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The roentgen equivalent in man (or mammal〔''The Effects of Nuclear Weapons'', Revised ed., US DOD 1962〕) (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 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. ==Usage== The rem and millirem are CGS units in widest use among the American public, industry, and government. SI units are the norm outside of the United States, and they are increasingly encountered within the US in academic, scientific, and engineering environments. The conventional units for dose rate is mrem/h. Regulatory limits and chronic doses are often given in units of mrem/yr or rem/yr, where they are understood to represent the total amount of radiation allowed (or received) over the entire year. In many occupational scenarios, the hourly dose rate might fluctuate to levels thousands of times higher for a brief period of time, without infringing on the annual total exposure limits. There is no exact conversion from hours to years because of leap years, but approximate conversions are: :1 mrem/h = 8766 mrem/yr :0.1141 mrem/h = 1000 mrem/yr The ICRP once adopted fixed conversion for occupational exposure, although these have not appeared in recent documents: :8 h = 1 day :40 h = 1 week :50 week = 1 yr Therefore, for occupation exposures of that time period, :1 mrem/h = 2000 mrem/yr :0.5 mrem/h = 1000 mrem/yr The US National Institute of Standards and Technology (NIST) strongly discourages Americans from expressing doses in rem, in favor of recommending the SI unit. The NIST recommends defining the rem in relation to the SI in every document where this unit is used. For US industries and US firms that do not require the sole use of SI, however, the unit rem is often preferred. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Roentgen equivalent man」の詳細全文を読む スポンサード リンク
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