Rem

Dose unit that takes into account the biological effectiveness of ionising radiation. In rem units, radiation doses are thus specified which, when absorbed in biological tissue, have the same biological effectiveness as when absorbing X-rays with an average specific ionization density of 100 ion pairs per microliter of water. Dose in rem = f times dose in rad, where f is the factor by which the "rem dose" of an ionizing radiation must be divided to obtain the equivalent absorbed X-ray dose (rad). The factor f is defined as "relative biological effectiveness". The unit for the equivalent dose is the sievert, 1 Sv = 100 rem; for extreme beta, gamma and X-rays: 1 rem = 1 rd.