Methodology for the radiological assessment of noble gases in non-human biota

A practical methodology is presented for the assessment of radon (222Rn) and thoron (220Rn) radiation dose rates to non-human biota, including animals and plants. This methodology is based on an allometric approach that scales allometrically the breathing rate and dimensions of the respiratory system as a function of whole body mass, and can calculate the dose rate to sensitive tissues and the whole body of terrestrial wildlife. We explain the derivation of the dose coefficients with all the underlying assumptions and we discuss the degree of conservatism associated with the method. A more complex, physiologically based model was developed for the plants, and we show here how it was used to demonstrate that the more simplified allometric methodology is reasonably accurate and conservative in terms of predicting internal and external dose rates to plant leaves. From the method’s application to a test case it is concluded that radon levels in some natural environments (such as mammals burrowing in soil) exceed background levels and no-effects dose benchmarks for non-human biota. This suggests that advised benchmark dose rates such as ICRP derived consideration reference levels for wildlife need to be better put into context with background dose rates, including exposure to radon, depending on the purpose of the benchmark and the assessment level. It is also clear that the contributions that radon and thoron make to radiological impact in wildlife need to be evaluated in line with emerging radiation effects data.