As regular readers will know, low-level radiation and its risks are sources of much discussion, polarised views and bad science.
It is dispiriting to read many articles – on both sides of the Atlantic – by media pundits and poorly-informed scientists about low-level radiation risks. These articles commonly assert, with little or no evidence, that there is nothing to worry about radiation and that nuclear projects are encumbered by overly strict safety limits. In particular, they usually state that no risks are seen below 100 mSv; that the Linear No-Threshold (LNT) model is wrong; and that there were only about 50 deaths at Chernobyl with no more expected.
There often seems to be a close relationship between the level of ignorance evidenced in articles on this subject and the over-confidence with which they are written.
For example, two such comments have been recently published in the US. One by a media pundit (http://www.forbes.com/sites/jamesconca/2013/01/11/like-weve-been-saying-radiation-is-not-a-big-deal/) and the other by a scientist and a layman (http://www.nytimes.com/2013/01/08/opinion/nuclear-accidents.html?_r=0). Of course, these writers are entitled to their opinions, but newspaper and journal editors should check these opinions before publishing them.
This post aims at providing editors with some balance and help in their checking.
But in recent months, a flurry of epidemiological studies go further than merely refuting ill-informed articles. They indicate adverse effects to people exposed to very low doses from medical CT scans and other clinical procedures; to infants living near nuclear power stations; and to Chernobyl clean-up workers. They even reveal adverse effects from background radiation to which all of us are exposed.
Together they reveal a pattern of higher-than-expected risks from very low exposures to radiation.
1. Background Radiation
Perhaps the most eye-opening of the recent studies concern background radiation. Most people think that background radiation levels (typically 2 to 3 mSv per year) are very low and are of little concern. But recent authoritative studies clearly indicate that background radiation is not harmless.
For example, a team of British scientists based at the University of Manchester, Imperial College and the UK Health Protection Agency has been examining this matter. Using two leukaemia risk models and estimates of red-bone-marrow doses received by children from background radiation, the team initially estimated that 20% of the childhood leukaemia cases in Great Britain were attributable to background radiation (Wakeford et al, 2009) – see references at end.
This surprising result was first refined to 15% of GB childhood leukaemias (Little et al, 2009) (Kendall et al, 2011), then the team predicted the risk rate from background gamma radiation. After conducting a large record-based case–control study with 27,000 cases and 37,000 controls to test associations between childhood cancer and natural background radiation, the authors estimated that the excess risk of childhood leukaemia was 12% per millisievert of cumulative red bone marrow dose from background gamma radiation (Kendall et al, 2012). The most recent comprehensive review (Wakeford, 2013) confirms these estimates.
Just to make sure the point gets across, these studies mean that all children will receive about 1 mSv of gamma radiation from background radiation each year and this results in their leukemia risk being increased by 12%.
It’s well known that leukemia is closely associated with radiation exposures and that children are more sensitive to radiation than adults. But the new evidence is not just from childhood leukemias, it comes from radon studies as well.
In Canada, following a survey of 14,000 homes with a geometric mean radon concentration of 42 Bq/m³, Chen et al (2010) from the Radiation Protection Bureau of Health Canada estimated that 16% of lung cancer deaths in Canada were attributable to indoor radon. However this fits slightly awkwardly with another large (7,000 cases and 14,000 controls) risk assessment of radon exposures (Darby et al, 2006) which estimated an excess relative risk of lung cancer of 16% (95% CI 0.05-0.31) at an average radon concentration of 100 Bq/m³. Whichever of these scientific teams turn out to be correct, the cancer risks from background radon exposures are still higher than were expected even just a few years ago.
Another very large Canadian study by Turner et al (2012) of over 800,000 Americans found that indoor radon was significantly associated with deaths from chronic obstructive pulmonary disease, ie chronic bronchitis and/or emphysema. The hazard ratio was 1.13 per 100 Bq·m?3 (95% CI = 1.05–1.21). There was a significant positive linear trend in deaths with increasing categories of radon concentrations (p<0.05). For comparison, the UK HPA’s recommended Action Level for radon is 200 Bq·m?3: indoor concentrations above this level require remediation.
And in areas with high levels of natural background radiation (usually from monazite sands), Møller and Mousseau (2012) studied radiation effects in local peoples and found increased risks in immunology, physiology, mutation and disease. They stated “.. if we see effects at these low levels, then we have to be thinking differently about how we develop regulations for … intentional exposures to populations, like the emissions from nuclear power plants, medical procedures, and even some x-ray machines at airports”.
2. Medical Exposures
Most exposures from medical diagnostic procedures are relatively low, and although their collective doses are increasing in most developed countries, in almost all cases they are justified by their clinical benefits. Nevertheless there have been a score or so of articles in scientific journals in recent years expressing concern about the risks of increased doses from CT scans, especially to children. Even the WHO has issued a draft report expressing the need for more vigilance.
In order to investigate these concerns, Pearce et al (2012) conducted a massive UK retrospective cohort study of computed tomography (CT) scans among 178,000 patients. The team estimated absorbed brain and red bone marrow doses per CT scan and assessed the excess incidence of leukaemia and brain tumours cancer with Poisson relative risk models. They observed a positive association between radiation dose from CT scans and leukaemia (excess relative risk [ERR] per mGy = 0·036, 95% CI 0·005–0·120; p=0·0097); and a positive association with brain tumours (0·023, 0·010–0·049; p<0·0001). They found CT scans caused statistically significant increases in cancer risks in under three-year olds: three head scans tripled their risk of brain cancer and five to ten scans tripled their risk of leukemia. Although the authors did not comment on these risks, there is no doubt that these are large risk increases from relatively small doses.
I shall be writing more on this matter in due course.
In Canada, similar risk increases were observed by Eisenberg et al (2011) after low-dose exposures from cardiac imaging in adult patients with acute myocardial infarction. For every 10 mSv from cardiac imaging, a 3% increase in cancer risk (RR= 1.03 per 10 mSv, 95% CI = 1.02–1.04) was observed. The authors stated “These results call into question whether our current enthusiasm for imaging and therapeutic procedures after acute myocardial infarction should be tempered.”
3. Leukemias in Chernobyl Clean-up Workers
Since the Chernobyl disaster in 1986, several studies have attempted to find increased leukemia risks among the tens of thousands of clean-up workers most of whom received relatively low doses, to little avail. This was due to the smallness of the studies and their lack of statistical power. However the latest study, (Zablotska et al, 2013) is very large (over 110,000 workers) and succeeded in finding statistically significant leukemia increases, even at the relatively low doses experienced by most of these adult workers (average dose = 92 mSv). The authors found a significant linear dose response for all leukemias with an ERR/Gy of 2.38 (95% CI: 0.49, 5.87).
4. Leukemias near Nuclear Power Stations
The final area is exposures from nuclear power stations.
Readers will be aware of my lectures showing that about 40 studies worldwide indicate increased leukemia risks among children within 5 km of nuclear power plants (NPPs). In particular, the important 2008 KiKK case-control study (discussed in Fairlie, 2009), which was commissioned by the German Government, found large increases in the risks of child leukemias and embryonal cancers near all German NPPs. This authoritative report led to geographical studies sponsored by the governments of France, UK, Switzerland and Germany. These have now been published and all four had similar findings, ie 30% to 40% increases in child leukemias near NPPs – see table from Körblein and Fairlie (2012) which contains the references to these four government studies.
Table: Studies of observed (O) and expected (E) childhood leukemias (under 5 year olds) within 5 km of NPPs
|Dataset||O||E||SIR=O/E||90% CI||one-sided p-value|
The important point here is that most scientists think that radiation exposures to local residents from NPPs are extremely small. Indeed, many nuclear scientists remain in denial about the relationship between proximity to NPPs and child leukemias despite the bountiful clear evidence which exists. Yet the evidence of child leukemias near NPPs fits well with the evidence emerging from background radiation and medical radiation.
The new studies mentioned in this post provide much food for thought. Readers should note that they are all very large studies commonly with over 100,000 data points. This means that they all have good statistical power, and the usual caveat of lacking statistical significance does not apply to them. Note also that these studies are mostly from government or academic sources- indeed some are by scientists who used to work in the nuclear industry. That is, we need to take their results very seriously.
Taken together, the new studies indicate that our current understandings about radiation risks, especially in infants and children, may be incorrect and they may need to be revised upwards. In particular, the current adult (absolute) ICRP risk for fatal cancer of 5% per Sv and the ICRP’s use of a dose and dose-rate effectiveness factor (DDREF) look increasingly out of date.
The new studies also mean that our public radiation limits and constraints may need to be revised.
Chen et al (2012) Canadian population risk of radon induced lung cancer: a re-assessment based on the recent cross-Canada radon survey. Radiat Prot Dosimetry 152 (1-3): 9-13. doi: 10.1093/rpd/ncs14. http://rpd.oxfordjournals.org/content/152/1-3/9.abstract.html?etoc
Darby S, et al (2006) Residential radon and lung cancer: detailed results of a collaborative analysis of individual data on 7148 persons with lung cancer and 14,208 persons without lung cancer from 13 epidemiologic studies in Europe. Scand J Work Environ Health; 32 Suppl 1:1-83. Erratum in: Scand J Work Environ Health. 2007 Feb;33(1):80.
Eisenberg et al (2011) Cancer risk related to low-dose ionizing radiation from cardiac imaging in patients after acute myocardial infarction. Canadian Medical Association Journal 183.4, 2011, 430-436. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3050947/pdf/1830430.pdf
Fairlie I (2009) Childhood Cancers Near German Nuclear Power Stations: hypothesis to explain the cancer increases. Medicine, Conflict and Survival Vol 25, No 3. 2009, pp 206–220.
Kendall GM, Little MP, Wakeford R (2011) Numbers and proportions of leukemias in young people and adults induced by radiation of natural origin. Leuk Res. Aug;35(8):1039-43. doi: 10.1016/j.leukres.2011.01.023. Epub 2011 Feb 21.
Kendall GM, Little MP, Wakeford R, Bunch KJ, Miles JCH, Vincent TJ, Meara JR and Murphy MFG (2012) A record-based case–control study of natural background radiation and the incidence of childhood leukaemia and other cancers in Great Britain during 1980–2006. Leukemia (5 June 2012) doi:10.1038/leu.2012.151).
Körblein A and Fairlie I (2012) French Geocap study confirms increased leukemia risks in young children near nuclear power plants. Letter to Editor. Int J of Cancer. September.
Little MP, Wakeford R, Kendall GM (2009) Updated estimates of the proportion of childhood leukaemia incidence in Great Britain that may be caused by natural background ionising radiation. J. Radiol. Prot. 29(4), 467–482.
Møller AP and Mousseau TA(2012) The effects of natural variation in background radioactivity on humans, animals and other organisms. Biological Reviews DOI: 10.1111/j.1469-185X.2012.00249.x
Pearce et al (2012) Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. The Lancet. June 7, 2012 DOI:10.1016/S0140-6736(12)60815-0, http://press.thelancet.com/ctscanrad.pdf
Turner MC et al (2012) Radon and COPD mortality in the American Cancer Society Cohort. Eur Respir J. 2012 May; 39(5): 1113–1119.
Wakeford R, Kendall GM and Little MP (2009) The proportion of childhood leukaemia incidence in Great Britain that may be caused by natural background ionizing radiation. Leukemia 23, 770–776.
Wakeford R (2013) The risk of childhood leukaemia following exposure to ionising radiation-a review. J Radiol Prot. 2013 Jan 7;33(1):1-25. [Epub ahead of print]
Zablotska et al (2013) Radiation and the Risk of Chronic Lymphocytic and Other Leukemias among Chornobyl Cleanup Workers. Environmental Health Perspectives. Volume 121 | number 1. Pp 59-65. January 2013.