Preneoplastic.

Cells with altered genetic stares that are not yet completely malignant.

Probability.

A number expressing the chance that a specified event will occur. It can range from 0 (indicating that the event is certain not to occur) to 1 (indicating that the event is certain to occur).

Proliferative cells.

Those cells in a tissue that divide by mitosis to become the expanding population that serves as the source for the fully differentiated cells which ultimately carry out the functions of the tissue.

R

rad.

A quantity of absorbed dose equal to 100 erg g-1. This is the traditional unit of dose and 1 rad = 0.01 gray (Gy).

Radiation.

Any combination of elementary particles that have sufficient kinetic energy to interact with and transfer energy to material that intercepts their path. If the energy transferred is sufficient to produce ionization in the material, it is classified as ionizing radiation.

Radiation exposure.

The total electrical charge of one sign produced in air by photons interacting in volume of air divided by the mass of air in that volume. It has the dimensions of charge divided by mass, Coulomb kg-1. The special unit of exposure is the Roentgen (R).

Radiation weighting factor (wR).

A modifying factor used to obtain equivalent dose from absorbed dose. It depends on the biological effectiveness of the specified radiation but does not depend on the tissue or organ under consideration.

Radioactivity.

A quantity of radioactive material based on the rate that the atoms or nuclei spontaneously transform and emit radiation. Radioactivity is described in terms of a becquerel (Bq), or curie (Ci).

Radon.

The element with atomic number 88. It is an inert gas within the same family of elements as helium, neon, argon, and xenon. Although there are several isotopes of radon, the most common is 222Rn which is part of the decay series that begins with 238U.

Radon decay products.

The four short-lived radioactive isotopes in the 238U series immediately following the decay of 222Rn. They are the metals, 218Po, 214pb, 214Bi, 214Po, and are also called radon daughters or radon progeny.

RCRA.

Resource, Conservation and Recovery Act regulates the generation, storage, transportation, treatment and disposal of hazardous substances.

rem.

A measure of equivalent dose H, that is obtained when the dose (rad) is multiplied by the radiation weighting factor (wR). For example, if wR = 1, then one rem is numerically the same as one rad.

Risk.

Conceptually, risk is a measure of the chance that a specified health outcome will occur. Various quantities are used to describe different aspects of risk. For example, the probability of disease is the chance that an individual will develop a specified disease during a selected interval of age (often the entire lifetime). Risks expressed as incidence rates, or mortality rates, are the number of persons who are expected to develop or die from the disease within a selected time interval and population group. This could be expressed as the number of cases or deaths per 100,000 persons per year.

Roentgen.

A quantity of radiation exposure equivalent to 2.58 × 10 -4 Coulomb kg-1. The roentgen is a traditional unit named on behalf of the German scientist W. K. Roentgen.

S

Safe Drinking Water Act (SDWA).

Legislation that originally became law in 1974 and was amended as recently as 1996. This act and its amendments are designed to provide safe drinking water for consumption by the public.

Senescence.

The stage in the life cycle of a cell when division stops and degenerative changes begin to occur. In human cells this happens after approximately 50 cell divisions.

SI units.

The International System of Quantities and Units derived by the General Conference of Weights and Measures. Base units are the meter, kilogram, second and coulomb. It is often referred to as the metric system.

Sievert (Sv).

A measure of equivalent dose, H, that is obtained when the Dose (Gy) is multiplied by the radiation weighting factor wR. For example, when wR = 1, then one Sv is numerically the



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