The information below is from Wikipedia concerning radiation, which is something people in Japan are currently learning more about at a rapid pace because of the previous 2 explosions at the Fukushima Dai-ichi nuclear power plant.
Frequently used SI multiples are the millisievert (1 mSv = 10−3 Sv) and microsievert (1 μSv = 10−6 Sv) or (1 mSv = 0.001 Sv) and (1 μSv = 0.000001 Sv).
The millisievert is commonly used to measure the effective dose in diagnostic medical procedures (e.g., X-rays, nuclear medicine, positron emission tomography, and computed tomography). The natural background effective dose rate varies considerably from place to place, but typically is around 2.4 mSv/year  (pdf).
For acute (that is, received in a relatively short time, up to about one hour) full body equivalent dose, 1 Sv causes nausea, 2-5 Sv causes epilation or hair loss, hemorrhage and will cause death in many cases. More than 3 Sv will lead to LD 50/30 or death in 50% of cases within 30 days, and over 6 Sv survival is unlikely. (For more details, see radiation poisoning.)