Sievert

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.

Definition

The unit gray measures absorbed radiation which is absorbed into any material. The unit sievert specifically measures absorbed radiation which is absorbed by a person. The equivalent dose to a person is found by multiplying the absorbed dose, in gray, by a weighting factor (w). The weighting factor (sometimes referred to as a quality factor) is determined by a combination of: the radiation type, the tissue absorbing the radiation, and other pertinent factors.

In terms of SI derived units:

1 Sv = 1 Gy • [w]  (where Sv=sievert, Gy=gray, w=weighting factor specific to each type of radiation and tissue).

1 Gy = 1 J / kg

therefore:

1 Sv = 1 J / kg • [w]

There is a misunderstanding about these units because both of them use J/kg. The CIPM in 1984 (Concerning the sievert (PV, 52, 31 and Metrologia, 1985, 21, 90) made it clear upon the suggestion of International Commission on Radiological Protection: $H=Q\cdot N\cdot D$ where

H is the dose equivalent (in sievert)

Q is a quality factor (dimensionless)

N is a product of any other multiplying factors (dimensionless)

D is the absorbed dose (in gray)

So the w weighting factor is the product of Q and N

Equivalency Weighting Factors

Radiation type and energy range	Factor
electrons, positrons, muons, or photons (gamma, X-ray)	1
neutrons <10 keV	5
neutrons 10–100 keV	10
neutrons 100 keV – 2 MeV	20
neutrons 2 MeV – 20 MeV	10
neutrons >20 MeV	5
protons other than recoil protons and energy >2 MeV	2
alpha particles, fission fragments, nonrelativistic heavy nuclei	20
Tissue type	Factor
bone surface, skin	0.01
bladder, breast, liver, esophagus, thyroid, other	0.05
bone marrow, colon, lung, stomach	0.12
gonads	0.20

Because the body has multiple tissue types a weighted sum or integral is often used.

Historically, the weighting factors for radiation type and tissue type were separated out as Q and N respectively. In 2002, the CIPM decided that the distinction between Q and N caused too much confusion and therefore deleted the factor N from the definition of absorbed dose in the SI brochure.

SI multiples and conversions

Frequently used SI multiples are the millisievert (1 mSv = 10⁻³ Sv = 0.001 Sv) and microsievert (1 μSv = 10⁻⁶ Sv = 0.000001 Sv).

Equivalent dose is measured in the United States in rem:

• 1 rem = 0.01 Sv = 10 mSv
• 1 mrem = 0.00001 Sv = 0.01 mSv = 10 μSv

• 1 Sv = 100 rem = 100,000 mrem (or millirem)
• 1 mSv = 100 mrem = 0.1 rem
• 1 μSv = 0.1 mrem

The rem and millirem (abbreviated mrem), as with other customary units in the United States, are in wider use among the general public, many industries, and government. However, SI units such as the sievert are frequently encountered in academic, scientific, and engineering environments.

The time derivative of the Sv (flux of ionising radiation) could be calculated from J/s (joule per second) which is W (watt). We cannot use for time derivative of sievert measuring it in W/kg, so a mixed unit is used instead: the mSv/h

Dose examples

Single dose examples

• 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
• International Commission on Radiological Protection recommended limit for volunteers averting major nuclear escalation: 500 mSv
• International Commission on Radiological Protection recommended limit for volunteers rescuing lives or preventing serious injuries: 1000 mSv

Hourly dose examples

• Average individual background radiation dose: 0.23μSv/h (0.00023mSv/h); 0.17μSv/h for Australians, 0.34μSv/h for Americans
• Highest reported level during Fukushima accident: 1000 mSv/h reported as the level at a pool of water in the turbine room of reactor two.

Yearly dose examples

• 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/yr
• Dose from Cosmic radiation (from sky) at sea level: 0.24 mSv/year
• Dose from Terrestrial radiation (from ground): 0.28 mSv/year
• Dose from Natural radiation in the human body: 0.40 mSv/year
• Dose from standing in front of the granite of the United States Capitol building: 0.85 mSv/year
• Average individual background radiation dose: 2 mSv/year; 1.5 mSv/year for Australians, 3.0 mSv/year for Americans
• Dose from atmospheric sources (mostly radon): 2 mSv/year
• Total average radiation dose for Americans: 6.2 mSv/year
• New York-Tokyo flights for airline crew: 9 mSv/year
• Dose from smoking 30 cigarettes a day Smoking: 13-60 mSv/year
• Current average dose limit for nuclear workers: 20 mSv/year
• Dose from background radiation in parts of Iran, India and Europe: 50 mSv/year
• Dose limit applied to workers during Fukushima emergency: 250 mSv/year

Dose limit examples

• Criterion for relocation after Chernobyl disaster: 350 mSv/lifetime
• In most countries the current maximum permissible dose to radiation workers is 20 mSv per year averaged over five years, with a maximum of 50 mSv in any one year. This is over and above background exposure, and excludes medical exposure. The value originates from the International Commission on Radiological Protection (ICRP), and is coupled with the requirement to keep exposure as low as reasonably achievable (ALARA) – taking into account social and economic factors.
• Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background.

Symptom benchmarks

Symptoms of acute radiation (dose received within one day):

• 0 – 0.25 Sv (0 – 250 mSv): None
• 0.25 – 1 Sv (250 – 1000 mSv): Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleen damaged.
• 1 – 3 Sv (1000 – 3000 mSv): Mild to severe nausea, loss of appetite, infection; more severe bone marrow, lymph node, spleen damage; recovery probable, not assured.
• 3 – 6 Sv (3000 – 6000 mSv): Severe nausea, loss of appetite; hemorrhaging, infection, diarrhea, peeling of skin, sterility; death if untreated.
• 6 – 10 Sv (6000 – 10000 mSv): Above symptoms plus central nervous system impairment; death expected.
• Above 10 Sv (10000 mSv): Incapacitation and death.

See also Radiation poisoning.

Explanation

Various terms are used with this unit:

• Dose equivalent
• Ambient dose equivalent
• Directional dose equivalent
• Personal dose equivalent
• Organ equivalent dose

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.

Given the linear no-threshold model of radiation response, the collective dose that a population is exposed to is measured in "man-Sievert" (man·Sv).

Q values

Here are some quality factor values based on ICRP 103 recommendations:

• Photons, all energies : Q = 1
• Electrons and muons, all energies : Q = 1
• Protons and charged pions : Q = 2
• Neutrons : Q is a continuous function of neutron energy
• Alpha particles and other atomic nuclei : Q = 20

There is some controversy that the Q=20 for alpha radiation is underestimated due to mistaken assumptions in the original work in the 1950s that developed those values. That original work neglected the component of the nucleus recoil radiation for alpha emitters.

N values

Here are some N values for organs and tissues:

• Bone marrow, colon, lung, breast, stomach: N = 0.12
• Gonads: N = 0.08
• Bladder, brain, salivary glands, 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

Spelling

The sievert is named after Rolf Maximilian Sievert. As with every SI unit named for a person, its symbol starts with an upper case letter (Sv), but when written in full, it follows the rules for capitalisation of a common noun; i.e., sievert becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

See also

• Becquerel (disintegrations per second)
• Counts per minute
• Curie (unit)
• Gray (unit)
• Ionizing radiation level examples - Example exposure scenarios
• Rad (unit)
• Rem (unit)
• Roentgen (unit)
• Rutherford (unit)
• Sverdrup (unit) (a non-SI unit of volume transport with the same symbol Sv as Sievert)
• Background radiation
• Relative Biological Effectiveness
• Radiation poisoning
• Linear Energy Transfer
• Orders of magnitude (radiation)

References

• 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)
• Introduction to Quantities and Units for Ionising Radiation National Physical Laboratory
• Radiation Protection Japanese Nuclear Emergency: EPA's Radiation Air Monitoring