sievert

The sievert (symbol: Sv) is the International System of Units (SI) SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray. It is named after Rolf Maximilian 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 the absorbed dose of radiation (D), absorbed by any material. The unit sievert measures the equivalent dose of radiation (H), having the same damaging effect as an equal dose of gamma rays.

Both the gray, with symbol Gy and the sievert, with symbol Sv are SI derived units, defined as a unit of energy (joule) per unit of mass (kilogram):

1 Gy = 1 Sv = 1 J / kg

This SI unit is named after Rolf Maximilian Sievert. As with every International System of Units (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. —Based on The International System of Units, section 5.2.

Dose equivalent

The equivalent dose to a tissue is found by multiplying the absorbed dose, in gray, by a weighting factor (W_R). The relation between absorbed dose D and equivalent dose H is thus:

.

The weighting factor (sometimes referred to as a quality factor) is determined by the radiation type and energy range.^[1]

where

H_T is the equivalent dose absorbed by tissue T

D_T,R is the absorbed dose in tissue T by radiation type R

W_R is the weighting factor defined by the following table

Thus for example, an absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv. The maximum weight of 30 is obtained for neutrons with L = 100 keV/μm.

Effective dose

The effective dose of radiation (E), absorbed by a person is obtained by averaging over all irradiated tissues with weighting factors adding up to 1:^[1][2]

.

For other organisms, weighting factors have been defined, relative to the effect on humans:

SI multiples and conversions

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

An older unit for the equivalent dose, is the rem,^[3] still often used in the United States. One sievert is equal to 100 rem:

• 1 rem = 0.01 Sv = 10 mSv
• 1 mrem = 0.01 mSv = 10 μSv
• 1 Sv = 100 rem
• 1 mSv = 100 mrem = 0.1 rem
• 1 μSv = 0.1 mrem

The conventional units for its time derivative is mSv/h.

Symptom benchmarks

See also Radiation poisoning.

Dose examples

Single dose examples

• Dental radiography: 0.005–0.03 mSv^[4][5]
• Average dose to people living within 16 km of Three Mile Island accident: 0.08 mSv during the accident^[6]
• Mammogram: Single Exposure, Equipment Mean: 2 mSv^[7]
• Mammogram: Procedural Mean, Equipment Variation: 4–5 mSv^[7]
• Brain CT scan: 0.8–5 mSv^[8]
• Chest CT scan: 6–18 mSv^[8]
• Gastrointestinal series^{[disambiguation needed ]} X-ray investigation: 14 mSv^[9]
• International Commission on Radiological Protection recommended limit for volunteers averting major nuclear escalation: 0.5 Sv = 500 mSv^[10]
• International Commission on Radiological Protection recommended limit for volunteers rescuing lives or preventing serious injuries: 1 Sv = 1000 mSv^[10]
• Fatal doses during Goiânia accident: 4.5–6 Sv = 4500–6000 mSv
• Non-fatal doses during Goiânia accident: 0–7 Sv = 0–7000 mSv

Hourly dose examples

• Average individual background radiation dose: 0.23 μSv/h (0.00023 mSv/h); 0.17 μSv/h for Australians, 0.34 μSv/h for Americans^[6][11][12]
• The hourly doses are 1.6 μSv/h (14 mSv/year) in the city of Fukushima and 0.062 μSv/h (0.54 mSv/year) in Tokyo as of May 25, 2011.^[13]
• Highest reported level during Fukushima accident: 433 Sv/h for the gas/steam inside the primary containment (drywell) of reactor unit 1 on August 19, 2011 (note the reading is not micro or milli Sv, but Sv/h).^[14]
• Highest dose rate measured in Finland during the Chernobyl disaster: 5 µSv/h ^[15]
• Measurements taken after Fukushima accident: Greater than 10 Sv/h for the ventilation shaft between reactors I and II (equipment used could only read up to 10 Sv/h)^[16][17]

Yearly dose examples

• Maximum acceptable dose for the public from any man made facility: 1 mSv/year^[18]
• Dose from living near a nuclear power station: 0.0001–0.01 mSv/year^[9][11]
• Dose from living near a coal-fired power station: 0.0003 mSv/year^[11]
• Dose from sleeping next to a human for 8 hours every night: 0.02 mSv/year^[11]
• Dose from cosmic radiation (from sky) at sea level: 0.24 mSv/year^[9]
• Dose from terrestrial radiation (from ground): 0.28 mSv/year^[9]
• Dose from natural radiation in the human body: 0.40 mSv/year^[9]
• Dose from standing in front of the granite of the United States Capitol building: 0.85 mSv/year^[19]
• Average individual background radiation dose: 2 mSv/year; 1.5 mSv/year for Australians, 3.0 mSv/year for Americans^[6][11][12]
• Dose from atmospheric sources (mostly radon): 2 mSv/year^[9][20]
• Total average radiation dose for Americans: 6.2 mSv/year^[21]
• New York-Tokyo flights for airline crew: 9 mSv/year^[12]
• Current average dose limit for nuclear workers: 20 mSv/year^[12]
• Dose from background radiation in parts of Iran, India and Europe: 50 mSv/year^[12]
• Dose from smoking 30 cigarettes a day: 60–80 mSv/year^[19][22]

Dose limit examples

• Criterion for relocation after Chernobyl disaster: 350 mSv/lifetime^[12]
• 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.^[23]
• Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background.^[23] However, experts including Professor Wade Allison of Oxford University argue that the dose limit can safely be raised to 100 millisieverts, based on current health statistics.^[24]
• Dose limit applied to workers during Fukushima emergency: 250 mSv.^[25]

History

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.^[26]

The older version of the definitions contained Q and N factors, corresponding to the current W_R and W_T, with values:

See also

• Becquerel (disintegrations per second)
• Counts per minute
• Curie (unit)
• Gray (unit)
• Ionizing radiation level examples - Example exposure scenarios
• Ionizing radiation units
• 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)

Notes

References

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