Millisievert

Millisievert

For other uses, see Sievert (disambiguation).

The sievert (symbol: Sv) is the International System of Units' (SI) derived unit of equivalent radiation dose, effective dose, and committed dose. Quantities that are measured in sieverts are designed to represent the stochastic biological effects of ionizing radiation. The sievert should not be used to express the unmodified absorbed dose of radiation energy, which is a clear physical quantity measured in grays. To enable consideration of biological effects, further calculations must be performed to convert absorbed dose into effective dose, the details of which depend on the biological context. This can be far more complicated than just multiplying by a weighting factor.

The sievert is of fundamental importance in radiation dosimetry, 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 equals 100 rem, an older unit of measurement still in widespread use. One sievert carries with it a 5.5% chance of eventually developing cancer.[1] Doses greater than 1 sievert received over a short time period are likely to cause radiation poisoning, possibly leading to death within weeks.

Definition

The gray and sievert units are both based on the measurement of the concentration of ionizing radiation absorbed per unit of a material's mass. Comparable to units of specific energy, the two SI units are expressed in terms of joules of absorbed ionizing energy per kilogram. However despite this shared commonality the gray and the sievert are only interchangeable for certain kinds of radiation.[2]

1 Gy = 1 J/kg
1 Sv = 1 J/kg equivalent

The gray is used to express the physical quantity of absorbed dose, that is, the quantity of energy absorbed per unit mass, in any material, while the sievert is used to express the biological equivalent dose to human tissue.

The sievert is used to compare the biological effects of the different forms of ionizing radiation. It is used when discussing equivalent dose (the external-source, whole-body exposure effects, in a uniform field), effective dose (which depends on the body parts targeted), and committed dose (whole body effects of ingested or inhaled radioactive material). The latter dose quantities are weighted averages of absorbed dose designed to be more representative of the stochastic health effects of radiation, and use of the sievert implies that appropriate weighting factors have been applied to the original absorbed dose measurement (in grays).[1]

As the sievert has a biological effects component, a corrective experimentally derived Weighting factor must be applied to convert the physical unit of the gray into the biological effects unit that is the sievert.

To bypass the complexity of tissue dependence in effective dose, the International Commission on Radiological Protection (ICRP) defined standard radiation weighting factors, independently of tissue type, to be used for risk and exposure assessment in radiology and the nuclear industry.[3] These values are conservatively chosen to be greater than the bulk of experimental values observed for the most sensitive cell types.

Weighting factors WR (formerly termed Q factor)
used to calculate equivalent dose
according to the 1992 ICRP report.[3]
Radiation Energy wR (also Q)
x-rays, gamma rays,
beta rays, muons
  1
neutrons <10 keV 5
10–100 keV 10
100 keV – 2 MeV 20
2–20 MeV 10
>20 MeV 5
protons, charged pions >2 MeV 2
alpha particles, nuclear fission products,
heavy nuclei
  20

Thus, for example, an absorbed dose of 100 mGy of x-rays gives an equivalent dose of 100 mSv, and similarly, a separate absorbed dose of 100 mGy of alpha radiation gives an equivalent dose of 2000 mSv, as alpha particles have a relative biological effectiveness value of 20. This may appear to lead to a paradox, if one looks back at the physical definition of the sievert, as this would suggest that the energy of the incident radiation field in joules has increased by a factor of 20 in this example when sieverts are used, 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. An equivalent dose of radiation is estimated to have the same biological effect as following an equal amount of absorbed dose of gamma rays.

Confusion can still be caused as there are two different radiation quantities that can both be measured in the same units of J/kg; the sievert and the gray. The sievert and the gray are different names for the same unit of delivery, but they are used in different contexts.[2]

If the equivalent dose is uniform throughout the organism, that is, if the radiation source equally impacts on the entire surface of the body, it will be equal to the body tissue specific effective dose. Otherwise, a weighted average will have to be taken to average out the radiation dose through the body while correcting for the different sensitivities of different tissues. See the article on effective dose for this calculation.

The internal or committed dose in radiation protection and medical radiology is a measure of the stochastic (i.e., probabilistic) health effect an individual is to expect due to the intake of radioactive material into their body. Also measured in sieverts, an internal dose from an internally located radioisotope is intended to carry the same effective probability of long term risk as an equivalent dose applied uniformly to the whole body from an external source, or the same amount of effective dose applied to part of the body.

SI units

This 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:[4]

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

An older unit for the equivalent dose is the rem,[5] still often used in the United States. One sievert is equal to 100 rem:

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

Health effects

Ionizing radiation has deterministic and stochastic effects on human health. Deterministic events happen with certainty, with the resulting health conditions occurring in every individual who received the same high dose. Stochastic 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 to reduce study noise by increasing statistical power.

The deterministic 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). A model of deterministic risk would require different weighting factors (not yet established) than are used in the calculation of equivalent and effective dose. To avoid confusion, deterministic effects are normally compared to absorbed dose in units of Gy, not Sv. See here for a discussion on the failure of finding deterministic risks below a Lethal Dose of 5 LD50.

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.[1] Individual studies, alternate 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.[6][7] However, the UNSCEAR report of 2012 states that no discernible effects of exposures below 0.1 Sv appear to exist, which is compatible with known cellular-repair mechanisms.[8]

According to Christopher Busby, at low doses, and dose rates, there is much less data, and much more controversy, regarding the possibility of cardiac and teratogenic effects, and the modelling of internal dose.[9] See the linear no-threshold hypothesis—however, this hypothesis was never based on true "low dose" studies[10] and Busby was discredited by the British Health Service for data falsifications and exploitation of Japanese parents after Fukushima emissions (2011).[11]

The International Commission on Radiological Protection (ICRP) 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.[1] For comparison, radiation levels inside the US capitol building are 0.85 mSv/a, close to the regulatory limit, because of the uranium content of the granite structure.[12] 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.

In 2012 the United Nations Scientific Committee on the Effects of Atomic Radiation stated that for typical background radiation levels (1–13 mSv per year) it is not possible to account for any health effects, and for exposures under 100 mSv, it is only possible in specific conditions.[13] See Lowest-observed-adverse-effect level and linear no-threshold model hypothesis.

Dose examples

Since 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.

Dose examples

0.098 μSv: banana equivalent dose, a whimsical unit of radiation dose[14][note 1]
0.25 μSv: U.S. limit on effective dose from a single airport security screening[15]
5 to 10 μSv: one set of dental radiographs[16]
80 μSv: average dose to people living within 16 km of Three Mile Island accident[17]
0.4 to 0.6 mSv: two-view mammogram, using weighting factors updated in 2007[18]
1.5 to 1.7 mSv: annual dose for flight attendants[19]
2 to 7 mSv: barium fluoroscopy, e.g. Barium meal, up to 2 minutes, 4–24 spot images[20]
10 to 30 mSv: single full-body CT scan[21][22]
68 mSv: estimated maximum dose to evacuees who lived closest to the Fukushima I nuclear accidents[23]
0.67 Sv: highest dose received by a worker responding to the Fukushima emergency[24][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[25]
21 Sv: fatal acute dose to Louis Slotin in 1946 criticality accident[26]
64 Sv: nonfatal dose to Albert Stevens spread over ~21 years, due to 1945 human experiment[27][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[28][note 2]
0.13 Sv/a (15 μSv/h avg) Ambient field inside most radioactive house in Ramsar, Iran[29][note 3]
(0.8 Sv/a) 0.09 mSv/h Natural radiation on a monazite beach near Guarapari, Brazil.[30]
(9 Sv/a) 1 mSv/h NRC definition of a high radiation area in a nuclear power plant, warranting a chain-link fence[31]
(0.24 kSv/a) 27 mSv/h close proximity to a 100 W radioisotope thermal generator[32]
(1.7 kSv/a) 0.19 Sv/h Highest reading from fallout of the Trinity bomb, 32 km away, 3 hours after detonation.[33][note 3]
(>90 kSv/a) >10 Sv/h most radioactive hotspot found in Fukushima I in areas normally accessible to workers[34][note 3]
(2.3 MSv/a) 270 Sv/h typical PWR spent fuel bundle, after 10 year cooldown, no shielding[35]
(90 MGy/a) 10 kGy/h immediate predicted activation of reactor wall in possible future fusion reactors.[36] After 100 years of decay, typical levels would be 2–20 mSv/h.[37] 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.[38]

Notes on examples:

History

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,[39] and announced in 1976 that it planned to formulate a suitable unit for equivalent dose.[40] The ICRP pre-empted the ICRU by introducing the sievert in 1977.[41]

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.[2]

See also

Notes

References

  • United Nations Scientific Community on the Effects of Atomic Radiation

External links