What's the point of measuring alpha and beta radiation in Sievert? As I understand it, one Sievert is an equivalent of 1 Joule absorbed by 1kg of body mass. But since alpha and beta radiation barely penetrate the human skin, most of the body mass isn't irradiated at all. So the dose in Sievert says nothing about the energy absorbed by the body, either absolute or per kg of body mass. It seems it would be more sensible to measure absorption by a square meter of human skin. Why isn't it done this way?