Radiation unit
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 Rontgen
, 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.
Usage
[
edit
]
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
[
edit
]
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]
History
[
edit
]
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]
Radiation-related quantities
[
edit
]
The following table shows radiation quantities in SI and non-SI units:
See also
[
edit
]
References
[
edit
]
- ^
"RADInfo Glossary of Radiation Terms"
.
EPA.gov
. United States Environmental Protection Agency. 31 August 2015
. Retrieved
18 December
2016
.
- ^
Morris, Jim; Hopkins, Jamie Smith (11 December 2015),
"The First Line of Defense"
,
Slate
, retrieved
18 December
2016
- ^
The Effects of Nuclear Weapons
, Revised ed., US DOD 1962
- ^
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.
- ^
Pradhan, A. S. (2007).
"Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations"
.
Journal of Medical Physics / Association of Medical Physicists of India
.
32
(3): 89?91.
doi
:
10.4103/0971-6203.35719
.
ISSN
0971-6203
.
PMC
3000504
.
PMID
21157526
.
- ^
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
.
- ^
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
.
- ^
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
.
- ^
"§ 20.1004 Units of radiation dose"
.
NRC Web
. Retrieved
29 January
2024
.
- ^
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
.
- ^
Peck, Donald J.; Samei, Ehsan.
"How to Understand and Communicate Radiation Risk"
. Image Wisely
. Retrieved
18 May
2012
.
- ^
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
.
- ^
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
.
- ^
Formerly Utilized Sites Remedial Action Program.
"Radiation in the Environment"
. US Army Corps of Engineers
. Retrieved
10 September
2017
.
- ^
"Information for Radiation Workers"
.
NRC Web
. Retrieved
29 January
2024
.
- ^
"Total Body Irradiation ≫ Radiation Oncology ≫ College of Medicine ≫ University of Florida"
. Retrieved
29 January
2024
.
- ^
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
.
- ^
Nucleonics
.
1
(2). 1947.
- ^
Parker, H.M. (1950). "Tentative Dose Units for Mixed Radiations".
Radiology
.
54
(2): 257?262.
doi
:
10.1148/54.2.257
.
PMID
15403708
.
- ^
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
.
- ^
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
.
- ^
Pradhan, A. S. (2007).
"Evolution of dosimetric quantities of International Commission on Radiological Protection (ICRP): Impact of the forthcoming recommendations"
.
Journal of Medical Physics / Association of Medical Physicists of India
.
32
(3): 89?91.
doi
:
10.4103/0971-6203.35719
.
ISSN
0971-6203
.
PMC
3000504
.
PMID
21157526
.
- ^
Handbook of Radiation Effects
, 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams
- ^
10 CFR 20.1003
. US Nuclear Regulatory Commission. 2009.
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Main articles
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Measurement
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Instruments and
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Protection techniques
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Organisations
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Regulation
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Radiation effects
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