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 × 1010 Bq.


The reduction in risk for low doses.


Deoxyribonucleic acid; the genetic material of cells.


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


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.


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.


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.


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


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


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.

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