# The Linear Hypothesis

It is very difficult to measure the incidence of cancer and other ailments caused by low level radiation. This is because the rates are very much lower than incidence of cancer from other (more usual) causes. One way of estimating the incidence is to give animals high doses of radiation, count how many cancers occur and assume that the number at low doses is proportionately lower. This is called the no threshold linear hypothesis.

The most thorough studies of the linear hypothesis have been made by Professor Bernard Cohen of the the Physics Department of the University of Pittsburgh in the US. He has found much evidence against it and describes it in his article Validity of the Linear No-Threshold Theory of Radiation Carcinogenesis at Low Doses.

The Uranium Institute in London, England has links to additional studies on its web page.

The Center for Nuclear Technology and Society at Worcester Polytechnic Institute offers much additional evidence against the linear hypothesis.

The linear hypothesis is unsupported by any direct evidence, and there is a lot of evidence against it. Here is some of it.

• The Atomic Bomb Casualty Commission composed of Japanese and American scientists has kept track of Hiroshima and Nagasaki survivors and estimated how much radiation each received. According to the linear hypothesis, these survivors should have had more cancer than has been observed.

• People living at high altitudes or at locations in India with high natural thorium levels should have had more cancer than has been observed.

• People in areas with high emissions of radon from granite and other rocks should have more cancer than has been observed. The EPA's estimate of the number of lung cancers caused by radon is based on the linear hypothesis and is almost certainly a large exaggeration. If so, the very expensive remedial measures proposed are a waste of money that would better be spent on getting rid of more serious threats to public health.

The rival to the linear hypothesis is the threshold hypothesis. The body repairs damage to cells from ionizing radiation. According to the threshold hypothesis radiation results in cancer only when there is enough radiation to overwhelm the threshold mechanism.

One could mix these hypothesis and assume that there is some linear component but that the major effect is a threshold effect. The above evidence suggests that the linear component is very small.

The linear hypothesis was chosen for regulatory purposes because (a) it is extremely conservative and (b) it isn't hard for the nuclear industry to meet standards based on it. It is politically very difficult to change it, because anti-nuclear organizations will accuse the regulatory authorities of jeopardizing public health.

The high estimates of cancer from the Soviet Chernobyl accident are based on the linear hypothesis.

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