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Linear Hypothesis
The linear hypothesis is a method for predicting damage done by low levels of ionizing radiation on biological material and the subsequent incidence of cancer. The hypothesis states that the number of cases of cancer caused by low levels of radiation can be estimated by giving animals high doses of radiation, then measuring the number of cases of cancer. Scientists then assume that the number of cases at a low dose is proportionately lower.
Assumptions
The hypothesis gets the "linear" part of its name from the assumption of a linear relationship between the amount of ionizing radiation and the rate of cancer that then develops.
"No threshold" comes from the assumption that there is no threshold below which ionizing radiation does not cause any cancer at all. A rival hypothesis is called the "threshold hypothesis," so "no threshold" is used to distinguish between the two.
Criticisms
There is very little direct evidence to support the linear hypothesis and much evidence against it. According to the Atomic Bomb Casualty Commission, survivors of the Hiroshima and Nagasaki bombs should have had more cancer than has been observed if the linear hypothesis were true. High altitudes and some areas of India have naturally high thorium levels, but fewer cases of cancer have been observed in these areas than is predicted by the linear hypothesis.
Alternatives
The main alternative to the linear hypothesis is the threshold hypothesis. According to the threshold hypothesis, there must be enough radiation to overwhelm the threshold mechanism for cancer to occur. Below this level, there are no cases of cancer. The threshold mechanism arises because the body repairs itself when its cells are damaged by radiation.