The main physical quantities used in radiological protection are the rate of nuclear transformation of radionuclides (the activity) and the energy absorbed by a unit mass of a substance from the radiation to which it is exposed (the absorbed dose). The unit of activity is the reciprocal second, representing the number of nuclear transformations (or disintegrations) per second, which is termed the becquerel (Bq). The unit of absorbed dose is the joule per kilogram, termed the gray (Gy).

The absorbed dose is the basic physical dosimetric quantity. However, it is not entirely satisfactory for radiation protection purposes because effectiveness in damaging human tissue differs for different types of ionizing radiation. Consequently, the absorbed dose averaged over a tissue or organ is multiplied by a radiation weighting factor to take account of the effectiveness of the given type of radiation in inducing health effects; the resulting quantity is termed the equivalent dose. The quantity equivalent dose is used when individual organs or tissues are irradiated, but the likelihood of injurious stochastic effects due to a given equivalent dose differs for different organs and tissues. Consequently, the equivalent dose to each organ and tissue is multiplied by a tissue weighting factor to take account of the organ’s radiosensitivity. The sum total of such weighted equivalent doses for all exposed tissues in an individual is termed the effective dose. The unit of equivalent dose and of effective dose is the same as that of absorbed dose, namely joule per kilogram, but the name sievert (Sv) is used in order to avoid confusion with the unit of absorbed dose (Gy).

When radionuclides are taken into the body, the resulting dose is received throughout the period of time during which they remain in the body. The committed dose is the total dose delivered during this period of time, and is calculated as a specified time integral of the rate of receipt of the dose. Any relevant dose restriction is applied to the committed dose from the intake.