Sievert
The sievert (symbol: Sv) is a derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is important in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation.

The sievert is used for radiation dose quantities such as equivalent dose and effective dose, which represent the risk of external radiation from sources outside the body, and committed dose which represents the risk of internal irradiation due to inhaled or ingested radioactive substances. The sievert is intended to represent the stochastic health risk, which for radiation dose assessment is defined as the probability of radiation-induced cancer and genetic damage. One sievert carries with it a 5.5% chance of eventually developing fatal cancer based on the linear no-threshold model.

To enable consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent dose and effective dose, the details of which depend on the radiation type and biological context. For applications in radiation protection and dosimetry assessment the International Commission on Radiological Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these. These are under continual review, and changes are advised in the formal "Reports" of those bodies.

Conventionally, the sievert is not used for high dose rates of radiation that produce deterministic effects, which is the severity of acute tissue damage that is certain to happen, such as acute radiation syndrome; these effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy). 

One sievert equals 100 rem. The rem is an older, non-SI unit of measurement. For electromagnetic radiation such as X-rays and gamma rays, the gray and sievert are equivalent (a sievert is the biological impact of one gray of electromagnetic radiation), but they diverge for types of radiation that are more biologically damaging, such as neutrons and alpha radiation, where one gray of absorbed radiation may produce 20 sieverts of biological effect.
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derived unit
Concept
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ionizing radiation
Concept
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International System of Units
Concept
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dosimetry
UNK
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radiation protection
Concept
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Rolf Maximilian Sievert
PER
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equivalent dose
Concept
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effective dose
UNK
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committed dose
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stochastic
Concept
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radiation-induced cancer
Concept
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linear no-threshold model
UNK
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absorbed dose
Concept
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International Commission on Radiological Protection
ORG
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International Commission on Radiation Units and Measurements
ORG
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acute radiation syndrome
Concept
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absorbed dose
Concept
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gray
Concept
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rem
Concept
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Sievert
Concept
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5.5
NUMBER
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20
NUMBER
instance of
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measured physical quantity