The sievert (symbol: Sv) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is a measure of the effect of low levels of radiation on the human body. Quantities that are measured in sieverts are designed to represent the stochastic health risk, which for radiation dose assessment is defined as the probability of cancer induction and genetic damage.
The sievert is used for radiation dose quantities such as equivalent dose, effective dose, and committed dose. It is used both to represent the risk of the effect of external radiation from sources outside the body, and the effect of internal irradiation due to inhaled or ingested radioactive substances.
Conventionally the sievert is not used for high levels of radiation which produce deterministic effects, which is the severity of acute tissue damage which is certain to happen. These effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy).
To enable consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent and effective doses, the details of which depend on the radiation type and biological context. For applications in radiation protection and dosimetry assessment the International Committee on Radiation Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these.
The sievert is of fundamental importance in dosimetry and radiation protection, and 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. One sievert carries with it a 5.5% chance of eventually developing cancer. Doses greater than 1 sievert received over a short time period are likely to cause radiation poisoning, possibly leading to death within weeks.
One sievert equals 100 rem. The rem is an older, non-SI unit of measurement.
To enable a comprehensive view of the sievert this article deals with the definition of the sievert as an SI unit, summarises the recommendations of the ICRU and ICRP on how the sievert is calculated, includes a guide to the effects of ionizing radiation as measured in sieverts, and gives examples of approximate figures of dose uptake in certain situations.
- 1 Definition
- 1.1 CIPM definition of the sievert
- 1.2 External operational and protection dose quantities
- 1.3 Calculating protection dose quantities
- 1.4 Operational quantities
- 2 Health effects
- 3 Dose examples
- 4 History
- 5 Radiation-related quantities
- 6 See also
- 7 Notes
- 8 References
- 9 External links
CIPM definition of the sievert
The SI definition given by the International Committee for Weights and Measures (CIPM) says:
"The quantity dose equivalent H is the product of the absorbed dose D of ionizing radiation and the dimensionless factor of Q (quality factor) defined as a function of linear energy transfer by the ICRU"
H = Q × D
The value of Q is not defined further by CIPM, but it requires the use of the relevant ICRU and ICRP recommendations to provide this value.
The CIPM also says that "in order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose D and the name sievert instead of joules per kilogram for the unit of dose equivalent H".
The gray - quantity "D"
- 1 Gy = 1 joule/kilogram - a physical quantity. 1 Gy is the deposit of a joule of radiation energy in a kg of matter or tissue.
The sievert - quantity "H"
- 1 Sv = 1 joule/kilogram - a biological effect. 1 Sv represents the equivalent biological effect of the deposit of a joule of radiation energy in a kg of tissue. The equivalence to absorbed dose is denoted by Q.
To calculate the value of Q, the ICRP and ICRU have generated procedures and data for calculating various dose quantities based on decades of experimental data and epidemiological studies. Whilst the CIPM uses the term "dose equivalent" to signify any modification of the physical dose into a biological effect quantity, only the ICRP and ICRU Operational Quantities now include the words dose equivalent.
External operational and protection dose quantities
The sievert is used to represent the biological effects of different forms of ionizing radiation on various tissues. The resultant complexity has required the creation and definition of a number of different radiation dose quantities by the ICRU. The relationships of the external physical and dose quantities as developed by the ICRU and the ICRP are shown in the accompanying diagram.
These are quantities in which no allowance has been made for biological effects. Radiation fluence is the amount of radiation particles impinging per unit area, kerma is the ionising effect of the radiation field, and absorbed dose is the amount of radiation energy deposited per unit mass.
The protection quantities are used to specify exposure limits to ensure that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided. These quantities are a calculated effect of radiation on organs of the human body using an anthropomorphic phantom. This is a 3D computational model of the human body which attempts to take into account a number of complex effects such as body self-shielding and internal scattering of radiation.
The values of protection quantities are used as "limiting quantities", which are designed to limit stochastic effects at low doses. As protection quantities are calculated and cannot practically be measured, operational quantities have been devised to relate them to practical radiation instrument and dosimeter responses.
Operational quantities are aimed at providing an estimate or upper limit for the value of the protection quantities related to an exposure. They are often used in practical regulations or guidance. These relate real-life operational instrument measurements and responses to the calculated protection quantities. Simple phantoms relate operational quantities to measured irradiation. According to the ICRP, the ICRU "sphere" phantom in most cases adequately approximates the human body as regards the scattering and attenuation of the radiation fields under consideration.
The "slab" phantom is used to represent the human torso for calibration of whole body dosimeters.
Instrument and dosimetry response
Calculating protection dose quantities
The sievert is used in external radiation protection when discussing equivalent dose (the external-source, whole-body exposure effects, in a uniform field), and effective dose (which depends on the body parts irradiated). These dose quantities are weighted averages of absorbed dose designed to be representative of the stochastic health effects of radiation, and use of the sievert implies that appropriate weighting factors have been applied to the absorbed dose measurement or calculation (expressed in grays).
The ICRP calculation provides two weighting factors to enable the calculation of protection quantities.
- 1. The radiation factor WR, which is specific for radiation type R - This is used in calculating the equivalent dose HT.
- 2. The tissue weighting factor WT, which is specific for tissue type T being irradiated. This is used to calculate the effective dose E.
When a whole body is irradiated uniformly only factor WR is used, but if the irradiation of a body is partial or non-uniform the factor WT is also used to calculate dose to each organ or tissue. These are then summed. The process is shown on the accompanying diagram. This approach calculates the biological risk contribution to the whole body, taking into account complete or partial irradiation, and the radiation type or types. The values of these weighting factors are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types, based on averages of those obtained for the human population.
Radiation type weighting factor WR
As the sievert has a biological effects component which depends on the radiation type, a corrective experimentally derived Weighting factor Wr, must be applied to convert the physical unit of the gray into the equivalent dose.
|Radiation||Energy||WR (formerly Q)|
|x-rays, gamma rays,
beta particles, muons
|neutrons||< 1 MeV||2.5 + 18.2·e-[ln(E)]²/6|
|1 MeV - 50 MeV||5.0 + 17.0·e-[ln(2·E)]²/6|
|> 50 MeV||2.5 + 3.25·e-[ln(0.04·E)]²/6|
|protons, charged pions||2|
Nuclear fission products,
The equivalent dose is calculated by multiplying the absorbed energy, averaged by mass over an organ or tissue of interest, by a radiation weighting factor appropriate to the type and energy of radiation. To obtain the equivalent dose for a mix of radiation types and energies, a sum is taken over all types of radiation energy dose.
- HT is the equivalent dose absorbed by tissue T
- DT,R is the absorbed dose in tissue T by radiation type R
- WR is the radiation weighting factor defined by regulation
Thus for example, an absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv.
This may appear to lead to a paradox, as this would suggest that the energy of the incident radiation field in joules has increased by a factor of 20, thereby violating the laws of Conservation of energy. However this is not the case, the sievert is used only to convey the fact that the biological effect of absorbing a gray of alpha particles would result in a 20 fold increase in the amount of biological effects that one would observe by absorbing a gray of x-rays. It is this biological component that is being expressed when using sieverts rather than the actual physical energy delivered by the incident absorbed radiation.
Tissue type weighting factor WT
The second weighting factor is the tissue factor WT, but is used only if there is has been non-uniform irradiation of a body. If the body has been subject to uniform irradiation, the effective doses summate to the equivalent dose, and only the Weighting factor WR is used. But if there is partial or non-uniform irradiation the calculation must take account of the organs irradiated. The ICRP values for WT are given in the table shown here.
|Organs||Tissue weighting factors|
|Red Bone Marrow||0.12||0.12||0.12|
|Remainder of body||0.30||0.05||0.12|
The article on effective dose gives the method of calculation. The absorbed dose is first corrected for the radiation type to give the equivalent dose, and then corrected for the tissue receiving the radiation. Some tissues like bone marrow are particularly sensitive to radiation, so they are given a weighting factor that is disproportionally large relative to the fraction of body mass they represent. Other tissues like the hard bone surface are particularly insensitive to radiation and are assigned a disproportionally low weighting factor.
The sum of effective doses to each organ or tissue of the body add up to the whole-body effective dose for the body. If only part of the body is radiated, then only the regions radiated are used to add up an effective dose which can be compared to organisms that receive an equivalent dose uniformly over its whole body.
The operational quantities provide the means of setting regulatory and guidance levels. Three external operational dose quantities had to be devised to relate real-life operational instrument measurements and responses to the calculated protection quantities. Also devised were two phantoms, The ICRU "slab" and "sphere" phantoms which relate these quantities to incident radiation quantities.
Ambient dose equivalent
This is used for area monitoring of penetrating radiation and is usually expressed as the quantity H*(10). This means the radiation is equivalent to that found 10 mm within the ICRU sphere phantom in the direction of origin of the field.
Directional dose equivalent
This is used for area monitoring of low penetrating radiation and is usually expressed as the quantity H'(0.07). This means the radiation is equivalent to that found at a depth of 0.07mm in the ICRU sphere phantom.
Personal dose equivalent
This is used for individual monitoring, such as with a personal dosimeter worn on the body. The recommended depth for assessment is 10 mm which gives the quantity Hp(10). In radiological protection practice value of omega is usually not specified as the dose is usually at a maximum at the point of interest.
Ionizing radiation has deterministic and stochastic effects on human health. Deterministic (acute tissue effect) events happen with certainty, with the resulting health conditions occurring in every individual who received the same high dose. Stochastic (cancer induction and genetic) events are inherently random, with most individuals in a group, failing to ever exhibit any causal negative health effects after exposure, while an indeterministic random minority do, often with the resulting subtle negative health effects being observable only after massive detailed epidemiology studies.
The use of the sievert implies that only stochastic effects are being considered, and to avoid confusion deterministic effects are conventionally compared to values of absorbed dose expressed by the SI unit gray (Gy).
Stochastic effects are those that occur randomly, such as radiation-induced cancer. The consensus of nuclear regulators, the nuclear industry, governments, some Academy of Sciences and the UNSCEAR, is that the incidence of cancers due to ionizing radiation can be modeled as increasing linearly with effective dose at a rate of 5.5% per sievert. Individual studies, alternative models, and earlier versions of the industry consensus have produced other risk estimates scattered around this consensus model. There is general agreement that the risk is much higher for infants and fetuses than adults, higher for the middle-aged than for seniors, and higher for women than for men, though there is no quantitative consensus about this.
The deterministic (acute tissue damage) effects that can lead to acute radiation syndrome only occur in the case of high doses (> ~0.1 Gy) and high dose rates (> ~0.1 Gy/h) and are conventionally not measured using the unit sievert. A model of deterministic risk would require different weighting factors (not yet established) than are used in the calculation of equivalent and effective dose. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report of 2012 states that no discernible acute tissue effects of exposures below 0.1 Sv appear to exist, which is compatible with known cellular-repair mechanisms.
The International Commission on Radiological Protection recommends limiting artificial irradiation of the public to an average of 1 mSv (0.001 Sv) of effective dose per year, not including medical and occupational exposures. For comparison, radiation levels inside the US capitol building are such that a human body would receive an additional dose rate of 0.85 mSv/a, close to the regulatory limit, because of the uranium content of the granite structure. According to the conservative ICRP model, someone who spent 20 years inside the capitol building would have an extra one in a thousand chance of getting cancer, over and above any other existing risk. (20 a·0.85 mSv/a·0.001 Sv/mSv·5.5%/Sv = ~0.1%) That "existing risk" is much higher; an average American would have a 10% chance of getting cancer during this same 20 year period, even without any exposure to artificial radiation. See natural Epidemiology of cancer and cancer rates. These estimates are, however, unmindful of every living cell's natural repair mechanisms, evolved over a few billion years of exposure to environmental chemical and radiation threats that were higher in the past and exaggerated by the evolution of oxygen metabolism—a challenging tradeoff made by life.
Significant radiation doses are not frequently encountered in everyday life, the following examples can help illustrate relative magnitudes. These are meant to be examples only, not a comprehensive list of possible radiation doses. An "acute dose" is one that occurs over a short and finite period of time, while a "chronic dose" is a dose that continues for an extended period of time so that it is better described by a dose rate.
|0.098||μSv:||banana equivalent dose, a whimsical unit of radiation dose representing the measure of radiation from a typical banana[note 1]|
|0.25||μSv:||U.S. limit on effective dose from a single airport security screening|
|5 to 10||μSv:||one set of dental radiographs|
|80||μSv:||average dose to people living within 16 km of Three Mile Island accident|
|0.4 to 0.6||mSv:||two-view mammogram, using weighting factors updated in 2007|
|1||mSv:||The U.S. 10 CFR § 20.1301(a)(1) dose limit for individual members of the public, total effective dose equivalent, per annum|
|1.5 to 1.7||mSv:||annual dose for flight attendants|
|2 to 7||mSv:||barium fluoroscopy, e.g. Barium meal, up to 2 minutes, 4–24 spot images|
|10 to 30||mSv:||single full-body CT scan|
|50||mSv:||The U.S. 10 C.F.R. § 20.1201(a)(1)(i) occupational dose limit, total effective dose equivalent, per annum|
|68||mSv:||estimated maximum dose to evacuees who lived closest to the Fukushima I nuclear accidents|
|0.50||Sv:||The U.S. 10 C.F.R. § 20.1201(a)(2)(ii) occupational dose limit, shallow-dose equivalent to skin, per annum|
|0.67||Sv:||highest dose received by a worker responding to the Fukushima emergency[note 1]|
|4.5 to 6||Sv:||fatal acute doses during Goiânia accident|
|5.1||Sv:||fatal acute dose to Harry Daghlian in 1945 criticality accident|
|21||Sv:||fatal acute dose to Louis Slotin in 1946 criticality accident|
|64||Sv:||nonfatal dose to Albert Stevens spread over ~21 years, due to a 1945 plutonium injection experiment by doctors working on the secret Manhattan Project.[note 1]|
Dose rate examples
All conversions between hours and years have assumed continuous presence in a steady field, disregarding known fluctuations, intermittent exposure and radioactive decay. Converted values are shown in parentheses.
|<1||mSv/a||<0.1||μSv/h||Steady dose rates below 0.1 µSv/h are difficult to measure.|
|1||mSv/a||(0.1||μSv/h avg)||ICRP recommended maximum for external irradiation of the human body, excluding medical and occupational exposures.|
|2.4||mSv/a||(0.27||μSv/h avg)||Human exposure to natural background radiation, global average[note 1]|
|24||mSv/a||(2.7||μSv/h avg)||Natural background radiation at airline cruise altitude[note 2]|
|0.13||Sv/a||(15||μSv/h avg)||Ambient field inside most radioactive house in Ramsar, Iran[note 3]|
|(0.8||Sv/a)||0.09||mSv/h||Natural radiation on a monazite beach near Guarapari, Brazil.|
|(9||Sv/a)||1||mSv/h||NRC definition of a high radiation area in a nuclear power plant, warranting a chain-link fence|
|(0.24||kSv/a)||27||mSv/h||close proximity to a 100 W radioisotope thermal generator|
|(1.7||kSv/a)||0.19||Sv/h||Highest reading from fallout of the Trinity bomb, 32 km away, 3 hours after detonation.[note 3]|
|(>90||kSv/a)||>10||Sv/h||most radioactive hotspot found in Fukushima I in areas normally accessible to workers[note 3]|
|(2.3||MSv/a)||270||Sv/h||typical PWR spent fuel bundle, after 10 year cooldown, no shielding|
|(90||MGy/a)||10||kGy/h||Immediate predicted activation of reactor wall in possible future fusion reactors. After 100 years of decay, typical levels would be 2–20 mSv/h. After approximately 300 years of decay the fusion waste would produce the same dose rate as exposure to coal ash, with the volume of fusion waste naturally being orders of magnitude less than from coal ash.|
Notes on examples:
- Noted figures are dominated by a committed dose which gradually turned into effective dose over an extended period of time. Therefore the true acute dose must be lower, but standard dosimetry practice is to account committed doses as acute in the year the radioisotopes are taken into the body.
- The dose rate received by air crews is highly dependent on the radiation weighting factors chosen for protons and neutrons, which have changed over time and remain controversial.
- Noted figures exclude any committed dose from radioisotopes taken into the body. Therefore the total radiation dose would be higher unless respiratory protection was used.
The sievert has its origin in the roentgen equivalent man (rem) which was derived from CGS units. The International Commission on Radiation Units and Measurements (ICRU) promoted a switch to coherent SI units in the 1970s, and announced in 1976 that it planned to formulate a suitable unit for equivalent dose. The ICRP pre-empted the ICRU by introducing the sievert in 1977.
The sievert was adopted by the International Committee for Weights and Measures (CIPM) in 1980, five years after adopting the gray. The CIPM then issued an explanation in 1984, recommending when the sievert should be used as opposed to the gray. That explanation was updated in 2002 to bring it closer to the ICRP's definition of equivalent dose, which had changed in 1990. Specifically, the ICRP had renamed the dose equivalent to equivalent dose, renamed the quality factor (Q) to radiation weighting factor (WR), and dropped another weighting factor 'N' in 1990. In 2002, the CIPM similarly dropped the weighting factor 'N' from their explanation but otherwise kept the old terminology and symbols. This explanation only appears in the appendix to the SI brochure and is not part of the definition of the sievert.
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). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (sievert), except in a situation where any word in that position 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.
Frequently used SI multiples are the millisievert (1 mSv = 0.001 Sv) and microsievert (1 μSv = 0.000001 Sv). The conventional units for its time derivative is mSv/h. Regulatory limits and chronic doses are often given in units of mSv/a or Sv/a, where they are understood to represent an average over the entire year. In many occupational scenarios, the hourly dose rate might fluctuate to levels thousands of times higher for a brief period of time, without infringing on the annual limits. The conversion from hours to years varies because of leap years and exposure schedules, but approximate conversions are:
- 1 mSv/h = 8.766 Sv/a
- 114.1 μSv/h = 1 Sv/a
Conversion from hourly rates to annual rates is further complicated by seasonal fluctuations in natural radiation, decay of artificial sources, and intermittent proximity between humans and sources. The ICRP once adopted fixed conversion for occupational exposure, although these have not appeared in recent documents:
- 8 h = 1 day
- 40 h = 1 week
- 50 weeks = 1 year
Therefore, for occupation exposures of that time period,
- 1 mSv/h = 2 Sv/a
- 500 µSv/h = 1 Sv/a
|100.0000 rem||=||100,000.0 mrem||=||1 Sv||=||1.000000 Sv||=||1000.000 mSv||=||1,000,000 µSv|
|1.0000 rem||=||1000.0 mrem||=||1 rem||=||0.010000 Sv||=||10.000 mSv||=||10000 µSv|
|0.1000 rem||=||100.0 mrem||=||1 mSv||=||0.001000 Sv||=||1.000 mSv||=||1000 µSv|
|0.0010 rem||=||1.0 mrem||=||1 mrem||=||0.000010 Sv||=||0.010 mSv||=||10 µSv|
|0.0001 rem||=||0.1 mrem||=||1 µSv||=||0.000001 Sv||=||0.001 mSv||=||1 µSv|
The following table shows radiation quantities in SI and non-SI units.
|Exposure (X)||röntgen||R||esu / 0.001293 g of air||1928||non-SI|
|Absorbed dose (D)||erg•g−1||1950||non-SI|
|Activity (A)||curie||c||3.7 × 1010 s−1||1953||non-SI|
|Dose equivalent (H)||röntgen equivalent man||rem||100 erg•g−1||1971||non-SI|
|Fluence (Φ)||(reciprocal area)||cm−2 or m−2||1962||SI (m−2)|
Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units, the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985.
- Background radiation
- Becquerel (disintegrations per second)
- Computational human phantom a means of modelling radiation protection quantities
- Counts per minute
- Linear Energy Transfer
- Orders of magnitude (radiation) – Example exposure scenarios
- Radiation poisoning
- Relative Biological Effectiveness
- Rutherford (unit)
- Sverdrup (a non-SI unit of volume transport with the same symbol Sv as sievert)
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