An acronym for "roentgen equivalent man", the rem is the absorbed dose of ionizing radiation that produces the same biological effect as 1 rad of X-rays or gamma rays. The rem was introduced as a result of the observation that some types of ionizing radiation, such as neutrons, produce a greater biological effect for the equivalent amount of absorbed energy than X-rays or gamma rays. In short, the rem is a measure of the biological effect of irradiation. For X-rays and gamma rays, the rem is equivalent to the rad. For other types of radiation, the number of rems equals the number of rads multiplied by a special factor (called the quality factor or relative biological effectiveness) that depends on the type of radiation involved. The rem has been superseded by the sievert in the SI system of units; 1 rem is equivalent to 0.01 sieverts.