Lower thresholds for lifetime health effects in mammals from high-LET radiation - Comparison with chronic low-LET radiation

Lower threshold dose rates and confidence limits are quantified for lifetime radiation effects in mammalian animals from internally deposited alpha-emitting radionuclides. Extensive datasets on effects from internal alpha-emitters are compiled from the International Radiobiological Archives. In total, the compiled database includes 257 records, which are analyzed by means of non-parametric order statistics. The generic lower threshold for alpha-emitters in mammalian animals (combined datasets) is 6.6·10^-5 Gy day^-1. Thresholds for individual alpha-emitting elements differ considerably: plutonium and americium - 2.0·10^-5 Gy day^-1; radium - 2.1·10^-4 Gy day^-1. Threshold for chronic low-LET radiation is previously estimated at 1·10^-3 Gy day^-1. For low exposures, the following values of alpha radiation weighting factor w_R for internally deposited alpha-emitters in mammals are quantified: w_R(α) = 15 as a generic value for the whole group of alpha-emitters; w_R(Pu) = 50 for plutonium; w_R(Am) = 50 for americium; w_R(Ra) = 5 for radium. These values are proposed to serve as radiation weighting factors in calculations of equivalent doses to non-human biota. The lower threshold dose rate for long-lived mammals (dogs) is significantly lower than comparing with the threshold for short-lived mammals (mice): 2.7·10^-5 Gy day^-1, and 2.0·10^-4 Gy day^-1, respectively. The difference in thresholds is exactly reflecting the relationship between the natural longevity of these two species. Graded scale of severity in lifetime radiation effects in mammals is developed, based on compiled datasets. Being placed on the severity scale, the effects of internal alpha-emitters are situated in the zones of considerably lower dose rates than effects of the same severity caused by low-LET radiation. RBE values, calculated for effects of equal severity, are found to depend on the intensity of chronic exposure: different RBE values are characteristic for low, moderate, and high lifetime exposures (30, 70, and 13, respectively). The results of the study provide a basis for selecting correct values of radiation weighting factors in dose assessment to non-human biota.