As previously noted, ionizing radiation is ubiquitous and all living things are, and have always been, exposed to naturally occurring radiation and radioactivity. Moreover, natural background levels of radiation vary widely from place to place; levels of radioactivity in soils, sediments, or pathways of exposure may result in elevated intake of natural radionuclides by biota. In this respect, it is important to understand that there may be practical difficulties in establishing protection criteria for radiation which is ubiquitous in the natural environment in comparison to many chemicals that are not naturally occurring.
In addition, nonhuman biota are exposed to external radiation from sources outside of the organism and from radiation arising from radionuclides taken into the organism. As noted earlier, many simplifying assumptions are needed to allow a practical estimation of doses from external and internal radiation, one example being the common assumption that radionuclides taken into the organism are uniformly distributed throughout the organism. Such assumptions introduce uncertainty into the assessment of risks to nonhuman biota.
Further uncertainty arises from the selection of a reference dose rate below which population-level effects are unlikely. As discussed in an earlier section, such data are not available for all biota and endpoints and professional judgments must be made to select the appropriate reference dose rate. At this time, the reference dose rates established by UNSCEAR are suggested for use in ecological risk assessments.
Recently, the European Community Sixth Framework Programme project ERICA (Environmental Risks from Ionizing Contaminants Assessment and Management), the successor to the FASSET project, adopted an ERA tiered methodology that requires risk assessment screening dose rate values for the risk characterization within tiers 1 and 2. ERICA adopted a 10 pGyh-1 screening dose rate based on the analysis of the chronic exposure data and incorporating safety factors.
The concept of an SI is widely used as the basis for risk assessment. Such an evaluation can be done using either very conservative assumptions about exposure or with more realistic assumptions about exposure conditions in a stepwise manner depending on the type of risk assessment. The key to the stepwise (tiered or graded) approach is to progress from a very conservative screen and, if necessary, move progressively to more realistic and less conservative assessments. The caveat in the process is that as one moves to more realistic assessment, there is an increasing demand for data and evaluations that involve detailed site-specific models.
ERAs are expected to continue evolving in light of recent developmental work (such as the ERICA project), the forthcoming update of the UNSCEAR report on effects of ionizing radiation on nonhuman biota, and further developments by ICRP in this area of emerging expertise. Moreover, the IAEA coordination group on the radiological protection of the environment provides a relevant platform for discussions about the international acceptance and harmonization of these developments as they arise. Although still under development, such approaches may find increasing utility in the future.
See also: Dose-Response.
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