The Sievert unit is based upon the Gray unit of radiation dose, but it also includes a weighting factor for the type of biological tissue that is exposed to rad

The Sievert unit is based upon the Gray unit of radiation dose, but it also includes a weighting factor for the type of biological tissue that is exposed to radiation. You can also read up on the linear no-threshold model of radiation exposure for more information. In terms of cancer risk, radiation doses on a range from 1 mSv to 100 mSv can definitely be compared, regardless of the duration of the exposure. There is a range of higher doses e.g. above 1 Gy that would cause acute radiation syndrome if received all at once, where different biological effects would predominate.