burns, organ damage, radiation sickness, and even death. Patients receiving radiation treatment for cancer often experience controlled acute radiation sickness because they receive relatively high levels of radiation. Infertility and cataract are two other examples of nonstochastic effects of radiation; cataract may not occur until several years after exposure. Doses to people near nuclear facilities are far below levels that would cause deterministic effects.

In the case of the effects of exposure to low levels of radiation (less than 0.1 Gy, or 100 mSv effective dose), the scientific uncertainty of radiation-induced cancer is considerable as there is little or no empirical knowledge. Despite the uncertainty, decisions are needed for use in setting standards for protection of individuals against the side effects of low-level radiation. Based on current scientific knowledge (or lack thereof), regulatory agencies in the United States currently use a model that describes radiation injury as a linear function of radiation dose that has no threshold; this is called the linear no-threshold (LNT) model. According to LNT, if a dose equal to 1 Gy gives a cancer risk X, the risk from a dose of 0.01 Gy is X/100, the risk from 0.00001 Gy is X/100,000, and so on. Thus, the risk of health effects including cancer risk is not zero regardless of how small the dose is.

In the LNT model, data from high levels of exposure where radiogenic cancers have been observed are used to extrapolate risks at lower doses where cancers have not been observed, and if they exist they are beyond the current science to observe and measure. One result of following the LNT model is that a very small estimated risk, when multiplied by a large number such as the population of the United States, results in an estimate of a substantial number of cases or deaths, which in reality may not exist.

Scientific groups such as the International Commission on Radiological Protection (ICRP), the National Council on Radiation Protection and Measurements (NCRP), and the National Research Council Committee on the Biological Effects of Ionizing Radiation (BEIR), repeatedly review and endorse the use of this model for assessing risk, which is used to set radiation protection standards and operating policies, such as the “as low as reasonably achievable” (ALARA) policy. This approach is often considered to be conservative and gives emphasis to public health. Data provided by the updated report of the atomic bombing survivors in Japan continue to be in support of the LNT model across the entire dose range. However, a concave curve was the best fit for data restricted to doses of 0-2 Gy. This resulted because risk estimates for exposure to 0.3-0.7 Gy were lower than those in the linear model (Ozasa et al., 2012). The finding was not explained.

Not all countries support the LNT model at this time, but in general it is perceived that with so much uncertainty about the effects at low doses, it is appropriate to continue with the LNT model that has been in place for several decades for purposes of radiation protection.



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