Sievert, definition

The risk index in Equation 6.2 is expressed in terms of risk (i.e., the probability that an adverse response will occur during an individual s lifetime). This definition is consistent with the fundamental objective of developing a risk-based hazardous waste classification system. However, the use of health risk per se in calculating the risk index presents some difficulties because risk is not proportional to dose for substances that cause deterministic effects. For this type of substance, the risk is presumed to be zero at any dose below a nominal threshold. Since the allowable dose should always be less than the threshold in order to prevent the occurrence of adverse responses, expressing the risk index in terms of risk would result in an indeterminate value and, more importantly, a lack of distinction between doses near the nominal thresholds and lower doses of much less concern. For any hazardous substance, including carcinogens for which risk is assumed to be proportional to dose without threshold, it is generally useful to express the risk index as the ratio of a calculated dose [e.g., sieverts, mg (kg d)-1] to an allowable dose that corresponds to an allowable risk ... 

In further support of this may be cited the observation of Sieverts,1 that the quantity of hydrogen absorbed by unit -weight of palladium is a function of the pressure and temperature only, and is quite independent of the superficial area of the metal. This would indicate that the absorption of nydrogen is an example of true solution rather than of definite chemical combination. 

It is assumed that within the range of exposure conditions usually encountered in radiation work, the risks of cancer and hereditary damage increase in direct proportion to the radiation dose. It is also assumed that there is no exposure level that is entirely without risk. Thus, for example, the risk factor for leukaemia is about 1 in 300 per sievert (see below for definition) of dose equivalent to the red bone marrow. In scientific notation, this is given as 3.4 x 10 per sievert. The mortality risk factor for all cancers from uniform irradiation of the whole body is about 3.4 x 10 per sievert for a UK working population, aged 20 to 64 years, averaged over both sexes. 

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