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It is time to scientifically challenge the old tenet stating that cancer risk is always proportional to dose, no matter how small. This seemingy blasphemous statement is based on new approaches that allow testing of the hypothesis that cancer risk is linearly proportional to dose with no threshold, the basis of much regulatory and assessment documentation. We hear much these days about the need for all assessments and regulations for risk to be based on sound and solid science. This has not been the case for physical and chemical cancer risks to humans.
For both physical and chemical exposure to agents that are thought to increase cancer risk, it has been traditional to state that responsible evaluations and recommendations should assume that all exposures, no matter what the amount, carry an associated cancer risk. This assumption allows estimation, for example, of the lifetime cancer risk of a single ionization or the risk from intake of a single molecule of a putative carcinogen. It further leads to the concept of a collective dose, where all the ionizations are added up in all the people, and the product
for example, person-roentgen equivalent man (rem) or person-sievert (Sv)
is related to (multiplied by) a cancer risk factor to give a potential population body count (1). Such a calculation is the origin of predictions, for example, that so many persons will die from radon exposure, or so many cancers will result from treating apples with a chemical.
As an extreme extrapolation, consider that everyone on Earth adds a 1-inch lift to their shoes for just 1 year. The resultant very small increase in cosmic ray dose (it doubles for every 2000 m in altitude), multiplied by the very large population of the Earth, would yield a collective dose large enough to kill about 1500 people with cancer over the next 50 years. Of course no epidemiological confirmation of this increment could ever be made, and although the math is approximately correct, the underlying assumptions should be questioned. Most of the environmental risks we now face from present or proposed activities probably are of this magnitude, and many of our policies say that prudence requires us to reduce these small values even further. We do not seem to have a...