Unidad para indicar la peligrosidad de una radiación, que debe su nombre al físico alemán Willhem Röentgen(1845-1923).Roentgen Equivalent Man (rem) es una unidad física utilizada antiguamente, y en la actualidad por los países anglosajones (aunque la están cambiando). Sus dimensiones son julios por kilogramo (J/kg). La unidad admitida en el Sistema Internacional de Unidades(SI) para medir esta cantidad es el sievert (Sv) con las mismas dimensiones que el rem. La equivalencia con la nueva unidad es 1 Sv = 100 rem.

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The roentgen equivalent in man (or mammal) (abbreviated rem; symbol rem, or often but incorrectly R) is an older, CGS unit of equivalent dose, effective dose, and committed dose. Quantities measured in rem are designed to represent the stochastic biological effects of ionizing radiation, primarily radiation-induced cancer. These quantities are a complex weighted average of absorbed dose, which is a clear physical quantity measured in rads. There is no universally applicable conversion constant from rad to rem; the conversion depends on relative biological effectiveness (RBE). The rem is defined since 1976 as equal to 0.01 sievert, which is the more commonly used SI unit outside of the United States. A number of earlier definitions going back to 1945 were derived from the roentgen unit, which was named after Wilhelm Röntgen, a German scientist who discovered X-rays. The acronym is now a misleading historical artifact, since 1 roentgen actually deposits about 0.96 rem in soft biological tissue, when all weighting factors equal unity. Older units of rem following other definitions are up to 17% smaller than the modern rem. One rem carries with it a 0.055% chance of eventually developing cancer. 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 the best indicator of ARS. 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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