Recommended RBE weighting factor for the ecological assessment of alpha-emitting radionuclides

Over 90% of the radiation dose received by biota exposed to radionuclides released from uranium mines and mills in Canada is from internally deposited alpha-emitters. Therefore, to properly characterize the potential environmental effects of ionizing radiation resulting from such exposures, it is necessary to take into account the greater relative biological effectiveness (RBE) of alpha particles. There is currently no consensus on the value of a radiation weighting factor to be used in calculating doses to non-human biota from exposures to alpha-particles. Values on the RBE of alpha particles have been derived in a broad range of experiments. Experimental results suggest that RBE values tend to be higher at the lower dose rates and when the irradiation is from radionuclides incorporated into tissue rather than external radionuclides. Therefore, to obtain an RBE value that has the greatest ecological relevance, the available data were reviewed and studies conducted in whole organisms at relatively low dose rates were given the most weight. Primary studies using endpoints such as mortality, reproduction and immune function provided the following RBE values: 40-50 (life shortening in mice); 47-377 (best estimate = 80) (oocyte mortality); 130-180 (hemopoiesis- acute gamma reference); 250- 360 (hemopoiesis- chronic gamma reference). Based on the available data, we recommend a RBE value of 40 for the ecological risk assessment of biota chronically exposed to alpha-emitting radionuclides. This value is representative of the RBE data on life shortening in mice and is in the low range of values obtained for other endpoints. A value of 40 for chronic exposures is realistic and is not considered overly conservative particularly because there are little data on the biological effects of alpha emitters on a wide range of aquatic and terrestrial species. (author)