For assessment purposes, ERAs commonly use the concept of a screening index (SI) which is simply the ratio of the estimated dose rate (to an individual biota) to the reference dose rate, viz.

Estimated dose rate Reference dose rate

This comparison assumes that the numerator and the denominator of the SI are based on a common assessment of dose relevant to the endpoint of interest (e.g., mortality, reproductive capacity).

When the estimated SI is below 1, it is considered that an effect to a (population of) biota is unlikely. When an SI is estimated to be greater than 1, an effect may be possible and further detailed evaluations are carried out to investigate whether the SI being greater than 1 is an artifact of undue conservative assumptions and the nature of the calculation, or whether an actual effect might be possible. At this time, the reference dose rates described in the previous section provide an appropriate basis for assessment.

The interpretation of SIs also needs to consider aspects such as the spatial and temporal distribution of radiation, the natural cycles of populations, and background radiation levels.

Arctic caribou provide one example of a situation where natural background can result in quite high doses to biota. Caribou and reindeer subsist on lichens during the winter months but must graze over a wide area to obtain enough nourishment. Lichens do not have a root system but are highly efficient at collecting and retaining nutrient material deposited on their surfaces. Furthermore, lichens do not turn over annually but integrate all deposited materials, including radionuclides, over several decades. Hence this food chain integrates radioactivity not only over time, but also over spatial area. Estimated radiation doses to various tissues of caribou taken from areas with natural radiation in Nunavat, Canada, are over the reference dose rate. Therefore, it can be important to consider natural background in ERA calculations.

It is evident that there is a wide range of complex factors to account for in the extrapolation of dose estimation from an individual level to the population level. When applying the SI approach to nonhuman biota at the individual level, great caution is therefore necessary about the interpretation of the predicted outcomes (as in most cases they would be overly conservative at the population level). A reliable interpretation also requires knowledge on the extrapolation from the individual level to the population level of both nonhuman biota dose rates and the related potential effects.

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