You’ll be sick from 2-10 sieverts in a short period (1-2 weeks). Over a long time, the body will repair some of the damage and you’ll need more to get sick.
100 mSv in a year is the lowest level at which any increase in cancer risk is clearly evident. Background is 2-3 mSv/year and and OSHA limits workplace exposure to 20 mSv/year.
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