Rems

Rems, short for roentgen equivalent man, are a unit of radiation dose equivalent used in radiological protection to express the potential biological effect of ionizing radiation on human tissue. The rem is part of the older system of units that also included the roentgen for exposure and the rad for absorbed dose. Dose equivalent H is obtained by multiplying the absorbed dose D by a radiation-weighting factor Q that accounts for radiation type and energy. The resulting value is expressed in rems.

In the International System of Units, dose equivalents are measured in sieverts (Sv), with 1 Sv equal to 100 rem. Therefore, 1 rem equals 0.01 Sv. The rem was designed to reflect biological risk rather than physical energy deposition, and its value varies with radiation type through the Q factor.

The rem system has largely been superseded by the sievert in international practice, and most modern regulatory guidance uses sieverts. However, rems may still appear in historical data, older regulations, or legacy documentation, and some U.S. occupational exposure records retain rem-based limits or equivalencies.

Note: Rem may be confused with REM, the acronym for rapid eye movement, in discussions about sleep. They are unrelated.