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Roentgen equivalent man

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roentgen equivalent man
Unit systemCGS units
Unit ofHealth effect of ionizing radiation
Named afterroentgen
1 rem in ...... is equal to ...
   SI base units   m2s−2
   SI derived unit   0.01 Sv

The roentgen equivalent man (rem)[1][2] is a CGS unit of equivalent dose, effective dose, and committed dose, which are dose measures used to estimate potential health effects of low levels of ionizing radiation on the human body.

Quantities measured in rem are designed to represent the stochastic biological risk of ionizing radiation, which is primarily radiation-induced cancer. These quantities are derived from absorbed dose, which in the CGS system has the unit rad. There is no universally applicable conversion constant from rad to rem; the conversion depends on relative biological effectiveness (RBE).

The rem has been defined since 1976 as equal to 0.01 sievert, which is the more commonly used SI unit outside the United States. Earlier definitions going back to 1945 were derived from the roentgen unit, which was named after Wilhelm Röntgen, a German scientist who discovered X-rays. The unit name is misleading, since 1 roentgen actually deposits about 0.96 rem in soft biological tissue, when all weighting factors equal unity. Older units of rem following other definitions are up to 17% smaller than the modern rem.

Doses greater than 100 rem received over a short time period are likely to cause acute radiation syndrome (ARS), possibly leading to death within weeks if left untreated. Note that the quantities that are measured in rem were not designed to be correlated to ARS symptoms. The absorbed dose, measured in rad, is a better indicator of ARS.[3]: 592–593 

A rem is a large dose of radiation, so the millirem (mrem), which is one thousandth of a rem, is often used for the dosages commonly encountered, such as the amount of radiation received from medical x-rays and background sources.



The rem and millirem are CGS units in widest use among the U.S. public, industry, and government.[4] However, the SI unit the sievert (Sv) is the normal unit outside the United States, and is increasingly encountered within the US in academic, scientific, and engineering environments, and have now virtually replaced the rem.[5]

The conventional units for dose rate is mrem/h. Regulatory limits and chronic doses are often given in units of mrem/yr or rem/yr, where they are understood to represent the total amount of radiation allowed (or received) 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 total exposure limits. The annual conversions to a Julian year are:

1 mrem/h = 8,766 mrem/yr
0.1141 mrem/h = 1,000 mrem/yr

The International Commission on Radiological Protection (ICRP) once adopted fixed conversion for occupational exposure, although these have not appeared in recent documents:[6]

8 h = 1 day
40 h = 1 week
50 week = 1 yr

Therefore, for occupation exposures of that time period,

1 mrem/h = 2,000 mrem/yr
0.5 mrem/h = 1,000 mrem/yr

The U.S. National Institute of Standards and Technology (NIST) strongly discourages Americans from expressing doses in rem, in favor of recommending the SI unit.[7] The NIST recommends defining the rem in relation to the SI in every document where this unit is used.[8]

Health effects


Ionizing radiation has deterministic and stochastic effects on human health. The deterministic effects that can lead to acute radiation syndrome only occur in the case of high doses (> ~10 rad or > 0.1 Gy) and high dose rates (> ~10 rad/h or > 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 rad, not rem.[9]

Stochastic effects are those that occur randomly, such as radiation-induced cancer. The consensus of the nuclear industry, nuclear regulators, and governments, is that the incidence of cancers caused by ionizing radiation can be modeled as increasing linearly with effective dose at a rate of 0.055% per rem (5.5%/Sv).[10] 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.[11][12] There is much less data, and much more controversy, regarding the possibility of cardiac and teratogenic effects, and the modelling of internal dose.[13]

The ICRP recommends limiting artificial irradiation of the public to an average of 100 mrem (1 mSv) of effective dose per year, not including medical and occupational exposures.[10] For comparison, radiation levels inside the United States Capitol are 85 mrem/yr (0.85 mSv/yr), close to the regulatory limit, because of the uranium content of the granite structure.[14] The NRC sets the annual total effective dose of full body radiation, or total body radiation (TBR), allowed for radiation workers 5,000 mrem (5 rem).[15][16]



The concept of the rem first appeared in literature in 1945[17] and was given its first definition in 1947.[18] The definition was refined in 1950 as "that dose of any ionizing radiation which produces a relevant biological effect equal to that produced by one roentgen of high-voltage x-radiation."[19] Using data available at the time, the rem was variously evaluated as 83, 93, or 95 erg/gram.[20] Along with the introduction of the rad in 1953, the ICRP decided to continue the use of the rem. The US National Committee on Radiation Protection and Measurements noted in 1954 that this effectively implied an increase in the magnitude of the rem to match the rad (100 erg/gram).[21] The ICRP introduced and then officially adopted the rem in 1962 as the unit of equivalent dose to measure the way different types of radiation distribute energy in tissue and began recommending values of relative biological effectiveness (RBE) for various types of radiation.[22] In practice, the unit of rem was used to denote that an RBE factor had been applied to a number which was originally in units of rad or roentgen.

The International Committee for Weights and Measures (CIPM) adopted the sievert in 1980 but never accepted the use of the rem. The NIST recognizes that this unit is outside the SI but temporarily accepts its use in the U.S. with the SI.[8] The rem remains in widespread use as an industry standard in the U.S.[23] The United States Nuclear Regulatory Commission still permits the use of the units curie, rad, and rem alongside SI units.[24]


The following table shows radiation quantities in SI and non-SI units:

Ionizing radiation related quantities
Quantity Unit Symbol Derivation Year SI equivalent
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7×1010 s−1 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1000000 Bq
Exposure (X) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit
röntgen R esu / 0.001293 g of air 1928 2.58×10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−1 1974 SI unit
erg per gram erg/g erg⋅g−1 1950 1.0×10−4 Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
Equivalent dose (H) sievert Sv J⋅kg−1 × WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 × WR 1971 0.010 Sv
Effective dose (E) sievert Sv J⋅kg−1 × WR × WT 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 × WR × WT 1971 0.010 Sv

See also



  1. ^ "RADInfo Glossary of Radiation Terms". EPA.gov. United States Environmental Protection Agency. 31 August 2015. Retrieved 18 December 2016.
  2. ^ Morris, Jim; Hopkins, Jamie Smith (11 December 2015), "The First Line of Defense", Slate, retrieved 18 December 2016
  3. ^ The Effects of Nuclear Weapons, Revised ed., US DOD 1962
  4. ^ Office of Air and Radiation; Office of Radiation and Indoor Air (May 2007). "Radiation: Risks and Realities". U.S. Environmental Protection Agency. p. 2. Retrieved 23 May 2012. In the United States, we measure radiation doses in units called rem. Under the metric system, dose is measured in units called sieverts. One sievert is equal to 100 rem.
  5. ^ Pradhan, A. S. (2007). "Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations". Journal of Medical Physics. 32 (3): 89–91. doi:10.4103/0971-6203.35719. ISSN 0971-6203. PMC 3000504. PMID 21157526.
  6. ^ Recommendations of the International Commission on Radiological Protection and of the International Commission on Radiological Units (PDF). National Bureau of Standards Handbook. Vol. 47. US Department of Commerce. 1950. Retrieved 14 November 2012.
  7. ^ Thompson, Ambler; Taylor, Barry N. (2008). Guide for the Use of the International System of Units (SI) (2008 ed.). Gaithersburg, MD: National Institute of Standards and Technology. p. 10. SP811. Archived from the original on 16 May 2008. Retrieved 28 November 2012.
  8. ^ a b Hebner, Robert E. (28 July 1998). "Metric System of Measurement: Interpretation of the International System of Units for the United States" (PDF). Federal Register. 63 (144): 40339. Retrieved 9 May 2012.
  9. ^ "§ 20.1004 Units of radiation dose". NRC Web. Retrieved 29 January 2024.
  10. ^ a b Icrp (2007). The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. Vol. 37. ISBN 978-0-7020-3048-2. Retrieved 17 May 2012. {{cite book}}: |journal= ignored (help)
  11. ^ Peck, Donald J.; Samei, Ehsan. "How to Understand and Communicate Radiation Risk". Image Wisely. Retrieved 18 May 2012.
  12. ^ United Nations Scientific Committee on the Effects of Atomic Radiation (2008). Effects of ionizing radiation : UNSCEAR 2006 report to the General Assembly, with scientific annexes. New York: United Nations. ISBN 978-92-1-142263-4. Retrieved 18 May 2012.
  13. ^ European Committee on Radiation Risk (2010). Busby, Chris; et al. (eds.). 2010 recommendations of the ECRR : the health effects of exposure to low doses of ionizing radiation (PDF) (Regulators' ed.). Aberystwyth: Green Audit. ISBN 978-1-897761-16-8. Archived from the original (PDF) on 21 July 2012. Retrieved 18 May 2012.
  14. ^ Formerly Utilized Sites Remedial Action Program. "Radiation in the Environment". US Army Corps of Engineers. Retrieved 10 September 2017.
  15. ^ "Information for Radiation Workers". NRC Web. Retrieved 29 January 2024.
  16. ^ "Total Body Irradiation » Radiation Oncology » College of Medicine » University of Florida". Retrieved 29 January 2024.
  17. ^ Cantrill, S.T; H.M. Parker (5 January 1945). "The Tolerance Dose". Argonne National Laboratory: US Atomic Energy Commission. Archived from the original on 30 November 2012. Retrieved 14 May 2012.
  18. ^ Nucleonics. 1 (2). 1947. {{cite journal}}: Missing or empty |title= (help)
  19. ^ Parker, H.M. (1950). "Tentative Dose Units for Mixed Radiations". Radiology. 54 (2): 257–262. doi:10.1148/54.2.257. PMID 15403708.
  20. ^ Anderson, Elda E. (March 1952). "Units of Radiation and Radioactivity". Public Health Reports. 67 (3): 293–297. doi:10.2307/4588064. JSTOR 4588064. PMC 2030726. PMID 14900367.
  21. ^ Permissible Doses from External Sources of Radiation (PDF). National Bureau of Standards Handbook. Vol. 59. US Department of Commerce. 24 September 1954. p. 31. Retrieved 14 November 2012.
  22. ^ Pradhan, A. S. (2007). "Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations". Journal of Medical Physics. 32 (3): 89–91. doi:10.4103/0971-6203.35719. ISSN 0971-6203. PMC 3000504. PMID 21157526.
  23. ^ Handbook of Radiation Effects, 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams
  24. ^ 10 CFR 20.1003. US Nuclear Regulatory Commission. 2009.