A unit of dose equivalent. Abbreviation, rem. Usually encountered as millirem, mrem. Originally defined by H. M. Parker as the quantity of radiation (of any mixture of types) that would produce the same biological damage in a human being as would result from absorption of 1 rep of X rays or gamma rays. It was subsequently redefined, as described below.

Some forms of radiation are more harmful than others. For example, a dose of 1 rad of alpha particles will do ten to twenty times as much damage as a dose of 1 rad of X rays, even though both exposures would deposit the same amount of energy in the tissue. To describe this difference, a factor called relative biological effectiveness (RBE) is used, which is the ratio of the size in rads of a dose of X rays to the size in rads of a dose of another type of radiation causing the same amount of damage. Thus the RBE of alpha particles is in the range 10 to 20 (depending on their energy, among other factors). RBE is determined experimentally.

Rems are then defined by dose in rems = dose in rads × RBE

The rem has been replaced by the sievert, 1 rem = ¹⁄₁₀₀ sievert, but the temporary continued use of the rem was sanctioned by the CIPM.

An example of the use of the rem are the exposure limits for workers set by the U.S. Occupational Safety and Health Administration (29 CFR 1910.96, last amended in 1993). The limits are given as the maximum permissible number of rems accumulated during a calendar quarter, for various parts of the body:

Permissible Accumulated Dose Exposed portion of body
1.25 Whole body, head and trunk, active blood forming organs, lenses of the eyes, gonads
7.5 Skin of whole body
18.75 Hands & forearms, feet & ankles

For an employee who is under the age of 18, the limits are 10% of those shown in the above table.

However, doses of up to 3 rems per calendar quarter are allowed, provided adequate records are kept and the dose to the whole body, added to the accumulated occupational exposure to the whole body, stated in rems, isn't more than the person's age minus 18, multiplied by 5.

The table above applies to chronic exposures to low levels of radiation. Exposure to a single powerful dose has different effects.

25—100 rems Probably no disabling illness.
100—200 rems Nausea, fatigue, vomiting over 125 rem. One percent reduction in life expectancy.
200—300 rems Probable recovery in 3 months.
300—600 rems Nausea, vomiting, diarrhea in first few hours. After a latent period of a week, loss of hair, malaise, fever, progressing to hemorrhage and emaciation in the third week. At about 500 rems, 50% die in 2 to 6 weeks.
more than 600 Almost all those exposed die within weeks.


roentgen equivalent, man, rem

The dose of any ionizing radiation that will produce the same biological effect as that produced by one roentgen of high voltage x-radiation. (A proposed unit; Parker)

National Research Council.
A Glossary of Terms in Nuclear Science and Technology.
New York: American Society of Mechanical Engineers, 1955.
Page 147.

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