed population. Also termed “attributable risk” or “risk difference.”

*Excess relative risk (ERR).*

The rate of disease in an exposed population divided by the rate of disease in an unexposed population minus 1.0.

*Exposure.*

The condition of having contact with a physical or chemical agent.

*Fibrosis.*

Damage to normal tissue that results in a modification of tissue structure but is not cancer.

*Fractionation.*

The delivery of a given dose of radiation as several smaller doses separated by intervals of time.

*Gamma radiation.*

Also gamma rays; short-wavelength electromagnetic radiation of nuclear origin, similar to X-rays but usually of higher energy (100 keV to several million electronvolts).

*Geometric mean.*

The geometric mean of a set of positive numbers is the exponential of the arithmetic mean of their logarithms. The geometric mean of a lognormal distribution is the exponential of the mean of the associated normal distribution.

*Geometric standard deviation (GSD).*

The geometric standard deviation of a lognormal distribution is the exponential of the standard deviation of the associated normal distribution.

*Germ cells.*

Reproductive cells such as the sperm and egg and their progenitor cells.

**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

*Gray (Gy).*

Special name of the SI unit of absorbed dose (see *Units*). 1 Gy = 1 J/kg = 100 rads.

*Half-life, biological.*

Time required for the body to eliminate half of an administered dose of any substance by metabolic processes of elimination; it is approximately the same for both stable and radioactive isotopes of a particular element.

*Half-life, radioactive.*

Time required for a radioactive substance to lose 50% of its activity by decay.

*ICRP (International Commission on Radiological Protection).*

An independent international organization that provides recommendations and guidance on protection against ionizing radiation.

*ICRU (International Commission on Radiation Units and Measurements).*

An independent international organization that provides recommendations and guidance on radiation quantities, units, and measurements.

*Incidence.*

Also, incidence rate; the rate of occurrence of a disease within a specified period of time, often expressed as a number of cases per 100,000 individuals per year.

*In utero.*

In the womb (*i.e.*, before birth).

*Inverse dose-rate effect.*

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

*In vitro.*

Cell culture conditions in glass or plastic containers.

*In vivo.*

In the living organism.

*Ionizing radiation.*

Radiation sufficiently energetic to dislodge electrons from an atom, thereby producing an ion pair. Ionizing radiation includes X- and gamma radiation, electrons (beta radiation), alpha particles (helium nuclei), and heavier charged atomic nuclei. Neutrons ionize indirectly by first colliding with components of atomic nuclei.

*Kerma* (*kinetic energy released in material*).

The kinetic energy transferred to charged particles per unit mass of irradiated medium by indirectly ionizing (uncharged) particles, such as photons or neutrons. Unit: gray (Gy). 1 Gy = 1 J/kg. If all of the kinetic energy is absorbed “locally,” the kerma is equal to the absorbed dose.

*Latent period.*

The time between exposure and expression of the disease. After exposure to a dose of radiation, there typically is a delay of several years (the latent period) before any cancer is observed.

*Life table.*

A table showing the number of persons who, of a given number born or living at a specified age, live to attain successivly higher ages, together with the numbers who die in each interval.

*Linear energy transfer (LET).*

Mean energy lost by charged particles in electronic collisions per unit track length. Unit: keV/mm.

*High-LET radiation.*

Neutrons or heavy, charged particles, such as protons or alpha particles, that produce ionizing events densely spaced on a molecular scale (*e.g., L* > 10 keV/μm; see *Unrestricted LET*).

*Low-LET radiation.*

X-rays and gamma rays or light, charged particles, such as electrons, that produce sparse ionizing events far apart on a molecular scale (*e.g., L* < 10 keV/μm).

*Restricted LET* (*L*_{Δ}). The mean energy lost per unit track length in electronic collisions with energy transfer not larger than Δ.

*Unrestricted LET* (*L*) or *Total collision stopping power,* the mean energy lost per unit track length in all electronic collisions.

*Linear (L) model or relationship (also linear dose-effect relationship).*

The linear model is a special case of the linear-quadratic model, with the quadratic coefficient equal to zero; the linear model expresses the effect (*e.g.*, cancer or mutation) as proportional to the dose (linear function of the dose).

*Linear-quadratic (LQ) model.*

Also, *linear-quadratic dose-effect relationship*; expresses the effect (*e.g.*, cancer) as the sum of two components, one proportional to the dose (linear term) and one proportional to the square of the dose (quadratic term). The linear term predominates at low doses; the quadratic term, at high doses.

*LNT model*.

Linear no-threshold dose-response for which any dose greater than zero has a positive probability of producing an effect (*e.g.,* mutation or cancer). The probability is calculated either from the slope of a linear (L) model or from the limiting slope, as the dose approaches zero, of a linear-quadratic (LQ) model.

*Lognormal distribution.*

When the logarithms of a randomly distributed quantity have a normal (Gaussian) distribution.

*LSS (Life Span Study)*.

Long-term study of health effects in the Hiroshima and Nagasaki atomic bomb survivors.

*Mechanistic basis.*

An explanation derived from a knowledge of the individual stages leading to an effect.

*Meta-analysis.*

An analysis of epidemiologic data from several studies based on data included in publications.

*Model.*

A schematic description of a system, theory, or phenomenon that accounts for its known or inferred properties and may be used for further study of its characteristics.

*Monte Carlo calculation.*

The method for evaluation of a probability distribution by means of random sampling.

*Mortality (rate).*

The frequency at which people die from a disease (*e.g.,* a specific cancer), often expressed as the number of deaths per 100,000 population per year.

**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

*Multiplicative effects.*

The combined effect of two agents is equal to the product of the effects of the two agents acting alone.

*NCRP (National Council on Radiation Protection and Measurements).*

U.S. Council commissioned to formulate and disseminate information, guidance, and recommendations on radiation protection and measurements.

*Neoplasm.*

Any new and abnormal growth, such as a tumor; neoplastic disease refers to any disease that forms tumors, whether malignant or benign.

*Nonstochastic.*

A description of effects whose severity is a function of dose; for these, a threshold may occur; some examples of somatic effects believed to be nonstochastic are cataract induction, nonmalignant damage to the skin, hematological deficiencies, and impairment of fertility.

*Normal distribution*.

The so-called bell-shaped curve of randomly distributed quantities; also referred to as a “Gaussian distribution.”

*Odds ratio (OR)*.

The odds of being exposed among diseased persons divided by the odds of being exposed among nondiseased persons.

*Oncogenes.*

Genes that encode the potential for cancer.

*Phenotype.*

The genetically and environmentally determined physical appearance of an organism.

*Photon.*

An electromagnetic quantum whose energy (*E*_{ph}) equals the product of the Planck constant (*h*) and its frequency (n). With the convenient units eV and s, and with the wave length λ in μm: *E*_{ph} = 4.136 × 10^{−15} ν = 1.24/λ.

*Pooled analysis.*

An analysis of epidemiologic data from several studies based on original data from the studies.

*Prevalence.*

The number of cases of a disease in existence at a given time per unit of population, usually 100,000 persons.

*Probability of causation.*

A number that expresses the probability that a given cancer, in a specific tissue, has been caused by a previous exposure to a carcinogenic agent, such as radiation.

*Projection model.*

A mathematical model that simultaneously describes the excess cancer risk at different levels of some factor such as dose, time after exposure, or baseline level of risk, in terms of a parametric function of that factor. It becomes a projection model when data in a particular range of observations are used to assign values to the parameters in order to estimate (or project) excess risk for factor values outside that range.

*Promoter.*

An agent that is not by itself carcinogenic but can amplify the effect of a true carcinogen by increasing the probability of late-stage cellular changes necessary to complete the carcinogenic process.

*Proof of principle.*

Proof-of-principle studies are those that extend or strengthen the validity of some elements of a hypothesis or a model using a system that is different from the one that provided the basis for the hypothesis or model.

*Proportional mortality ratio.*

The ratio of the percentage of a specific cause of death among all deaths in the population being studied divided by the comparable percentage in a standard population.

*Protraction.*

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

*Quadratic-dose model.*

A model that assumes that the excess risk is proportional to the square of the dose.

*Quality factor (Q).*

An LET-dependent factor by which the absorbed dose is multiplied to obtain (for radiation protection purposes) the dose equivalent that corresponds roughly to the absorbed dose from X- or gamma rays that causes the same degree of biologic effect. Absorbed dose (Gy) × *Q* = dose equivalent (Sv).

*Rad.*

A special unit of absorbed dose, now replaced by the SI unit gray (see *Units*). 1 rad = 0.01 Gy = 100 erg/g.

*Radiation.*

Energy emitted in the form of waves or particles by radioactive atoms as a result of radioactive decay or produced by artificial means, such as X-ray generators.

*Radioactivity.*

The property of nuclide decay in which particles or gamma radiations are usually emitted.

*Artificial radioactivity.* Man-made radioactivity produced by fission, fusion, particle bombardment, or electromagnetic irradiation.

*Natural radioactivity.* The property of radioactivity exhibited by more than 50 naturally occurring radionuclides.

*Radiogenic.*

Caused by radiation.

*Radioisotope.*

A radioactive atomic species of an element with the same atomic number and usually identical chemical properties.

*Radionuclide.*

A radioactive species of an atom characterized by the constitution of its nucleus.

*Relative biologic effectiveness (RBE).*

The ratio *D*_{ref}/*D*, where *D* is the absorbed dose of a specified radiation and *D*_{ref} is the absorbed dose of a sparsely ionizing reference radiation (γ-rays or X-rays) that produces the same level of effect. When the magnitude of the dose *D* is not specified, the RBE is meant to be the low-dose limit of the ratio *D*_{ref}/*D* (this low-dose *RBE* equals the low-dose effectiveness (initial slope) of the specified radiation to that of the reference radiation).

*Relative risk (RR)*.

The rate of disease in an exposed population divided by the rate of disease in an unexposed population. Also termed “rate ratio.”

*Rem* (*rad equivalent man*).

A special unit of dose equivalent, now replaced by the SI unit sievert (see *Units*). 1 rem = 0.01 Sv.

**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

*Retinoblastoma.*

An eye tumor that is an example of an inherited malignant tumor with a dominant autosomal gene inheritance pattern.

*Risk.*

A chance of injury, loss, or detriment; a measure of the deleterious effects that may be expected as the result of an action or inaction.

*Risk assessment.*

The process by which the risks associated with an action or inaction are identified and quantified.

*Risk coefficient.*

The increase in the annual incidence or mortality rate per unit dose: (1) absolute risk coefficient is the increase of the incidence or mortality rate per unit dose; (2) relative risk coefficient is the fractional increase above the baseline incidence or mortality rate per unit dose.

*Risk estimate.*

The increment of the incidence or mortality rate projected to occur in a specified exposed population per unit dose for a specified exposure regime and expression period.

*Sievert (Sv).*

Special name of the SI unit of dose equivalent (see *Units*). 1 Sv = 1 J/kg = 100 rem.

*SI units.*

Units of the International System of Units as defined by the General Conference of Weights and Measures in 1960. They are the base units, such as meter (m), kilogram (kg), second (s), and their combinations, which have special names (e.g., the unit of energy, 1 J = 1 kg m^{2}/s^{2}, or absorbed dose, 1 Gy = 1 J/kg = 1 m^{2}/s^{2} (see *Units*).

*Solid cancers.*

Solid cancers include all malignant neoplasms other than those of the lymphatic and hematopoietic tissue.

*Somatic cells.*

Nonreproductive cells.

*Specific activity.*

Activity of a given nuclide per unit mass of a compound, element, or radioactive nuclide.

*Specific energy* (*z*)*.*

The energy per unit mass actually deposited in a microscopic volume in a single energy deposition event or at a given absorbed dose. This is a stochastic quantity as opposed to its average, the absorbed dose, *D.* The mean energy imparted by ionizing radiation to a medium per unit mass. Unit: 1 Gy = 1 J/kg.

*Standardized morbidity ratio* or *Standardized mortality rate (SMR).*

The ratio (multiplied by 100) of the mortality rate from a disease in the population being studied divided by the comparable rate in a standard population. The ratio is similar to a relative risk times 100.

*Stochastic.*

Effects whose probability of occurrence in an exposed population (rather than severity in an affected individual) depends on dose; stochastic effects are commonly regarded as having no threshold; hereditary effects are stochastic; some somatic effects, especially cancers, are regarded as being stochastic.

*Suppressor gene.*

A gene that can suppress another gene such as an oncogene. Changes in suppressor genes can lead to expression by genes such as oncogenes.

*Synergistic effect.*

Increased effectiveness results from an interaction between two agents, so that the total effect is greater than the sum of the effects of the two agents acting alone.

*Target cells.*

Cells in a tissue that have been determined to be the key cells in which changes occur in order to produce an end point such as cancer.

*Threshold hypothesis.*

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

*Transformed cells.*

Tissue culture cells changed from growing in an orderly pattern exhibiting contact inhibition to growing in a pattern more like that of cancer cells.

*Uncertainty.*

The range of values within which the true value is estimated to lie. It is a best estimate of possible inaccuracy due to both random and systemic errors.

*Random Errors.* Errors that vary in a nonreproducible way around a limiting mean. These errors can be treated statistically by use of the laws of probability.

*Systemic Errors.* 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.

*Units of dose.*

Also known as dosimetric units.

*UNSCEAR* (*United Nations Scientific Committee on the Effects of Atomic Radiation*).

A UN committee that publishes periodic reports on sources and effects of ionizing radiation.

*Variability.*

The variation of a property or quantity among members of a population. Such variation is inherent in nature and is often assumed to be random; it can then be represented by a frequency distribution.

**Suggested Citation:**"Glossary." National Research Council. 2006.

*Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2*. Washington, DC: The National Academies Press. doi: 10.17226/11340.

*Weighted dose* (*d*)*.*

The dose to A-bomb survivors, roughly adjusted to account for the increased effectiveness of the small neutron absorbed dose contribution. The weighted dose equals the gamma-ray absorbed dose to a specified organ plus the neutron absorbed dose multiplied by a weighting factor that has usually been set equal to 10 in analyses by the Radiation Effects Research Foundation (RERF). Unit: 1 Sv = 1 J/kg.

*X-radiation.*

Also X-rays; penetrating electromagnetic radiation, usually produced by bombarding a metallic target with fast electrons in a high vacuum.