Reports in the media of the nuclear plant disasters in Japan often cite the amount of radioactivity released in units called millisieverts. Not knowing what a millisievert is, I resorted to Wikipedia for the needed clarification. We reproduce some of their explanation here as a public service.
The sievert (symbol: Sv) is the SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Sievert, a Swedish medical physicist renowned for work on radiation dosage measurement and research into the biological effects of radiation.
As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Sv). When an SI unit is spelled out in English, it should always begin with a lower case letter (sievert), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
For those used to older units such as the rem, (a unit for the equivalent dose), one sievert = 100 rem; 1 rem = 0.01 Sv = 10 mSv.
- Dental radiography: 0.005 mSv
- Average dose to people living within 16 km of Three Mile Island accident: 0.08 mSv during the accident
- Mammogram: 3 mSv
- Brain CT scan: 0.8–5 mSv
- Chest CT scan: 6–18 mSv
- Gastrointestinal series X-ray investigation: 14 mSv
- Maximum acceptable dose for the public from any man-made facility: 1 mSv/year
- Dose from living near a nuclear power station: 0.0001–0.01 mSv/year
- Dose from living near a coal-fired power station: 0.0003 mSv/year
- Dose from sleeping next to a human for 8 hours every night: 0.02 mSv/year
- Dose from cosmic radiation at sea level: 0.24 mSv/year
- Dose from terrestrial radiation: 0.28 mSv/year
- Dose from natural radiation in the human body: 0.40 mSv/year