Risk Estimates of Low-Level Ionizing Radiation.
There has never been any doubt about the disastrous short - and long-term effects of exposure to high doses of ionizing radiation. However, the public acceptance of billion dollar investments in the fifties and sixties in a runaway build-up of nuclear weapons production, civilian nuclear energy, as well as nuclear medicine, was based on the confident assurances of enthusiastic radiation experts that added exposures at dose levels acceptable to industry would not be found detrimental to human health.
Premature evaluations of the delayed effects of radiation among the Japanese survivors, as well as transference to humans of radiation effects in animals, lead to strongly held optimistic convictions. For decades, any challenge to these tenets based on extrapolations by epidemiologic studiey among populations with occupational and environmental exposures, were received with enmity and rejection. Nevertheless, optimistic expectations had to be scaled back continuously as the appearance of long delayed excess cancer cases among the Japanese survivor population (see Table 1).
Our present regulations for radiation protection depend almost exclusively on the recommendations of the international commission on radiological protection (ICRP). The ICRP itself relies on published data and recommendation of it's subcommittees. The main source of information on radiation risk and radiation hazards taken from the experience of the atomic bomb survivors. The official risk estimates of this source have changed over the years. In 1977 the ICRP published a figure of 1,25 additional cancer death per 10,000 person cSv. This figure was revised in 1990 to 5. Other organisations and independent scientists arrived at much higher risk estimates (see. Table 1).
|Source||Cancer death per 10,000 person cGy|
|BEIR (1972)||1,17 - 6,2|
|UNSCEAR (1977)||0,7 - 1,7|
|BEIR (1980)||1,5 - 5|
|Charles et al. (1983)||1,0 - 4,4|
|Preston + Pearce (1987)||5,8 - 18|
|UNSCEAR (1988)||4,2 - 11|
|BEIR V (1990)||5,4 - 12,4|
|Nussbaum + Köhnlein (1990)||25|
The population of Hiroshima and Nagasaki was exposed to an acute very short radiation flash and experienced extrem and traumatic hardship in the weeks and months after that event. The very strong and resistent survived situation at a higher rate than the weaker individuals among the unborn, the very young and the old. Thus the surviving Hiroshima/Nagasaki collective represents an extraordinarly selected healthy population quite, uncomparable to a normal city population.
The radiation situation at the working place (Uranium miners, nuclear workers, medical personal etc.) is completely different from the a-bomb survivors exposure. Radiation risk derived from one type of exposure condition cannot be applied to a completely different exposure condition. There is no universal dose effect relationship.
Already for these reasons one has to challenge the normally used procedure of the official commissions. Furthermore the radiation standards were derived from the Hiroshima/Nagasaki cohorts exposed to rather high doses (> 100 cGy). Occupational radiation exposure is normally at much lower doses (< a few cGy per year). Thus for risk estimates at low doses one has to extrapolate from the observed cancer and mutation rates at high doses. Different procedures have been proposed in the past for this extrapolation (see Figure 2). In the last twenty years there has been a steady growth in the number of independent investigations of populations exposed to low level ionizing radiation where adverse health effects have been found. Radiation effects have been confirmed repeatedly in cohorts exposed to diagnostic x-rays, nuclear weapons fallout, and occupational exposure rangeng from unanium miners, nuclear dockyard workers, workers in other nuclear facilities to flying personal and cabin attendents of civil air lines.
Time is now running out for the risk estimates supporting the official interagency policy that low-level ionizing radiation is "harmless". The reason for this is that during the "harmless" era there were many unnecessary and avoidable exposures to low-level radiation.
Newer studies reported 0.05 Sv as the doubling dose for leukemia, lung cancer and other cancer forms while the official estimates in interagency reports are still well over 1 Sv.
This again suggests that the actual risks are more than 20 times the official ones. The agencies responsible for setting radiation protection standards refuse to use the new data for risk estimates because to there opinion the older data like the Japanese A-bomb data is the best available. There is, however, no biostatistical warrant for this claim. From a scientific standpiont a population of healthy worker who never been exposed to high doses of radiation is much more informative than a population of sick persons or survivors of the A-bomb who may have been exposed to one Sv and more. Continuous and concurrent dosimetry for monitoring uranium miners, nuclear dockyard workers and workers in other nuclear facilities is far superior to retrospectve dosimetry that is based on assumptions which are now in serious question. Finally, good statistical practice says that you never extrapolate far beyond the range of the data when good data in the right range is available.
With the much better data and direct risk estimates available today, scientific evaluation of radiation risks should replace the obsolete older estimates by the newer ones. That this did not happen in the latest reports of the official commissions suggests that official estimates are no longer a scientific product but rather a political one.
|Characteristics of the data||Data available today||Data used in official reports|
|Who are the persons under study?||Nuclear workers under normal working conditions; Uranium miners; medical X-ray exposed persons||Survivors of A-bomb, persons with grave disease requiring therapeutic X-ray|
|What are the dosages used?||Low-level radiation||Dosages in most subjects well over 1 Sv|
|What is the qualitiy of the dosimetry for persons under study?||Continuous monitoring of the exposure with the recording of relevant data for each individual||Retrospective estimates of exposures without adequate crosschecks or control of the dosimetry|
|What is the quality of the follow-up for the persons under study?||Virtually complete with full deaths certificates and other information||Incomplete and often inadequate follow-up and poor quality of information on individuals|
|What was the quality of information used for comparison?||Unexposed reference populations; national standard mortality ratios||Biased comparison series (e.g. in some A-bomb comparisons, persons composed up to 0,1 Sv are used as controls)|
|What assumptions are necessary for estimates of doubling dose or other quantitative measures of health effects?||Estimates can be made directly without any strong assumption or extrapolation beyond the range of data available||Estimates require assumpti-ons of dubious "linear" or other hypotheses and are merely guesswork|
As long as official risk estimates for radiation technologies ranging for Uranium mining to permanent disposal of radioactive waste are different by more than an order of magnitude from those of modern epidemiological studies one has to invoke the "primary principle":
In dealing with potentially hazardous technologies the benefit of the doubt must go to the public and not to the technologies.
In practical terms this means that the critics of a technology must persent present sound evidence of the hazard and after this the burden of proof shifts to the proponents to show that the technology is save.
If we consider that 0.02 Sv per year exposure is currently allowed for nuclear workers by the national nuclear regulatory commissions then a doubling dose for lung cancer and leukemia will accumulate within a three years time. A dose that doubles the risk of a fatal disease in such a short time is a serious public health hazard. Since the proponents of nuclear technologiey failed to prove that such exposures are save permissible doses should reduced drastically as claimed for many years.