|In its submission to the ICRP on its 2006 Draft Recommendations, the Australasian Radiation Protection Society (ARPS) compares ARPS 2005 Position Statement on risks from exposure to low levels of ionizing radiation with the approach taken to this matter in the Draft Recommendations, and comments inter alia that:
“If the ICRP does not at least allow the possibility that the LNT hypothesis, while being a practical guide for radiation protection practice, may not apply at …. dose rates [around natural background levels], it could be faced by the accusation in another 15 years that it has grossly overestimated risks from low doses, and thereby given rise to undue concerns about low level radiation.”
I have recently been reviewing a number of papers on the biological effects of low level radiation for presentation at the 15th Pacific Basin Nuclear Conference (15PBNC), which is to be held in Sydney, 15-20 October 2006). Amongst these is a paper entitled “Cancer and Low Dose Responses in Vivo: Implications for Radiation Protection” by R E J Mitchel, a leading research worker in this field, working at the Chalk River Laboratory of AECL in Canada. Some extracts from Dr Mitchel’s paper are as follows:
“For malignant transformation in human and rodent cells, the protective effect of low doses is dose independent for all doses up to about 100 mGy, when given at low dose rate. Above about 300 mGy, these protective effects give way to an increased risk of malignant transformation, suggesting detrimental effects outweigh protective effects at this point.”
“….different tissues appear to have different thresholds at which protection turns to detriment. The results suggest that protective adaptive responses may predominate at typical public and occupational exposure levels, but that at doses around 100 mGy detrimental effects may overcome the protection. High doses at high dose rates do not induce the protective response, although relatively high total doses received at low dose rates may be effective.”
“Adaptive responses to low doses (typically 1 - 100 mGy) have been shown to increase cellular DNA double-strand break repair capacity, reduce the risk of cell death, reduce radiation or chemically-induced chromosomal aberrations and mutations, and reduce spontaneous or radiation-induced malignant transformation in vitro. Elevated DNA repair capacity after low dose exposure is a response that has been tightly conserved throughout evolution, appearing in single-cell eukaryotes, simple eukaryotes, insects, plants, amphibians, and mammals including human cells, suggesting that it is a basic response critical to life.”
“…. increasing dose by adding low doses to high doses decreases risk…. However, once past the upper dose threshold, increased dose could increase risk, as currently assumed.”
“These observations show that low dose responses are non linear and that the biological processes occurring in cells in response to low doses and dose rates can be fundamentally different from those that result from exposure to high doses. These observations undermine the concept of DDREF and indicate that at low doses DDREF becomes infinite.”
These findings stand in stark contrast to the view expressed by the ICRP in Clause 55 of its Draft Recommendations, that:
“…. in the low dose range under 100 mSv it is scientifically reasonable to assume that the increase in the incidence of cancer or hereditary effects will rise in direct proportion to an increase in the absorbed dose in the relevant organs and tissues.”
Dr Mitchel’s paper is not an isolated publication of new or controversial findings. It is part a very substantial body of scientific literature which refutes the LNT assumption. This body of literature includes:
• earlier publications by Dr Mitchel and by many other reputable scientists over many years;
• most of the papers on this subject that have been submitted for presentation at 15PBNC and of those that were presented at 14PBNC (two years ago); and
• many of the publications that were considered by the ARPS Working Group which drafted the Society’s Position Statement on Risks from Exposure to Low Levels of Ionizing Radiation (mentioned above and appended to the ARPS submission on the ICRP Draft Recommendations).
The ICRP appears neither to have refuted this body of work nor to have recognised it in the Draft Recommendations.