At the height of the Fukushima Daiichi crisis, there was a lot of discussion in the comments of this blog about the impact of radiation on human health. In particular, there was a debate about whether the linear no-threshold hypothesis (LNT) or the hormetic dose response (RH) was more scientifically robust model for assessing the implications of a Chernobyl or Fukushima release event.
In short, the LNT hypothesis says that there is a directly proportional relationship between the dose received and the probability of biological damage from ionizing radiation. (That is, there is no safe level.) The RH model, by contrast, posits that low-dose radiation (at or somewhat above background levels) is actually beneficial to health, perhaps because of stimulation of natural repair mechanisms in the body. (High doses still are still detrimental). The following figure (from Luckey, cited below) illustrates the RH:
Now that the situation in Japan has (more-or-less) stabilised and radiation levels are low, it is probably useful to revisit this topic. First, the radiation update from Fukushima from WNN:
Provisional analyses based on radiation dose rates at the site boundary show that emissions to air have reduced by a factor of two million compared to those at the height of the crisis, when the torus suppression chamber of unit 2 ruptured on 15 March. Someone standing at the western border of the power plant today could expect to receive a maximum of 1.7 millisieverts per year (mSv/y) from airborne radioactivity from the three ruined reactors. This compares to the 2.4 mSv/y average that people worldwide receive from background sources, and the operational limit for nuclear power plants in Japan to limit public exposure to 1.0 mSv/y.
Dose rates from emissions drop dramatically away from the site: five kilometres away the maximum rate from newly released radiation is 0.3 mSv/y; ten kilometres away it is 0.09 mSv/y; and 20 kilometres away it is 0.03 mSv/y. It is important to note that these figures apply only to the rate of release of radiation now, and do not include the effects of any materials already deposited on the ground, some of which will continue to emit radiation for many years. There are areas totalling about 1000 square kilometres where dose rates have been elevated beyond 20 mSv/y due to caesium-137 deposited on the ground.
… and some relevant recent posts/comment threads on BNC about LNT and RH:
Some useful (short) briefing papers that I suggest you read before engaging in this discussion are this PDF paper by clinical radiobiologist Prof David Wigg (University of Adelaide) and the recent “Biological Effects of Ionizing Radiation: a Perspective for Japan” (2-page PDF) by Prof Sir Samurai T.D.Luckey (yes, that really is his name). Some quotes from the two papers:
The most rigorous epidemiological study of the effects of low exposure to radiation workers was the Nuclear Shipyards Workers Study initiated by the USA Department of Energy in which 71 000 workers were examined. There were two exposed groups with doses less than or greater than the equivalent of 5 years’ background radiation. These were compared with similar workers with no exposure. The higher dose group had lower cancer death rates and lower death rates from all causes. Similar findings of lower deaths from all causes were shown in the British Radiologists Study of all British radiologists between 1900 and 1980.
The failure of the LNT hypothesis at low doses is supported by mathematical theory, which predicts that under conditions of low dose, the outcome of extremely complex phenomena cannot be predicted by a simple linear equation. The LNT hypothesis is highly unscientific when applied to low dose irradiation and its application has had profound and undesirable consequences that are with us today.
Conclusions about the effects of acute exposure are generally based on data on the Japanese survivors of the atomic bombs (see Figure 2). The RERF compared the cancer mortality of Hiroshima and Nagasaki survivors with that of people who were 3-10 km from ground zero (“in-the-city controls”). These controls received some radiation from the bombs, and many went into the bombed areas while residual radiation was high. Total [all-cause] “mortality rates in 120,321 atomic bomb survivors were not increased at doses <490 mSv.
The cancer mortality rate of the 7,430 survivors of Hiroshima and Nagasaki who received 10-19 mSvwas 68.5% ( <.01) of that of controls. The 28,423 survivors (69% of all survivors) who received 2,000 mSv.
I know the two examples above are clearly arguing in support of RH over LNT, but that’s because I found it difficult to find similar opinionated review works, backed by citations and written by a relevant expert in the field of radiobiology, who argues the LNT case. If you know of them, let me know, and I’ll amend this post to include a citation that reflects this position too. The best I have come up with is the National Academy of Sciences’ Biological Effects of Ionizing Radiation (BEIR) report — an expert panel who reviewed available peer reviewed literature:
the committee concludes that the preponderance of information indicates that there will be some risk, even at low doses
A brief summary of the controversy around that, and similar statements on LNT and RH, can be read here.
Actually, on that point, what I’m particularly seeking in the comments on this BNC post is to find/cite examples from the peer-reviewed medical/clinical/experimental literature (abstract links or full PDF papers) that either support or reject the LNT or RH. Meta-analysis [quantitative synthesis of individual studies] would be particularly useful, since they are statistical syntheses. Less useful are equivocal conclusions such as accepting the use of the LNT because it is a ‘null’ model that can be used in the absence of compelling evidence supporting (or rejecting) the RH model.
A major problem, of course, is that because we are talking about the potential impact (detrimental or beneficial) of very low-dose radiation, individual effects are typically small — thus large-scale epidemiological studies are required to come up with statistically meaningful signals. These data are also difficult to analyse (e.g., removing potential biases caused by other mortality factors, stratifying the sample to cover representative cross-sections of the populations under study, etc.) and obtaining precise individual doses for subjects is also challenging.
A real breakthrough in resolving this issue would be if proxy methods could be developed and deployed to measure doses of individuals over defined periods or even lifetimes. Then, very effective control-impact and continuous dose studies could be designed. Some tantalising possibilities exist, such as new microchip technology or using teeth as natural biodosimeters. Perhaps, in the near future, this may lead to more robust answers — and more scientifically informed public policy.
Finally, if you’d rather watch than read, I thoroughly recommend a Horizon documentary, which is available in full on YouTube, called “Nuclear Nightmares”. The first part is here, and links to the remaining 4 parts are on that page (it is a total of ~50 min):
I look forward to the comments that follow. Please remember to follow the BNC commenting rules by sticking to the topic at hand, and, where possible, supporting your statements with references. A certain amount of opining and philosophy on radiation and public opinion etc. is also acceptable here, but please try not to drift too far into the realm of speculation.