of dose is unknown. The linear no-threshold model is used to extrapolate the expected number of extra deaths caused by exposure to environmental radiation, and it therefore has a great impact on public policy. The model is used to translate any radiation release, like that from a "dirty bomb", into a number of lives lost, while any reduction in radiation exposure, for example as a consequence of radon detection, is translated into a number of lives saved. When the doses are very low, at natural background levels, in the absence of evidence, the model predicts via extrapolation, new cancers only in a very small fraction of the population, but for a large population, the number of lives is extrapolated into hundreds or thousands, and this can sway public policy. A linear model has long been used in health physics to set maximum acceptable radiation exposures. The United States-based National Council on Radiation Protection and Measurements (NCRP), a body commissioned by the United States Congress, recently released a report written by the national experts in the field which states that radiation's effects should be considered to be proportional to the dose an individual receives, regardless of how small the dose is. A 1958 analysis of two decades of research on the mutation rate of 1 million lab mice showed that six major hypotheses about ionizing radiation and gene mutation were not supported by data. Its data was used in 1972 by the Biological Effects of Ionizing Radiation I committee to support the LNT model. However, it has been claimed that the data contained a fundamental error that was not revealed to the committee, and would not support the LNT model on the issue of mutations and may suggest a threshold dose rate under which radiation does not produce any mutations. Fieldwork The LNT model and the alternatives to it each have plausible mechanisms that could bring them about, but definitive conclusions are hard to make given the difficulty of doing longitudinal studies involving large cohorts over long periods. A 2003 review of the various studies published in the authoritative Proceedings of the National Academy of Sciences concludes that "given our current state of knowledge, the most reasonable assumption is that the cancer risks from low doses of x- or gamma-rays decrease linearly with decreasing dose." A 2005 study of Ramsar, Iran (a region with very high levels of natural background radiation)