With the 25th anniversary of Chernobyl falling on April 26, a debate is brewing over the estimated death toll from the nuclear disaster. The debate has erupted with a heated exchange between prominent British columnist George Monbiot and anti-nuclear campaigner Dr Helen Caldicott.
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 risk of cancer. 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, medical studies are unlikely to be able to demonstrate a statistically-significant rise 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 was the 2006 report of the Committee on the Biological Effects of Ionising Radiation (BEIR) of the US National Academy of Sciences.
The BEIR report reviewed the available data and concluded: “The Committee judges that the balance of evidence from epidemiologic, animal and mechanistic studies tend to favour a simple proportionate relationship at low doses between radiation dose and cancer risk.”
It said “the 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 BEIR said that uncertainty remains because of the unavoidable methodological difficulties: “It should be noted however, that 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.”
Likewise, the BEIR committee noted in its 1990 report: “The committee recognises that its risk estimates become more uncertain when applied to very low doses. Departures from a linear model at low doses, however, could either increase or decrease the risk per unit dose.”
Governments clearly believe that low-level radiation is to be avoided, hence the 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.
A few scientists argue that low-level radiation is harmless or even beneficial. Their voice is amplified by the nuclear industry.
In Australia, for example, uranium mining and exploration companies such as Toro Energy, Uranium One and Heathgate Resources have funded speaking tours by overseas scientists who claim that low-level radiation exposure is harmless or beneficial.
If the mining and exploration companies truly believe that claim, one wonders whether their occupational health and safety procedures are up to scratch.
Back to Chernobyl. There is general agreement that 30-60 people died in the immediate aftermath of the accident. Beyond that, medical studies generally don't indicate a statistically-significant rise 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.
Dr Elizabeth Cardis from the International Agency for Research on Cancer says: “The total lifetime numbers of excess cancers will be greatest among the ‘liquidators’ (emergency and recovery workers) and among the residents of ‘contaminated’ territories, of the order of 2000 to 4600 among each group (the size of the exposed populations is 200,000 liquidators and 6,800,000 residents of ‘contaminated’ areas).
“These increases would be difficult to detect … 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 evidence is lacking and thus the death toll from Chernobyl was just 30-60. 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. It's simple — use estimates of the total radiation exposure, and multiply by a standard risk estimate based on the LNT model.
The International Atomic Energy Agency (IAEA) estimates a total collective dose of 600,000 person-Sieverts over 50 years from Chernobyl fallout (IAEA Bulletin, Vol.38, No.1, 1996). A standard risk estimate from the International Commission on Radiological Protection (ICRP) 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 and 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 9,000 to 93,000 deaths.
That 10-fold difference seems significant, but it is easily enough explained by the differing approaches and assumptions used in the various studies. For example, whether they consider radiation exposure across Europe or just in the most heavily contaminated countries of Eastern Europe.
And of course that 10-fold difference is peanuts compared with the many orders of magnitude separating Monbiot’s 43 and Caldicott's 985,000.
To briefly note some of the most important studies:
Reports by the UN Chernobyl Forum and the World Health Organisation in 2005-06 estimated up to 4000 eventual deaths among the higher-exposed Chernobyl populations and an additional 5000 deaths among populations exposed to lower doses in Belarus, the Russian Federation and Ukraine.
A study by Cardis et al. reported in the International Journal of Cancer estimates 16,000 deaths.
British radiation scientists Dr Ian Fairlie and Dr David Sumner estimate 30,000 to 60,000 deaths.
A 2006 report, commissioned by Greenpeace and involving 52 scientists, 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 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. We simply need to accept and acknowledge that the 30,000 figure arrived at by multiplying the IAEA dose estimate with the ICRP risk estimate may understate or overstate the true death toll.
This debate has a broader significance. The Chernobyl debate feeds into studies comparing the hazards of different energy sources. These studies generally attempt to quantify the hazards, expressing them in deaths per gigawatt-year of electricity.
A recent paper by the Australian “Choose Nuclear Free” partner groups summarises the comparative assessments. It reaches three main conclusions.
First, the greatest hazards — the link between fossil fuels and global warming, and the link between nuclear power and nuclear weapons — cannot be meaningfully measured.
Second, it is also difficult to accurately measure hazards such as particulate emissions from coal plants, and routine radiation emissions across the nuclear fuel cycle (which are probably responsible for far greater radiation exposure than the Chernobyl accident).
Third, notwithstanding the above, coal and nuclear power are clearly far more hazardous than renewables — even without considering global warming and nuclear weapons proliferation.
Factor in global warming and nuclear proliferation, and coal and nuclear are extremely hazardous indeed.