The
radiation absorbed dose is how much
radiation hits your
tissues. Because different tissues
absorb radiation differently, human
exposure has to be measured using a
weighted average over the different tissues of the body. The distinction between absorbed
dose (measured in
grays) and absorbed dose equivalent (measured in
sieverts) is absolutely critical to
health and safety.
A person's total annual absorbed exposure is of the order of one millisievert. This comes from cosmic radiation, from radon decay from the rocks underneath us, from medical practices like x-rays, and from flying in jets. Workers in nuclear power plants have a slightly higher exposure. Some people because of how and where they work will get perhaps five millisieverts a year. If you're near a nuclear power plant when an accident happens you might suddenly get a flash dose of ten; a single dose of twenty to fifty millisieverts is survivable but you might start worrying about your fertility. When you're close enough that you get a 100 mSv dose or more is when you're likely to have to worry about radiation sickness and possible death.