(1) The act or condition of being subject to irradiation. Exposure should not be used as a synonym for dose. Dose is a measure of the effects of exposure. Exposure can be divided into categories according to its nature and duration (see exposure situations) or according to the source of the exposure, the people exposed and/or the circumstances under which they are exposed (see exposure, types of). External Exposure: Exposure to radiation from a source outside the body. Contrasted with internal exposure. Internal Exposure: Exposure to radiation from a source within the body. Contrasted with external exposure. (2) The sum of the electrical charges of all of the ions of one sign produced in air by X rays or gamma radiation when all electrons liberated by photons in a suitably small element of volume of air are completely stopped in air, divided by the mass of the air in the volume element. Unit: C/kg (in the past, the roentgen (R) was used). (3) The time integral of the potential alpha energy concentration in air, or of the corresponding equilibrium equivalent concentration, to which an individual is exposed over a given period (e.g. a year). Used in connection with exposure to radon and thoron progeny. The SI unit is J·h/m3 for potential alpha energy concentration or Bq·h/m3 for equilibrium equivalent concentration. (4) "The product of the air concentration of a radionuclide to which a person is exposed and the time of exposure. More generally, when the air concentration varies with time, the time integral of the air concentration of a radionuclide to which a person is exposed, integrated over the time of exposure". This definition, quoted verbatim from ICRP G3, reflects a loose usage of exposure found particularly in the context of airborne radon. This usage is listed here for information, but it is discouraged