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Measuring radiological risks To measure the risk of biological damage, regardless of the type of radiation (alpha, beta, gamma or neutron) or exposure (internal or external, partial or total) involved, the effective dose, often called 'dose' and expressed in sieverts, is used. It is an overall indicator enabling all types of exposure to be measured on the same scale.
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The effective dose E (in sieverts), often erroneously called 'whole body dose' or simply 'dose', is equal to the absorbed dose D (in grays) multiplied by the radiation weighting factor WR and the tissue weighting factor WT. WT indicates the radiosensitivity of each organ.
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  The sievert is a relatively 'large' unit and there is a wide range of exposure levels. Consequently a submultiple, the millisievert, is often used (1 mSv = 0.001 Sv). The sievert has replaced the rem since 1975 (1 sievert = 100 rems).
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  Individual exposure occurs only while the body is actually being subjected to radiation. The total individual dose received is the product of the dose rate (dose delivered per unit of time) multiplied by the exposure time.
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  The collective dose, expressed in man-sieverts, gives a measure of the extent to which a group of people or a population has been exposed. It is equal to the sum of the individual doses. Example: A collective dose of 1 man-sievert corresponds to 1000 persons who have each received 1mSv or to 10 persons who have each received 100 mSv.
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