Some types of ionizing radiation produce greater biological effects than others for the same amount of energy imparted. The rem is a common unit that relates the dose of any radiation to the biological effect of that dose. Therefore, to relate the absorbed dose of specific types of radiation, a "quality factor" must be multiplied by the dose in rad.

To indicate the dose an individual receives in the unit rem, the word "rem" follows immediately after the magnitude, for example, "50 rem." One thousandth of a rem is abbreviated as "mrem" or "millirem."