Saturday, November 10, 2012

Evidence for Linear No-Threshold Theory?

A new study has just been released that casts doubt on the theory of radiation hormesis, the idea that low doses of ionizing radiation can be good for you. It seems to provide some evidence for the Linear No Threshold theory, which assumes that cancer risk is directly proportional to the amount of radiation taken in, so that there will always be some risk, even as very low doses. What this theory means is that while the risk for the individual may be very low, over a large number of people there will be some cancers that are induced by radiation.

The model has always been controversial, and extremely difficult to measure, because at such very low levels of risk it is difficult to separate confounding factors in statistical studies. Even things like different populations being more reliable in self-reporting smoking rates can confuse things so that results are not reliable.

This Linear No-Threshold Theory (LNT) has always been doubted by pro-nuclear experts. After all, what other substance in the universe has new threshold for effect? Even poisons like arsenic have a minimum does below which no effects at all can be expected. Wade Allison sums up this skepticism:

"In earlier decades knowledge of cell biology was too primitive to provide confident understanding, and adequate evidence of the effect of radiation on humans was no available to corroborate any particular view. In their absence, and for lack of anything better, the knowledge gap was bridged by a rule of thumb - a model in science speak. This is the LNT model. This assumes that clinical damage is in simple proportion to the initial radiation energy dose. No justification was given for it, but it was a reasonable working hypothesis at the time. Despite the poor state of knowledge, a start had to be made somewhere."
(From Radiation and Reason, p. 44)

Now the most rigorous support for LNT, at least to my knowledge, has come in the form of a study of 110,645 people involved in the cleanup after Chernobyl. It concluded that despite the relatively low doses of radiation received, there was a significant increase in the risk of getting leukemia.

We are talking about a very small number of cases, 117 cases of leukemia. It was estimated that 16% of leukemia cases were attributed to radiation exposure, that is, 19 cases. The cleanup workers also received considerably more radiation that residents of Fukushima. Just to put this in perspective, 19 cases is 0.0019 % of the deaths that can be attributed to coal dust every year.

Nevertheless it has thrown the pro-nuclear community into a bit of a tizzy. Any evidence for LNT will be more fuel for people who say that any level of radiation is dangerous. The inevitable claim will be that there is actually some risk for people up at Fukushima. If you crunched the numbers you might come up with 2 or 3 cases of leukemia, at a guess.

You can read the paper here. If it is deliberately anti-nuclear, it is massively more sophisticated that the propaganda put by Arnie Gundersen or Helen Caldicott.  Certainly it is beyond my skill set to critique.

You can read an early response by a pro-nuclear blogger here. I will be watching carefully how this all goes down

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