With the 25th anniversary of Chernobyl falling on 26 April, debate is brewing over the estimated death toll from the nuclear disaster. Indeed the debate has erupted with a heated exchange between journalist George Monbiot and anti-nuclear campaigner Helen Caldicott. In the wake of Fukushima, Monbiot has been advocating strongly for nuclear power as the best way to reduce carbon emissions. Along the way, he’s set out to debunk what he sees as the prevailing myths about going nuclear. One of these is the Chernobyl death toll.
The debate over the Chernobyl death toll turns on the broader debate over the health effects of low-level ionising radiation — and in particular the cancer risk it poses. The weight of scientific opinion holds that there is no threshold below which ionising radiation poses no risk and that the risk is proportional to the dose: the "linear no-threshold" (LNT) model.
Uncertainties will always persist. In circumstances where people are exposed to low-level radiation, epidemiological studies are unlikely to be able to demonstrate a statistically significant increase in cancer rates. This is because of the "statistical noise" in the form of widespread cancer incidence from many causes, the long latency period for some cancers, limited data on disease incidence, and various other data gaps and methodological difficulties.
Notwithstanding the difficulties, there is growing scientific confidence in the LNT model. An important study in this regard is the 2006 report of the Committee on the Biological Effects of Ionising Radiation (BEIR) of the US National Academy of Sciences.
The BEIR report comprehensively reviewed available data and concluded that:
"The balance of evidence from epidemiologic, animal and mechanistic studies tend to favor a simple proportionate relationship at low doses between radiation dose and cancer risk. … [T]he risk of cancer proceeds in a linear fashion at lower doses without a threshold and … the smallest dose has the potential to cause a small increase in risk to humans."
The report notes that uncertainty remains because of the unavoidable methodological difficulties: "Even with the increased sensitivity the combined analyses are compatible with a range of possibilities, from a reduction of risk at low doses to risks twice those upon which current radiation protection recommendations are based."
Governments clearly believe that low-level radiation is to be avoided — hence the 20 kilometre evacuation zone around the Fukushima nuclear plant in Japan, widespread restrictions on food and water consumption, and the growing number of countries imposing restrictions on the importation of food from Japan.
Back to Chernobyl. There is general agreement that 30 to 60 people died in the immediate aftermath of the accident. Beyond that, epidemiological studies generally don’t indicate a statistically significant increase in cancer incidence in populations exposed to Chernobyl fallout. Nor would anyone expect them to because of the data gaps and methodological problems mentioned above.
Elizabeth Cardis from the International Agency for Research on Cancer explains that emergency and recovery workers and residents of contaminatted territories will have the greatest total lifetime numbers of excess cancers, noting that "these increases would be difficult to detect epidemiologically against an expected background number of 41,500 and 800,000 cases of cancer respectively among the two groups."
For a few marginal scientists and nuclear industry spruikers, that’s the end of the matter. The death toll from Chernobyl was 30 to 60. Full stop. But for those of us who prefer mainstream science, we can still arrive at a scientifically defensible estimate of the Chernobyl death toll — even if epidemiological studies don’t help — by using estimates of the total radiation exposure, and multiplying by a standard risk estimate based on the LNT model.
The International Atomic Energy Agency estimates a total collective dose of 600,000 person-Sieverts over 50 years from Chernobyl fallout (see the IAEA Bulletin, Vol.38, No.1, 1996). A standard risk estimate from the International Commission on Radiological Protection is 0.05 fatal cancers per Sievert. Multiply those figures and we get an estimated 30,000 fatal cancers. Now let’s recall that, according to the BEIR report, the LNT model may overstate risks or understate them by a factor of two. Thus the estimated death toll ranges from something less than 30,000 — up to 60,000.
A number of studies apply that basic methodology — based on collective radiation doses and risk estimates — and come up with results varying from 9000 to 93,000 deaths. While that tenfold difference seems significant, it is explained by the differing approaches and assumptions used in the various studies. For example, whether or not they consider radiation exposure across Europe or just in the most heavily contaminated countries of Eastern Europe. (And of course that tenfold difference is peanuts compared to the many orders of magnitude separating Monbiot’s 43 and Caldicott’s 985,000.)
Monbiot says he asked Helen Caldicott for sources about the Chernobyl death toll. Here are some of the most important studies which he didn’t mention in his article. Firstly, reports by the UN Chernobyl Forum (pdf) and the World Health Organisation in 2005-06 estimated up to 4000 eventual deaths among the higher-exposed Chernobyl populations and an additional 5,000 deaths among populations exposed to lower doses in Belarus, the Russian Federation and Ukraine.
A study by Elizabeth Cardis and her colleagues published in 2006 in the International Journal of Cancer estimates 16,000 deaths. Research published in 2006 by UK radiation scientists Ian Fairlie and David Sumner estimated 30,000 to 60,000 deaths. And finally, a 2006 report commissioned by Greenpeace estimates a death toll of about 93,000.
So where do Monbiot and Caldicott fit in the context of these scientific studies of the Chernobyl death toll? They don’t fit anywhere at all. Caldicott relies on a Russian report titled Chernobyl: Consequences of the Catastrophe for People and the Environment. Suffice it here to note that the study uses a loose methodology to arrive at an unlikely conclusion.
Monbiot sides with the marginal scientists in arguing that low-level radiation is harmless. He cites a report from the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to claim that the "official death toll" from Chernobyl is 43. But the UNSCEAR report made no effort to assess the effects of widespread low-level radiation exposure. Specifically, the report states:
"The Committee has decided not to use models to project absolute numbers of effects in populations exposed to low radiation doses from the Chernobyl accident, because of unacceptable uncertainties in the predictions. It should be stressed that the approach outlined in no way contradicts the application of the LNT model for the purposes of radiation protection, where a cautious approach is conventionally and consciously applied."
So UNSCEAR cites "unacceptable uncertainties in the predictions" as its reason for shying away from an assessment of the impacts of widespread radiation exposure. The rest of us needn’t be so shy. And we shouldn’t be shy about criticising Monbiot or anyone else who claims that just 30 to 60 people died as a result of Chernobyl.
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