The sievert (symbol Sv) is an SI derived unit of equivalent dose or effective dose (of radiation), and so is dependent upon the biological effects of radiation as opposed to the physical aspects, characterised by the absorbed dose (measured in grays).

The unit is named after Rolf Sievert, a Swedish medical physicist who was well-known for his work on radiation dosage measurement together with his research into the biological effects of radiation.

The equivalent dose to a tissue is found by multiplying the absorbed dose by a "quality factor" Q, dependent upon radiation type, and by another factor N, dependent on all other pertinent factors. N depends upon the part of the body irradiated, the time and volume over which the dose was spread, even the species of the subject.

To avoid any risk of confusion between the absorbed dose and the equivalent dose, one must use the corresponding special units, namely the gray instead of the joule per kilogram for absorbed dose and the sievert instead of the joule per kilogram for the dose equivalent.

1 Sv is equal to 100 rems. If you assume Q and N equal to 1, then 1 Sv ≈ 107,185 R.

The millisievert (mSv) 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 3.5 mSv/year.

For a full body equivalent dose, 1 Sv causes slight blood changes, 2-5 Sv causes nausea, hair loss, hemorrhage and will cause death in many cases. More than 6 Sv will lead to death in less than two months in more than 80% of cases, and much over 4 is more likely than not to cause death. See radiation poisoning for a more complete analysis of effects of various dosage levels.

Collective dose is measured in "man-sieverts" (man.Sv).

Here are some quality factor Q values:

• Photons, all energies : Q = 1
• Electrons and muons, all energies : Q = 1
• Neutrons,
  • energy < 10 keV : Q = 5
  • 10 keV < energy < 100 keV : Q = 10
  • 100 keV < energy < 2 MeV : Q = 20
  • 2 MeV < energy < 20 MeV : Q = 10
  • energy > 20 MeV : Q = 5
• Protons, energy > 2 MeV : Q = 5
• Alpha particles and other atomic nuclei : Q = 20

Here are some N values for organs and tissues:

• Gonads: N = 0,20
• Bone marrow, colon, lung, stomach: N = 0,12
• Bladder, brain, breast, kidney, liver, muscles, oesophagus, pancreas, small intestine, spleen, thyroid, uterus: N = 0,05
• Bone surface, skin: N = 0,01

And for other organisms, relative to humans:

• Viruses, bacteria, protozoans: N ≈ 0,03 – 0,0003
• Insects: N ≈ 0,1 – 0,002
• Molluscs: N ≈ 0,06 – 0,006
• Plants: N ≈ 2 – 0,02
• Fish: N ≈ 0,75 – 0,03
• Amphibians: N ≈ 0,4 – 0,14
• Reptiles: N ≈ 1 – 0,075
• Birds: N ≈ 0,6 – 0,15
• Humans: N = 1

Sources

• Comité international des poids et mesures (CIPM) 1984, Recommendation 1 (PV, 52, 31 and Metrologia, 1985, 21, 90)
• Abdeljelil Bakri, Neil Heather, Jorge Hendrichs, and Ian Ferris; Fifty Years of Radiation Biology in Entomology: Lessons Learned from IDIDAS, Annals of the Entomological Society of America, 98(1): 1-12 (2005)