In the SI system (the internationally agreed system of units), three main units are used to measure levels of ionizing radiation – the becquerel (Bq), the gray (Gy), and the sievert (Sv). The becquerel is the SI unit of spontaneous activity of a radioactive source (see radioactivity), such as uranium.
In medicine, the most commonly used units are the gray and the sievert. The gray is the SI unit of radiation that is actually absorbed by an tissue or substance as a result of exposure to radiation. Because some types of radiation affect biological organisms more than others, the sievert is used as a measure of the impact of an absorbed dose. It uses additional factors, such as the kind of radiation and its energy, to quantify the effects on the body of equivalent amounts of different types of absorbed energy.