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Saturday, March 13, 2010


Extracts from Professor Bernard Cohen's paper
Validity of the Linear No-Threshold Theory of Radiation Carcinogenesis at Low Doses

on the alleged damage caused by low level radiation:

Basis for the Linear No-Threshold Theory

The principal basis for the LNT is theoretical, and very simple. A single particle of radiation hitting a single DNA molecule in a single cell nucleus of a human body can initiate a cancer. The probability of a cancer initiation is therefore proportional to the number of such hits, which is proportional to the number of particles of radiation, which is proportional to the dose. Thus, the risk is linearly dependent on the dose; this is the LNT.

The problem with this very simple argument is that factors other than initiating events affect the cancer risk. Our bodies have biological defence mechanisms (BDMs) which prevent the vast majority of initiating events from developing into a fatal cancer...

There is plenty of very direct and obvious evidence on this. For example, the number of initiating events is roughly proportional to the mass of the animal; more DNA targets means more hits. Thus, the simple theory predicts the cancer risk to be proportional to the mass of the animal. But experience indicates that the cancer risk in a given radiation field is roughly the same for a 30 gram mouse as for a 70 000 gram man, and there is no evidence that elephants are more susceptible than either.

If only the number of hits (which is proportional to the number of initiating events) were relevant (regardless of the mass of the target), then our very definition of dose in terms of radiation hits per unit mass of the target would be misleading. Another obvious example of the failure of the simple basis for the LNT is in the spectacular increase in cancer incidence with age. Young people experience cancer initiating hits as frequently as old people, but the probability for a cancer to develop is much higher in old people...

Effects of Low Level Radiation on Cancer Development

The effects of LLR on several different measures of the immune response are ...the immune response is increased by LLR. There is at least one study of this effect over a wide range of radiation doses (Ref 10). It reports increases in immune response by 80% in vitro and by 40% in vivo at about 20 rad followed by a rapid decrease to well below the unirradiated level at doses of about 50 rad.

All of the work reported in this section suggests that LLR is protective against cancer; quite the opposite of what is expected from the LNT. Not only has the simple basis for the LNT collapsed, but also there is a large body of evidence indicating that a more complete treatment of the problem would show a decrease in risk with increasing dose in the low dose region.

Risk vs. Dose Data from Human Exposures

IARC study (Ref 15) of 95 673 monitored radiation workers in the USA, the UK and Canada. For all cancers except leukaemia, there were 3830 deaths but no excess over the number expected. The risk is reported as -0.07 per Sv with 90% confidence limits (-0.4, +0.3). There is surely no support for the LNT here...

The data on leukaemia among A-bomb survivors ... data strongly suggest a threshold above 20 cSv.

for breast cancer among Canadian women exposed to x-ray fluoroscopic examinations for tuberculosis ... there seems to be a decrease in risk with increasing dose, at least up to 20 cSv.

... bone and head cancers among dial painters, chemists, and others occupationally exposed to ingested radium ... there were no tumours among those with exposures below 1000 cGy ... give no support for the LNT, and are strongly suggestive of a threshold behaviour. Moreover, this threshold behaviour is strongly supported with much better statistics by data from injection of radioactivity into animals.

.... the percentage of all deaths that are from cancer varies in the USA from 22% in New England to 17% in the Rocky Mountain States (where radiation levels are highest)...

no excess cancer has been found in the high radiation areas of India or Brazil. But all such effects can easily be explained by potential...

Obtaining Indisputable Evidence

According to the LNT, {radon in homes] is responsible for at least 10% of all lung cancers, and a known confounding factor, cigarette smoking, is responsible for nearly all of the rest. Another advantage is that levels of radon in homes vary much more widely than natural gamma radiation.

There have been numerous case-control studies of the relationship between radon in homes and lung cancer... results from different studies have been inconsistent. This work has given no statistically significant information on the validity of the LNT in the low dose region... A different approach, specifically designed for testing the LNT, was carried out by the present author and is described in the following section.

University of Pittsburgh Study

...We compiled hundreds of thousands of radon measurements from several sources to give the average radon level (r) in homes for 1729 US counties, well over half of all US counties and comprising about 90% of the total US population

We see a clear tendency for m [age adjusted lung cancer rates] to decrease with increasing r [radon level], in sharp contrast to the increase expected from the supposition that radon can cause lung cancer, shown by the line labelled "theory".

The report goes through a number of possible confounding factors which could distort the result & concludes

Since they are typical of the largest confounding effects one can plausibly expect, it is extremely difficult to imagine a confounding effect that can explain the discrepancy. Requirements on such an unrecognised confounding factor were listed, and they make its existence seem extremely implausible.

Since no other plausible explanation has been found after years of effort by myself and others, I conclude that the most plausible explanation for our discrepancy is that the linear no-threshold theory fails, grossly over-estimating the cancer risk in the low dose, low dose rate region. There are no other data capable of testing the theory in that region.

An easy answer to the credibility of this conclusion would be for someone to suggest a potential, not implausible, explanation based on some selected variables. Either they or I will then calculate what values of those variables are required to explain our discrepancy. We can then make a judgement on the plausibility of that explanation. To show that this procedure is not unreasonable, I offer to provide a not implausible explanation for any finding of any other published ecological study. This alone demonstrates that our work is very different from any other ecological study, and therefore deserves separate consideration.

I have highlighted the last paragraph because this is precisely how science works. Measurements & theories are tested to destruction, or otherwise. Anybody maintaining the "official" LNT theory or even disputing its opposite, the hormesis theory, simply has to be prepared to put up an alternative, plausible, explanation for these measurements. Anybody arguing against them without at least putting forward a testable hypothesis, no matter how well supported they are by the great & good in government, is simply showing no respect for scientific principles.

I assume, from the fact that it isn't recorded, that no supporter of the official line has done so. - Neil

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