Occupationally Exposed Workers
Background
The best scientific evidence of human radiation effects initially came from epidemiologic studies of atomic bomb survivors in Hiroshima and Nagasaki. While no evidence of genetic effects has been found, these studies showed a roughly linear relationship between the induction of cancer and extremely high dose-rate single high doses of atomic bomb radiation. This was consistent with the knowledge that ionizing radiation can damage DNA in linear proportion to high-dose exposures and so produce gene mutations known to be associated with cancer. In the absence of comparable low dose effects it was prudent to propose tentatively the no threshold hypothesis that extrapolates linearly from effects observed at very high doses to the same effects at very low doses.
It was accepted in 1959 by the International Commission on Radiological Protection (ICRP)1 and afterwards adopted by national radiation protection organizations to guide regulations for the protection of occupationally exposed workers and the public.2 This hypothesis that all radiation is harmful in linear proportion to the dose, is the principle used for collective dose calculations of the number of deaths produced by any radiation, natural or generated, no matter how small. The National Council of Radiation Protection and Measurements Report 121, quot;Principles and Application of Collective Dose in Radiation Protection," summarizes the basis for adherence to linearity of radiation health effects:3 "Taken as a whole, the body of evidence from both laboratory animals and human studies allows a presumption of a linear no threshold response at low doses and low-dose rates, for both mutations and carcinogenesis.
Therefore, from the point of view of the scientific bases of collective doses for radiation protection purposes, it is prudent to assume the effect per unit dose in the low-dose region following single acute exposures or low-dose fractions in a linear response. There are exceptions to this general rule of no threshold, including the induction of bone tumors in both laboratory animals and in some human studies due to incorporated radionuclides, where there is clearly evidence for an apparent threshold. However, few experimental studies, and essentially no human data, can be said to prove or even to provide direct support for the concept of collective dose with its implicit uncertainties of nonthreshold linearity and dose-rate independence with respect to risk.
The best that can be said is that most [sic] studies do not provide quantative data that, with statistical significance, contradict the concept of collective dose. Ultimately, confidence in the linear no threshold dose-response relationship at low doses is based on our understanding of the basic mechanisms involved. Genetic effects may result from a gene mutation, or a chromosome aberration. The activation of a dominant acting oncogene is frequently associated with leukemias and lymphomas, while the loss of suppressor genes appears to be more frequently associated with solid tumors. It is conceptually possible, but with a vanishing small probability, that any of these effects could result from the passage of a single charged particle, causing damage to DNA that could be expressed as a mutation or small deletion.
It is a result of this type of reasoning that a linear nonthreshold dose-response relationship cannot be excluded. It is this presumption [sic], based on biophysical concepts, which provides a basis for the use of collective dose in radiation protection activities." NCRP Report 121 summarizes that while some studies "provide quantitative data that, with statistical significance, contradict the concept of collective dose," "ultimately, confidence in the linear no threshold dose-response relationship at low doses [LNT hypothesis] is based on our understanding of the basic mechanisms involved."
Current understanding of the basic biologic mechanisms involved and recent data will be examined after presenting some of the statistically significant epidemiologic data that contradict the LNT hypothesis. The biologic data also contradict "the presumption, based on biophysical concepts, which provides a basis for the use of collective dose in radiation protection activities."