Sievert

Similar to the unit gray, sievert is a unit for measuring radiation dose quantities. What gray does not tell us is how dangerous that radiation is, or in other words, how much of it is absorbed by human tissue. Therefore we use sievert as an equivalent unit for gray to consider the danger of various radiations.

Gray is only the actual physical dose:

1 Gy = 1 J/kg

But this does not tell us how dangerous the radiation is. For instance, 1 Gy of alpha radiation is 20 times more dangerous than 1 Gy of gamma radiation. 1 Gy (J/kg) of alpha radiation corresponds to 20 Sv of the equivalent dose: