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Thursday, March 01, 2012
Trying to Inject Some Evidence-Based Science into the Radiation Debate
 
BY GERRY A. THOMAS, PHD

Professor of Molecular Pathology

Department of Surgery and Cancer

Imperial College London

Hammersmith Hospital

 

The first anniversary of the earthquake, tsunami, and resultant release of radiation from the damaged nuclear power plant at Fukushima in Japan takes place this month, as does the 26th anniversary of the accident at the Chernobyl nuclear power plant. These two accidents have dented public confidence in the safety of nuclear power and have prompted a revival in the public debate over human exposure to radiation in general.

 

Is the public right to be so concerned? We are a successful species inhabiting a naturally radioactive world and must have evolved protective mechanisms to deal with the effects of natural radiation. In contrast, we enjoy voluntary exposure to other carcinogens (e.g., tobacco) to which we have not yet managed to evolve protective mechanisms, and yet many seem able to accept the associated health risks.

 

Can we inject some evidence-based science into the radiation debate in order to find out just how dangerous man-made exposure to radiation might be, and turn this into some sort of risk concept that we can equate to our daily lives? In general, individuals seem to accept the use of radiation when a direct beneficial effect can be associated with it -- e.g., the use of radiation in medical tests and therapies. However, there appears to be much less acceptance of the risk when the possibility of exposure (to often much lower doses) results from emissions from the nuclear industry.

 

It is generally accepted that there is a linear, no threshold, dose-response curve to radiation. There is no doubt that this is so at higher levels of radiation exposure and that appropriate precautions must be taken to safeguard health. However, at much lower doses (less than 1 mSv), epidemiological studies lose their power to detect increases in cancer above the background level that we all accept is associated with a variety of environment and genetic factors. There are also uncertainties around single and accumulated doses of radiation.

 

Fractionated doses are given with therapeutic intent so as to limit immediate damage to surrounding normal tissue during radiotherapy using x-rays. But can we draw conclusions from this on low, fractionated doses that might be received from isotopic contamination following a nuclear accident? We are left with a great feeling of insecurity about assigning a “safe” level of exposure.

 

Our ability to measure extremely small amounts of radiation only increases this insecurity. But surely if we can measure it accurately, we should be able to define its effect on the human body accurately too?

 

It is interesting that we do not seek to define safety with respect to drug use in the same way. Many drugs – e.g., paracetamol -- have relatively small margins between beneficial and potentially toxic doses. Our safety margins with radiation appear to be much larger. Why do we interpret risk so differently in these two scenarios?

 

As a society, we have chosen to take a precautionary approach to radiation, particularly where this involves exposure of the healthy population at large. There is nothing intrinsically wrong in this approach, as we have no scientific evidence on which to base an alternative approach --- or do we? What evidence do we have from exposure of large populations of healthy people to radiation?

 

Hiroshima and Nagasaki

The majority of those who died following the atomic bombs in Hiroshima and Nagasaki died from flash burns or other injuries; 15-20% died as a result of acute radiation sickness. Long term studies (1950-1997) find that out of the 9,335 cancer deaths in the population of 86,572, only 440 (5%) of the solid cancers and 103 of the 310 leukemias were attributed to radiation exposure. Only a very small proportion (0.8%) of non-cancer related deaths were also associated with radiation.

 

In addition, there are no observable inherited effects in the subsequent generation.

 

The decrease in life expectancy is 2.6 years for those who received the highest doses, and 21 days for those who received the lowest doses. Since the majority of the population received low doses, the average loss of life expectancy is four months. The argument can be made that the full consequences will be known only when the last of the exposed generation have reached old age, but we can already ask the question: Does radiation exposure at this level cause as many health problems as we thought?

 

Chernobyl and Thyroid Cancer

The only proven radiobiological effect from Chernobyl has been an increase in thyroid cancer in those who were young at the time of the accident. The increase was rapid, and is still apparent today, although the level of thyroid cancer is back to that prior to the accident for those born after 1987, when radioactive iodine had disappeared from the environment.

 

There appears to be little difference in the type or clinical outcome of radiation-induced thyroid cancer when compared with age-matched controls. Thyroid cancer is very amenable to treatment and although 30% of patients may suffer a relapse, only 1% may eventually die of their disease.1 Of the approximately 6,000 diagnosed cases since 1986, only 15 have so far proved fatal..2 Many of these cases would have been prevented if better measures to limit exposure to radioiodine had been put in place.

 

There is, thus far, no evidence for increases in other diseases in the exposed population at large.

 

Now that the human population has been reduced as a result of the establishment of the exclusion zone, the thriving natural environment around the reactor accident suggests that the presence of higher than background levels of cesium-137 in the environment poses little risk to human or animal health. Lifespan studies, similar to those carried out in Japan, would be needed in order to identify any further minor health effects. However, these will be expensive to perform, and may well be expected to give results that will not satisfy the concerns of those who have already made up their minds.

 

So, what have we learned from Chernobyl? That cancer risk is determined by the age at exposure and concentration of radioisotopes in particular tissues. Low-dose exposure to cesium-137, even over a long period of time, is perhaps not as deleterious to health as we would have predicted.

 

The one thing we appear not to have learned is how to deliver information about radiation risk to an exposed population. There have been considerable psychological consequences, unrelated to the direct radiation risks, from the Chernobyl accident, which have been inadequately quantified.

 

Fukushima

And now, let’s consider Fukushima. Here the release of radiation was much lower than that which emerged from Chernobyl (approximately one quarter with respect to radioactive cesium). And the measures introduced by the Japanese government to limit exposure will have made a huge difference to effects on the population’s health:

  • evacuation from the areas closest to the plant;
  • recommendation to stay indoors with windows closed;
  • not to hang washing outside, for those further away; and
  • to limit access to contaminated milk and vegetables.

There were various factors peculiar to Japan that further reduced the amount of radioiodine detected in the thyroids of Japanese children: It was cold at the time of the accident and cows were, in general, in barns rather than in pasture; the Japanese population is iodine replete (in contrast to the areas around Chernobyl, where people were moderately iodine deficient); and the Japanese have little dairy produce in their diet.

 

We know from studies on populations irradiated with curative intent and from the Chernobyl population, that the child’s thyroid is the most sensitive organ to the effect of radiation. The Japanese have launched an extensive

monitoring program to screen the 360,000 children who were exposed to radioiodine, and to follow them up with regular checks every two years until age 20 and every five years thereafter.

 

Despite all of this, there is still considerable anxiety over the possible long-term health effects. But we cannot escape the fact that few members of the population in Japan will have received a total dose of radiation (even allowing for the longer lived isotopes released such as Cs-137) greater than that of a single CT scan. Even in the most heavily contaminated areas around Chernobyl, the whole-body doses for those residing in the 50 km exclusion zone over a 19-year period equate to 50mSv, and for those living further away, 10 mSv.

 

Compare this with the average dose of 7-9 mSv for a single CT scan or 1 mSv per year (70-80 mSv over a lifetime) in terms of exposure to natural background radiation, and this provides some context for understanding the health risks posed by these accidents.

 

Of course, we do not yet have the scientific proof that even a single CT scan does not increase the possible risk of developing cancer by an infinitesimally small amount. However, there are a number of studies currently under way focusing on long-term effects on children following CT scans. If there is to be any risk that can be assessed with confidence, we have to turn to a young cohort that we already know is more susceptible for detrimental effects on health from radiation exposure, and try to extrapolate from there.

 

Fear the Greatest Risk

There is no doubt that the greatest risk to the exposed Japanese population is fear of radiation, rather than any direct effect on health. But an enormous amount of money and effort will go into trying to identify the infinitely small radio biological effect on this population. Unfortunately, as with Chernobyl, it is likely that much less funding will be invested into identifying the much greater effect of the myths surrounding the real health effects of radiation.

 

Radiation risk must be put into context. The consequences for the most exposed group of atomic bomb survivors was an average loss of life expectancy significantly lower (2.6 years) than that caused by severe obesity or smoking (10 years).3  We cannot escape the fact that exposure to radio iodine in fallout from a nuclear accident potentially has an effect on thyroid cancer in the young. However, this is rarely fatal, and, more importantly, we know how we can limit this exposure.

 

A rational debate about the effects of medical radiation and nuclear power generation means putting the risks and benefits into perspective based on sound scientific studies. Unfortunately, all too often, it seems that when radiation

knocks at the door, science and rational thinking go out of the window.

 

 

REFERENCES

1.     Chernobyl 25 years on: The long term health effects. Special issue of Clinical Oncology, May 2011, (Vol. 23, Issue 4. http://www.journals.elsevierhealth.com/periodicals/chernobyl

 

2.     UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation, 2008 Report to the General Assembly, Effects of Ionizing Radiation. http://bit.ly/xqVi7z  

3.     Smith JT: Are passive smoking, air pollution and obesity a greater mortality risk than major radiation incidents? BMC Public Health 2007; 7:49 doi:10.1186/1471-2458-7-49  http://bit.ly/zqjQbL