When a person is exposed to a radiation field, the radiation acts on the body by releasing its energy into the body: It is said that the person has an accumulated dose. Doses can be expressed in grays or sieverts. The absorbed dose of energy is expressed in grays or milligrays and corresponds to the energy absorbed per kilogram of body tissue. On the other hand, the effective dose, which is expressed in sieverts, relates to the sum of the absorbed contributions of energy per kilogram of body tissue, multiplied by weighting factors that take into account the biological effect of the different types of radiation as well as the varying radiation sensitivity of individual organs and tissues. Since a dose of one sievert is a very large value, the doses that usually occur are often expressed in millisieverts (one thousandth of a sievert, also written as mSv) or microsieverts (one millionth of a sievert, also written as μSv).