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Correspondence |
In the January 2005 issue of BJR, Cameron [1] suggests that moderate doses of radiation increase longevity. His main arguments are from epidemiological studies of ecological design and also from cohort studies of workers.
It is well known that ecological studies, which lack data on individual causes of death and individual exposures to the putative risk factors, are prone to statistical problems such as confounding [2]. Cameron quotes an ecological study [3] which compared three Rocky Mountain States (Idaho, Colorado, New Mexico) with three Gulf States (Louisiana, Mississippi, Alabama) in the USA. Cancer mortality was about 26% higher in the Gulf States, while natural background was lower by a factor of about three. Cameron argues that these differences are too extreme for confounding to be a plausible explanation. But this is based on the assumption that natural background radiation is a dominant cause of cancer. This is very far from the truth.
Radiation is a minor factor in determining overall cancer rates. The importance of factors such as smoking and urban/rural lifestyle was pointed out by Archer [4] in a critique of the Jagger paper. Jagger [5] accepted that the differences in cancer rates between the Rocky Mountain and the Gulf States was probably due to these factors. He regarded the lesson to be drawn as no more than that natural background radiation is not an important cause of cancer.
It is certainly true that natural background radiation does not account for a large proportion of cancers and that it would not be expected that the effects of natural radiation could be detected in ecological studies. Consider a hypothetical study in which the highly exposed population received about twice the radiation exposure of the control population, say an extra 3 mSv pa. Over a lifetime, the extra dose would be about 200 mSv, very roughly equating to an increased risk of cancer mortality of 1% [6]. It is highly unlikely that this excess could be detected above the normal background cancer mortality of 20–25%.
Natural background radiation is thus not a dominant cause of cancer. However, it is important to remember that factors which may have a relatively minor impact on mortality are not unimportant if they are controllable. Part of the natural background radiation is almost impossible to avoid, but the component from radon, which can give very significant doses, can and should be controlled. A large pooling of case/control studies from Europe [7] has demonstrated a risk of lung cancer down to exposures of 100–199 Bq m–3. There was no evidence for a threshold and if one exists it must lie below 150 Bq m–3. The present Action Level in the UK is 200 Bq m–3, but it is being reviewed [8].
Cameron also points to cohort studies of US shipyard workers [9, 10] and British radiologists [11]. Cohort studies are less subject to confounding than are ecological studies. However, it is a common observation that strong selection effects apply and make it difficult to interpret comparisons between the group being studied and reference populations. This frequently manifests itself as the well known "Healthy Worker Effect" [12].
Matanoski et al [9, 10] studied workers at US nuclear shipyards. They reported significantly lower standardized mortality rates (SMRs) in those overhauling nuclear-powered ships with cumulative effective doses greater than 5 mSv than in those with lower doses, and in the latter group compared with non-radiation shipyard workers. However, the authors of the study did not suggest that these results provide evidence for a beneficial effect of radiation but instead regarded selection bias as a more likely cause. It would be perverse to try to override this interpretation and suggest instead that radiation increases longevity.
Cameron also cites a study of British radiologists [11] in which it was reported that those employed after 1920 had lower SMRs than other contemporary medical doctors. However, a large proportion of these other doctors would have been General Practitioners who are known to have smoked more than other doctors [13]. In their paper, Berrington et al [11] focused on internal trends in mortality risks with time since entry into the radiology profession (as an indication of cumulative dose), rather than relying solely on external comparisons of SMRs with other groups of doctors or the more general population. Thus they found evidence for an increasing trend in risk of cancer mortality with time since first registration with a radiological society; for example, there was a 41% excess risk among those registered for more than 40 years.
Cameron also argues that life evolved in the presence of significant levels of natural background radiation and must be adapted to repair the damage that it causes. He draws the conclusion that "It is illogical to suggest that radiation damage to one cell may cause cancer". But it is indisputable that radiation is a cause of cancer and it is hard to see why a mechanism involving a rare kind of damage to a single cell should be less "logical" than any other.
Yours etc.,
National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK
Footnotes
Since receiving this letter from the NRPB we are very sorry indeed to learn of the death of Professor John Cameron. 
(The Editors do not hold themselves responsible for opinions expressed by correspondents)
Received for publication March 8, 2005. Accepted for publication March 22, 2005.
References
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