Radiation therapy is responsible for extending the life of many people, and the application of radiation and radiopharmaceuticals has led to greatly improved medical diagnoses and subsequent treatment of injuries and diseases. More indirectly, applications of radiation and radionuclides in industry have led to economic benefits that have improved living standards and, consequently, better health. But the question as posed often is referring to small doses of radiation and the direct effects on the individuals exposed. Here, the story is much more complicated
All living organisms have evolved and exist in the presence of ionizing radiation. Consequently, it is logical to expect that organisms have “learnt” or “adapted” to cope with the damaging effects of ionizing radiation and, indeed, biological processes have been found that do repair damage to cellular constituents like that caused by overexposure to potentially toxic agents like ionizing radiation. One can press this idea further and ask whether the stimulation of such repair processes results in the organism being able to cope better with other subsequent potentially toxic agents. This can include further radiation exposure, or other completely different agents including heat, metals, and chemicals. This type of effect has actually been observed in many studies with micro-organisms, plants and animals. One could even ask whether such stimulation has become an essential feature of life and that without it, organisms will not be as healthy and, further, that a little more ionizing radiation might have a beneficial effect on the life of an organism – in other words, a “hormetic” effect (from “hormesis” – a well-known effect where low doses of common toxins, like arsenic, are good for you but high doses aren’t). There is certainly evidence for all these effects from particular radiation experiments with animals. The question is to what extent they may be generally applicable to humans.
The damaging effects of high doses of radiation on health are well documented. Indeed the effectiveness of radiation therapy relies on killing cancer cells. Doses of ionizing radiation lower than those used in therapy, but still hundreds of times higher than occupational and environmental levels, are known to increase the likelihood that some kinds of cancer will occur sometime (years) after the exposure. This was recognized, more than 50 years ago in the increased incidence of leukemia in various groups that had been exposed to high doses of radiation: children after they had been exposed to X-rays for various medical reasons during infancy or prenatal care, radiologists, and the Japanese survivors of the Hiroshima atomic bomb. Therefore, scientists concluded at the time (in 1958) that the most conservative approach for protection was to assume that the incidence of leukemia would be proportional to the accumulated radiation dose, although there could be a dose below which there would be no negative effect. At high doses (about 100 chest X-rays all at once or about 100 mSv) data generally support the idea that the increased incidence of some cancers is proportional to the accumulated radiation dose. It has been possible to estimate the relationship between radiation dose and increased incidence of cancers (the “risk coefficient”) that do appear to be induced by high doses of radiation. The Japanese survivors of the atomic bombs in Hiroshima and Nagasaki have provided most information for this.
For very low doses, like we are exposed to every day from nature, or from dental X-rays, airplane flights, or working near radiation like nuclear medicine and nuclear power stations (i.e. doses below 100 mSv), it is not possible to determine from A-bomb survivor data what the relationships are between radiation dose and cancer. However, just to be safe, the approach for protection purposes assumes all radiation can be detrimental and should be avoided or kept to levels “as low as reasonably achievable” (ALARA).
In other words, the cancer risk is assumed to be proportional to radiation dose, all the way from zero to doses where effects have actually been observed. This is the “linear non-threshold” model (LNT) that is the basis for regulations on radiation around the world. It is generally assumed that this model is “reasonably conservative” – that is, it errs on the safe side without going overboard. It has the practical advantage that protection against radiation can be considered on its own, without knowledge of previous radiation dose (e.g. from a medical examination, or from radon at home).
There is dissent on the appropriateness of the LNT model, however, in both directions, and advocates of the conflicting positions can point to experimental data in support of their claims. The suggestion that the LNT model under-estimates the actual risk is based on some selected data from population studies (childhood leukemia from nuclear test fallout) and on experimental observations that some animal cells that are close to a cell that has been hit by radiation appear to be genetically affected even though they have not been hit. (This is called the “bystander” effect.) However, this suggestion is generally regarded by radiobiologists and epidemiologists as having no sound scientific basis. There is the view that there is a threshold below which radiation doses have no harmful effect. There is also the view that some increases in radiation exposure have a beneficial effect on health. Even with the much more extensive data now available than was the case when the LNT model was first adopted, it is still not possible to determine unequivocally whether there is or there is not an increased risk of cancer at a doses of a few tens of mSv.
However, everyday our understanding of genetics, biochemistry, and processes involved in cancer formation become stronger, and scientists will continue to collect information that will increase our knowledge about low-dose effects. Currently, there is strong evidence that shows that not all radiation may be detrimental. For example, the risk of cancer following consumption of radioactive paint used historically for making watches glow in the dark (radium dial painters) showed that low doses did not cause bone cancer. In the case of the Japanese atomic bomb survivors, the incidence of leukemia at low doses was not significantly raised. Some scientists have interpreted the leukemia data as indicating a reduced incidence in the low dose range but, in reality, the statistical variation weakens this interpretation. There have been other studies, such as of human populations that live in regions with high natural background radiation (tens or hundreds of mSv per year), or of animals in the region around the Chernobyl nuclear power plant, where the absence of effects or of beneficial effects have been claimed, but none of these studies have overcome statistical limitations and practical difficulties. Evidence for beneficial effects on human health from radiation exposures continues to be much sought after but, so far, it remains elusive.
From the above discussion there are two implications for protection against ionizing radiation:
- Whether there are harmful or beneficial effects from small increases in radiation dose above our normal background levels, any such effects are very small, may well be zero, and are lost in impacts of everyday living.
- Given the absence of unequivocal evidence for any harmful or beneficial effects at low doses, there is no justification for trying to estimate, with the scientifically-accepted risk coefficient mentioned above, increases in cancers in a population that has received small increases in radiation doses.
Osborne, Richard.(Sept 2008). Can radiation have health benefits? http://www.nuclearfaq.ca/cnf_sectionD.htm