Roentgen equivalent man

Roentgen equivalent man is an old unit that is used for measuring exposure to radiation. Usually it is abbreviated as "rem".In the early 1980s it was replaced by the Sievert. 

1 Sievert is equal to 100 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.The Effects of Nuclear Weapons, Revised ed., US DOD 1962

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.
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