Units to measure radioactivity for protective purposes include the becquerel (Bq), the gray (Gr) and the sievert (Sv). The becquerel measures the intensity of the radiation, i.e. how many nuclei decay or disintegrate per second. One Becquerel is equal to one nucleus decay once per second. The radiation dose absorbed by a person (that is, the amount of energy deposited in human tissue by radiation) is measured using the gray (Gy). The risk that a person will suffer health effects from an exposure to radiation is measured using the sievert (Sv).