Appendix 4.1

Comments by Australian Radiation Laboratory

on Reduced-Biological Effectiveness at Low-Rate, Low-Dose Exposures (DREF): An Unwarranted Conjecture, by Wolfgang Kohnlein and Rudi Nussbaum

The general assertion in this paper is that the conventional radiation protection recommendations and methodologies for controlling exposure radiation, both in Australia and overseas will underestimate the cancer induction risk for low exposure rates and low total exposure. The inference is that current standards for limiting radiation exposure may be inadequate. The paper is selective in the choice of supporting literature, but the general line of argument deserves consideration. In view of time restraints the present comments are restricted to potential impact on the risk assessment for uranium mining.

There are two potential pathways for radiation exposure during uranium mining: external gamma-ray exposure and internal exposure to radionuclides through inhalation or ingestion. There are two different exposure groups: workers and the general public. For workers the current radiation dose limit is 20 mSv per year. For members of the public the incremental radiation dose arising from mining activities is restricted to 1 mSv per year. This is additional to the radiation dose from natural background of approximately 2 mSv per year.

The radiation risk for external exposure is based on variety of epidemiological studies of high exposure and high exposure rate, particularly the study on the A-bomb survivors. The content of the Kohnlein paper deals with the extrapolation of these studies to low exposure/low exposure rate situations. It should be noted that the increase in the risk estimates from the A-bomb study has been due to improvements more to changes in the dosimetry (how much and what type of radiation) and to the use of a multiplicative model instead of an additive model, rather than to the affects of long delayed and increasing mortality rates, as suggested in Kohnlein's paper. Kohnlein points out in his paper that the A-bomb survivor studies are consistent with a linear, non-threshold dose response and he argues against the use of a reduction factor (DREF) for low dose rate and low dose exposures.

The crucial issue raised in this paper is his assertion that the risk factors for these low dose rate exposures are in fact up to a factor of 10 higher than the values that the linear hypothesis would project. This issue needs to be considered in the light of the two types of exposure pathways.

- for uranium mining situations, the two principal sources for internal exposure are inhalation of radon progeny and inhalation of radioactive dust. There is an extensive literature on the increased incidence of lung cancer amongst uranium miners, and the derived risk factors range across a factor of four about a mean value of 2.5 per cent per WLM (BEIR IV, 1988). A recent paper by Lubin et al. (1995), pooled the results of 11 cohort studies of underground miners to investigate the presence of an inverse dose rate effect. They found that the data confirmed an inverse dose-rate effect for high-LET radiation, but there appeared to be a diminution of the effect below 50 WLM. The results of this study would not support the claim that risk factors for radon progeny inhalation should be increased by at least a factor of 10, even for low dose-rate exposures.

- for radioactive dust, the risk assessments are based on the use of dosimetric models of the inhalation the radioactive material. The most recent model is that devised by a Task Group of the International Commission on Radiological Protection (ICRP66). For the one case where the model can be compared with the results of epidemiological studies (radon progeny), the model produces risks that are a factor of 3 higher than the uranium miner worker studies. If, as suggested by Kohnlein, the Relative Biological Effectiveness of high-LET radiation should be increased by more than a factor of 10, then the difference between the model predictions and the epidemiological results would blow out to a factor of 30.

Kohnlein's work highlights the complexity of assessing risk for radiation exposure, particularly for high-LET radiation and for low doses and dose-rates. An increase in the values used for radiation risk assessment, particularly at low dose rates, has to be consistent with the observed cancer rates and the contribution to the cancer risk arising from the natural background radiation. If there was a significant increase in the risk factors at these low dose rates then variations in natural background levels should lead to changes in cancer rates: this has not been found to be the case. In their paper on Radiation and exposure rate, Darby and Doll (1990) state:

". . . for non-smokers an upper limit (for radon risk) can be derived from the observation that their lifetime risk of lung cancer of about 0.6 per cent is, with few exceptions, remarkably constant in all populations for whom data is available, despite a variation of at least twofold in the average national exposure to radon indoors."

Dr Stephen B Solomon

Leader

Health Physics Group

Australian Radiation Laboratory

09 May 1997

References

Committee on the Biological Effects of Ionizing Radiations (BEIR IV). National Academy Press, Washington, DC, 1988.

Lubin, Boice, Edling, Hornung, Howe, Kunz, Kusiak, Morrison, Radford, Samct, Tirmarche, Woodwaed and Shu, Radon-exposed underground miners and inverse dose-rate (protraction enhancement) effects. Health Physics 69(4):494-500, 1995.

International Commission on Radiological Protection (ICRP) Human Respiratory Tract Model for Radiological Protection. A Report of Committee 2 of the ICRP. Oxford:Pergamon Press; ICRP Publication 66, Ann. ICRP 24(1/4) (1994).

Darby and Doll. Radiation and exposure rate. Nature 344:824, 1990.