Named for Swedish clinical physicist Rolf Maximillian Sievert (AD 1896-1960) this is a unit of measure intended to replace the rem (rad equivalent mammals or man) in measuring radiation exposure. It is a SI-derived unit expressing the amount of radiation needed to apply 1 joule of energy to each kilogram of human tissue (Gy or one gray). This is not the same for every type of radiation so it is multiplied for dimensionless Q factors (quality) and N (the product of any other multiplying factors. Sv = Q*N*Gy One sievert of radiation produces a constant biological effect regardless of the type of radiation.
The International Commission on Radiological Protection officially sets both Q and N. Q is set at 1 for x-rays, gamma rays, and beta particles; 10 for neutrons; and 20 for alpha particles. N is a factor that takes into account the distribution of energy, i.e. internal radiation is more dangerous than radiation from outside the body.
One hundred rems is roughly equivalent to 1.0 Sv for x-rays of energy ranging from 200 to 250 keV. However usually the millisievert (mSv) or microsievert (µSv) are used since one sievert is inconveniently large.
Average exposure is between 1 and 3 mSv, primarily from natural background radiation and medical x-rays. It is thought that upto a total of 60 mSv over a year is not dangerous and no study had yet found any danger below 50 mSv in the course of a year. Though it should be noted that no one would recommend getting this much on purpose. Indeed above 10 mSv/yr a person should think about reducing his exposure.
10 sieverts in a short period (less than 24 hours) would result in death from radiation poisoning. 1 sievert (or 1,000 mSv) would result in sickness, but would be unlikely to result in death immediately. Though this would result in a significantly higher risk of cancer. If not repeated a burst of upto 20 mSv is believed to be totally safe. In between these values is a grey area best expressed as an increasing chance of death or cancer. A person could die from an exposure of just 2 sieverts... but it is far from a sure thing.
The current limit in the nuclear industry is 25 µSv exposure in a hour.