The Effects of Low Level Radiation
Report of the British Pugwash Group
18 April 2000
Chair: Sebastian Pease
Rapporteur: Peter Nicholls, British Institute of Radiology, London, UK
ROGER Clarke (Chair, International Commission on Radiological Protection) spoke first on "Low level radiation". Less than a year after the discovery of X-rays by Roentgen (1895), guidelines to prevent dermal burns were developed by Fuchs. Serious international efforts had to await the end of WWI. In 1934 the accepted limit was set at 0.2 roentgens/day--about 25 times the level deemed acceptable today. In those days and for some time thereafter (the reporter has seen recent colour TV footage of people sitting in abandoned US uranium mines 'for their health') low levels of radiation and radioactivity were regarded as beneficial (radioactive underwear was the rage...).
After the bombs everything changed. By 1955 it was recognised that for at least two groups of exposed victims--radiologists themselves and the survivors of Hiroshima and Nagasaki--cancer (especially leukemia) risks were increased.
Now the ICRP guidelines are that "any risk must be kept much smaller than that from other hazards" and "the probability of developing radiation-dependent diseases, characteristically cancers, is directly proportional to the dose received". That is, there is no threshold. By the late 1970s the question had become one as to what is 'reasonable'. Utilitarian cost-benefit analysis was in vogue. The key questions were seen as: How many lives will be saved? What will it cost? Protect society, it was thought, and the individual WILL be protected.
But by the time the 1990s arrived the emphasis had changed. A concern for individual risk was uppermost. An important question was that of inequity. It was not acceptable if a single individual was at high risk even if the population at large were relatively safe. Standards must therefore address the question of the individual risk.
Now that we are in the 2000s the focus has become one of looking at individual risks, sometimes from single sources. But the threshold effect is still debated. The French Academy (the reporter notes the dependence of French industry upon nuclear power and of French military prestige upon nuclear weapons) has produced a report that says such a threshold exists. In the US Senate, Sen. Pete Domenici has introduced a resolution demanding recognition of such a threshold by bodies such as ICRP. Yet, says, ICRP, there is no threshold.
There are two ways of looking at the evidence:
- The epidemiological. For A-bomb survivors we have data down to 50-100 mGy (milligrays). It is argued that there are no excess cancer cases at these levels (some say below 200 mGy). We may note that the average "natural" background is 3 mGy for a lifetime exposure of about 200 mGy. For radium workers we have data at similar levels. No definable risk can be demonstrated at low doses although some have found that risks in utero increase for exposures as low as 10 mGy. As Joseph Rotblat pointed out in the discussion, the numbers of such cases (both populations and victims of disease) are too low for statistics to tell us anything reliably one way or the other.
- The molecular biological. DNA is the target. The cell can repair damaged DNA. But only single strand breaks in the double stranded material can reliably be repaired. Double breaks (common from radiation 'hits') can leave the molecule damaged or mutated. Under such conditions the probability of cancer seems to be increased for a single mutation. Hence, no threshold. The cell engages in adaptive responses to insult ("hormesis"). This, together with evidence for radiation-induced changes in apoptosis (controlled cell death) and immune surveillance, has suggested that low levels of damage may actually be advantageous to the tissue or at least unthreatening (radioactive underwear makes a come-back??).
Nonetheless in Clarke's view no evidence at the cellular level is available seriously to challenge the ICRP position of 'no threshold'. In the new era of 'equity-based ethics' individuals have acquired 'rights' to certain levels of protection--how much, the 'stakeholders' themselves must decide, not the experts or the government. Protect the individual, we now say, and society will automatically be protected--a reversal of the older doctrine. The result? The maximum dosage is now set at 0.3 mSv (millisieverts), giving a possible cancer risk (assuming no threshold) of 1:10^5 and amounting to 10% of the 3 mSv natural background exposure. But note that on Cornish granite the natural exposure goes to 10 mSv or even to 100 mSv in some pockets of radon accumulation. Those of us who take international flights may or may not wish to be reminded that that gives a substantial added 'natural'(?) exposure--possibly of greatest concern in the case of flight crews.
The rules require continuous dialogue. Assessment of risks as a percentage of natural background may be the most useful. This then also enables us to consider the question of environmental radiation protection policy--an area which current human-focused guidelines do not address. Because the environment is not one of individuals, such risks are direct and not statistical in nature. What will be the effect on oak trees? Or shellfish? Note that some organisms are much less sensitive to radiation than are human beings (cockroaches are the famous example) but others more so (including some plants and perhaps trees).
But justifying acceptable levels of radiation involves invoking more than science; it is also a matter of policy into which technical radiological issues are but a minor input. At the moment all we can say technically is: (i) we must control doses to all those most exposed to risk; and, (ii) such doses must be ALARP (as low as reasonably practical).
In discussion this reporter was surprised to hear that there seem to be no firm guidelines as to acceptable levels of radionuclides in consumer products. Some, of course, are deliberately radioactive (e. g. smoke alarms), others by accident (newsprint a possible case). There is a voluntary code but no governmental instructions. The National Radiological Protection Board (of which Dr. Clarke is Director) does regularly monitor the air, food samples and public water samples for us. Whatever comfort that provides.
Douglas Holdstock (Secretary, Medact) then dealt with the specific question of "Depleted Uranium". Depleted uranium (DU), left over after weapons or reactor 235U has been extracted, contains 99.8% 238U ('natural' uranium is 99.3% 238U, 0.7% 235U and a small amount of 234U). 300 tons of DU were released in the second Gulf War and about 7-10 tons in Kosovo. It is not a reactor product and contains no fission products. DU shells release up to 1kg of burning dust on impact, giving possible rise to both chemical and radiological effects.
What are the chemotoxicity dangers? Uranium is a heavy metal like a number of others (lead, cadmium etc.) and 1mg is dangerous for kidney function. But to get 1mg U to the kidney 50mg would have to be inhaled, an amount not likely to be taken up by anyone other than an unfortunate crew member of a stricken tank. In any case the description of 'Gulf War Syndrome' illnesses does not include kidney-related complaints.
What are the radiological dangers? 238U has a half life of 4.5 byr, and 235U of 0.71 byr compared to 24 kyr for 239Pu. This means that 238U, and even 235U are hardly radioactive (the weapons' explosive effect is due to nuclear fission, quite a different process). Still, what radioactivity there is involves alpha-emission, a possible inducer of 'genomic instability', as discussed by Dr. Clarke. But 100mg of U would be needed for a significant radiation dose and the descriptions of 'Gulf War Syndrome' suggest a pattern of multiple causes, to which the indiscriminate use of insecticides inside tents and the injection of anti-nerve gas cocktails including substances like prostigmine are the most likely major contributors. The reported symptoms reminded Dr. Holdstock of the recognized syndrome suffered by farmers exposed to extensive amounts of sheep dip chemicals.
Should then DU be banned from weapons? There is an inhumane weapons convention. The use of any weapon must pass the 'principle of justification'. The reporter notes that some military authorities or services have decided against using DU, perhaps because their own personnel are uncomfortable about it--but good can be done by stealth. It seems therefore not to be seen as militarily decisive. Although it may not be as poisonous as some think, it is, even just as a weapon, unpleasant or 'unknightly' (the occupants of an attacked tank have little opportunity of surrender). Its use blurs the distinctions between conventional, chemical and nuclear weapons. The absence of any preceding discussion of its development at civilian levels (our knowledge of its existence may have come courtesy of a sharp-eyed Gulf reporter and a talkative soldier) was another indication of failed civilian control of the military. A ban may therefore be a political as much as an ethical desirability. But doubtless the debate will continue.
- Clarke, R. H., & Holdstock, D. (2000) Summaries (British Pugwash Group, 63A Gt. Russell St., London WC1B 3BJ, UK).
- Fetter, S., & von Hippel, F. (1999) Bulletin of the Atomic Scientists, 55, #6, 42-45, and, for another set of viewpoints:
- Laka Foundation (1999) Depleted Uranium: a post-war disaster for environment and health. (Laka Foundation, Ketelhuisplein 43, 1054 RD Amsterdam, Netherlands).
Department of Biological Sciences
Central Campus, University of Essex
Colchester, CO4 3SQ