Whole body exposure, radiation limits

For whole body exposure, the non-occupational exposure limit is 100 mrem/year. This is in addition to the 360 mrem/yr received, on average, by individuals in the U.S. from natural background radiation and manmade radiation sources. The 100 mrem/year limit also applies to individuals under age 18 who work in the vicinity of radiation sources. 

In setting radiation limits, it is customary to assume that, for exposure of the whole body to radiation, the effect produced is proportional to the absorbed dose. Since this assumption ignores the possibility of damaged tissue repairing itself during long-term exposure to low radiation doses, it is believed to represent a conservative approach to the estimation of possible damage. Somatic effects where the probability of injury (e.g., the initiation of cancer) is proportional to the dose are known as stochastic effects. In contrast to these, an effect such as cataract of the eye is nonstochastic in this case, a threshold dose exists such that no cataract will be induced if the absorbed dose is below this amount. 

The Leggett (1992) model was developed to predict tissue doses and whole-body dose to people who may be exposed to americium. The model is considered an updated version of the ICRP (1989) model for americium, which has been used to establish risk-based limits of intake of241 Am (ICRP 1989). The Leggett (1992) and ICRP (1989) models predict similar long-term average doses of americium to the liver and skeleton for an injection exposure and would be expected to predict similar radiation risks and risk-based intake limits (Leggett 1992). Descriptions of applications of the Leggett (1992) model in risk assessment have not been reported. 

The whole body tolerable dose of radiations has been set at a maximum of 5 rem per yeeir (Le., 0.1 rem per week). This whole body dose has been set up considering the more sensitive tissues such as the gonads and the bone marrow (as a general rule fast dividing tissues are more sensitive to radiation). There are, however, other tissues which are not so sensitive. Thus, hands and forearms, which are most involved in exposure while carrying out biolo cal experiments, can tolerate a higher exposure. The tolerable exposure limit for these organs is set up at 75 rem per year and up to 20 rem in any 13-week period. 

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