Editing Roentgen equivalent man

{{Short description|Radiation unit}}
{{About|the [[Centimetre–gram–second system of units|CGS]] unit of [[equivalent dose]]|the historical equivalent of modern [[kerma (physics)|kerma]]|roentgen (unit)}}{{For|the light metro system of Montreal|Réseau express métropolitain}}
{{Use dmy dates|date=April 2022}}
{{Infobox Unit
| bgcolour =
| name = roentgen equivalent man
| image = 
| caption = 
| standard = [[CGS]] units
| quantity = [[Equivalent dose|Health effect of ionizing radiation]]
| symbol = rem
| dimension = 
| namedafter = [[Roentgen (unit)|roentgen]]
| units1 = [[SI base unit]]s
| inunits1 = [[metre|m]]<sup>2</sup>⋅[[second|s]]<sup>−2</sup>
| units2 = [[SI derived unit]]
| inunits2 = 0.01 [[Sievert|Sv]]
}}
The '''roentgen equivalent man''' ('''rem''')<ref name=EPA2>{{cite web|title=RADInfo Glossary of Radiation Terms|publisher=United States Environmental Protection Agency|website=EPA.gov|url=https://www.epa.gov/enviro/radinfo-glossary-radiation-terms|access-date=18 December 2016|date=31 August 2015}}</ref><ref name=Slate>{{citation|date=11 December 2015|title=The First Line of Defense|first1=Jim|last1=Morris|first2=Jamie Smith|last2=Hopkins|work=Slate|url=http://www.slate.com/articles/news_and_politics/politics/2015/12/nuclear_weapons_plants_are_killing_the_people_who_once_guarded_them_a_center.html|access-date=18 December 2016}}</ref> is a [[Centimetre–gram–second system of units|CGS unit]] of [[equivalent dose]], [[effective dose (radiation)|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 (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&nbsp;[[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)|roentgen unit]], which was named after [[Wilhelm Röntgen]], a German scientist who discovered [[X-ray]]s. 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.<ref name="remdef">''The Effects of Nuclear Weapons'', Revised ed., US DOD 1962</ref>{{rp|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 radiation|background]] sources.

==Usage==
The rem and millirem are CGS units in widest use among the U.S. public, industry, and government.<ref name=EPA>{{cite web|title=Radiation: Risks and Realities|url=http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=000003JH.txt|publisher=U.S. Environmental Protection Agency|access-date=23 May 2012|author1=Office of Air and Radiation |author2=Office of Radiation and Indoor Air|page=2|date=May 2007|quote=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.}}</ref> 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.<ref>{{Cite journal |last=Pradhan |first=A. S. |date=2007 |title=Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations |journal=Journal of Medical Physics |volume=32 |issue=3 |pages=89–91 |doi=10.4103/0971-6203.35719 |doi-access=free |issn=0971-6203 |pmc=3000504 |pmid=21157526}}</ref>

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_(astronomy)|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:<ref>{{cite book|title=Recommendations of the International Commission on Radiological Protection and of the International Commission on Radiological Units|year=1950|publisher=US Department of Commerce|url=http://www.orau.org/ptp/Library/NBS/NBS%2047.pdf|access-date=14 November 2012|series=National Bureau of Standards Handbook|volume=47}}</ref>
: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.<ref name=NIST>{{cite book|last1=Thompson|first1=Ambler|last2=Taylor|first2=Barry N.|title=Guide for the Use of the International System of Units (SI)|year=2008|publisher=[[National Institute of Standards and Technology]]|location=Gaithersburg, MD|url=https://www.nist.gov/pml/pubs/sp811/index.cfm|edition=2008|access-date=28 November 2012|archive-url=https://web.archive.org/web/20080516081348/http://www.physics.nist.gov/Pubs/SP811/sec05.html|id=SP811|page=10|archive-date=16 May 2008|url-status=live}}</ref> The NIST recommends defining the rem in relation to the SI in every document where this unit is used.<ref name="fedreg63">{{cite journal|last=Hebner|first=Robert E.|title=Metric System of Measurement: Interpretation of the International System of Units for the United States|journal=Federal Register|date=1998-07-28|volume=63|issue=144|page=40339|url=http://www.gpo.gov/fdsys/pkg/FR-1998-07-28/pdf/98-16965.pdf|access-date=9 May 2012}}</ref>

== 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 (>&nbsp;~10&nbsp;rad or >&nbsp;0.1&nbsp;Gy) and high dose rates (>&nbsp;~10&nbsp;rad/h or >&nbsp;0.1&nbsp;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.<ref>{{Cite web |title=§ 20.1004 Units of radiation dose. |url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1004.html |access-date=2024-01-29 |website=NRC Web |language=en-US}}</ref>

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).<ref name="ICRP103">{{Cite book|title=The 2007 Recommendations of the International Commission on Radiological Protection|journal=Annals of the ICRP|year=2007|volume=37|series=ICRP publication 103|issue=2–4|url=http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103|access-date=17 May 2012|isbn=978-0-7020-3048-2|last1=Icrp}}</ref> 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.<ref name="peck">{{cite web |last1=Peck |first1=Donald J.|title=How to Understand and Communicate Radiation Risk |url=https://www.imagewisely.org/Imaging-Modalities/Computed-Tomography/How-to-Understand-and-Communicate-Radiation-Risk|publisher=Image Wisely |access-date=18 May 2012 |author2=Samei, Ehsan}}</ref><ref name="UNSCEAR">{{cite book|author=United Nations Scientific Committee on the Effects of Atomic Radiation|title=Effects of ionizing radiation : UNSCEAR 2006 report to the General Assembly, with scientific annexes|year=2008|publisher=United Nations|location=New York|isbn=978-92-1-142263-4|url=http://www.unscear.org/unscear/en/publications.html|access-date=18 May 2012}}</ref> There is much less data, and much more controversy, regarding the possibility of [[cardiac]] and [[teratogenic]] effects, and the modelling of [[Effective dose (radiation)|internal dose]].<ref name="ECRR2010">{{cite book|last=European Committee on Radiation Risk|title=2010 recommendations of the ECRR : the health effects of exposure to low doses of ionizing radiation|year=2010|publisher=Green Audit|location=Aberystwyth|isbn=978-1-897761-16-8|url=http://www.euradcom.org/2011/ecrr2010.pdf|edition=Regulators'|editor=Busby, Chris|access-date=18 May 2012|display-editors=etal|url-status=dead|archive-url=https://web.archive.org/web/20120721220541/http://www.euradcom.org/2011/ecrr2010.pdf|archive-date=21 July 2012}}</ref>

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.<ref name=ICRP103 /> 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.<ref name="FUSRAP" >{{cite web|last=Formerly Utilized Sites Remedial Action Program|title=Radiation in the Environment|url=http://www.fusrapmaywood.com/projmain.html|publisher=US Army Corps of Engineers|access-date=10 September 2017}}</ref> The [[Nuclear Regulatory Commission|NRC]] sets the [[Total effective dose equivalent|annual total effective dose]] of full body radiation, or total body radiation (TBR), allowed for radiation workers 5,000 mrem (5 rem).<ref>{{Cite web |title=Information for Radiation Workers |url=https://www.nrc.gov/about-nrc/radiation/health-effects/info.html |access-date=2024-01-29 |website=NRC Web |language=en-US}}</ref><ref>{{Cite web |title=Total Body Irradiation » Radiation Oncology » College of Medicine » University of Florida |url=https://radonc.med.ufl.edu/patient-care/information-for-patients/cancer-treatment-results/total-body-irradiation/ |access-date=2024-01-29 |language=en}}</ref>

==History==
The concept of the rem first appeared in literature in 1945<ref>{{cite web|last1=Cantrill|first1=S.T|author2=H.M. Parker|title=The Tolerance Dose|date=1945-01-05|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA322447|archive-url=https://web.archive.org/web/20121130121433/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA322447|url-status=dead|archive-date=30 November 2012|access-date=14 May 2012|publisher=US Atomic Energy Commission|location=Argonne National Laboratory}}</ref> and was given its first definition in 1947.<ref>{{cite journal|journal=Nucleonics|year=1947|volume=1|issue=2}}</ref> 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 (unit)|roentgen]] of high-voltage x-radiation."<ref>{{cite journal|last=Parker|first=H.M.|title=Tentative Dose Units for Mixed Radiations|journal=Radiology|year=1950|volume=54|issue=2|pages=257–262|doi=10.1148/54.2.257|pmid=15403708}}</ref> Using data available at the time, the rem was variously evaluated as 83, 93, or 95 [[erg]]/gram.<ref>{{cite journal|last=Anderson|first=Elda E.|title=Units of Radiation and Radioactivity|journal=Public Health Reports|date=March 1952|volume=67|issue=3|pages=293–297|doi=10.2307/4588064|pmc=2030726|pmid=14900367|jstor=4588064}}</ref> 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).<ref>{{cite book|title=Permissible Doses from External Sources of Radiation|publisher=US Department of Commerce|url=http://www.orau.org/ptp/Library/NBS/NBS%2059%20Addendum.pdf|access-date=14 November 2012|page=31|date=24 September 1954|series=National Bureau of Standards Handbook|volume=59}}</ref> 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.<ref>{{Cite journal |last=Pradhan |first=A. S. |date=2007 |title=Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations |journal=Journal of Medical Physics |volume=32 |issue=3 |pages=89–91 |doi=10.4103/0971-6203.35719 |doi-access=free |issn=0971-6203 |pmc=3000504 |pmid=21157526}}</ref> 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.<ref name="fedreg63" /> The rem remains in widespread use as an industry standard in the U.S.<ref>''Handbook of Radiation Effects'', 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams</ref> The United States [[Nuclear Regulatory Commission]] still permits the use of the units [[Curie (unit)|curie]], rad, and [[rem (unit)|rem]] alongside SI units.<ref>{{cite book|title=10 CFR 20.1003|year=2009|publisher=US Nuclear Regulatory Commission|url=https://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1004.html}}</ref>

==Radiation-related quantities==
The following table shows radiation quantities in SI and non-SI units:
{{Radiation related quantities}}

==See also==
*[[Roentgen equivalent physical]]
*[[Banana equivalent dose]]
*[[Health threat from cosmic rays]]
*[[Orders of magnitude (radiation)]]

==References==
{{Reflist}}
{{Radiation protection}}
{{Ionising radiation related quantities}}

{{DEFAULTSORT:Roentgen Equivalent Man}}
[[Category:Units of radiation dose]]
[[Category:Radiation health effects]]
[[Category:Radiobiology]]
[[Category:Non-SI metric units]]
[[Category:Wilhelm Röntgen|Equivalent]]
[[Category:Equivalent units]]