It should also be kept in mind that the sievert is a radiation dose unit used for a variety of arbitrarily defined dose quantities. Grays and sieverts are both

It should also be kept in mind that the sievert is a radiation dose unit used for a variety of arbitrarily defined dose quantities. Grays and sieverts are both J/kg, but you get sieverts by using weighting factors depending on the radiation type, tissue type, organ, and which dose quantity you are calculating. Most of the dose values reported are referring to "effective dose", which attempts to sort of normalize all exposures in terms of full body exposures, e.g. comparing a hand exposure with a chest exposure. This has a number of issues, but as you can imagine, in radiation protection situations, the exact site of exposure is not always well defined.