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OCR for page 317
Issues in Risk Assessment
EXPOSURE ASSESSMENT
B. Leaderer and D. Porcella
The exposure assessment group agreed that, for applicability to ecological problems, the definition of exposure assessment should be generalized to accommodate both nonchemical and chemical stressors. The following definition was proposed: assessment of the extent and nature of the stressor and its co-occurrence with the target. Stressors can be physical, chemical, or biological. Examples of physical stresses are habitat loss, thermal loadings, and UV radiation. Chemical stressors include toxicants and nutrients. Biological stressors include species introductions and pest organisms.
Targets for exposure assessment can be at any level of biological organization, from individual organisms to ecosystems and the biosphere. Exposure assessment can involve direct measurement, indicators of exposure, and modeling. Extent refers to the magnitude and spatiotemporal distribution of the stressor. Nature refers to the characteristics peculiar to the stressor (e.g., physical and chemical properties of a chemical contaminant).
Methods of Measuring Stressors for Ecological Exposure Assessment
The group identified a wide range of methods applicable to measuring ecological exposures. The most obvious methods are the same kinds of direct and indirect methods used in human exposure assessment, including measurements of environmental contaminant concentrations in media to which organisms are exposed, measurements of uptake or body burden, and measurements of biochemical markers correlated with contaminant exposure.
Larger-scale tools for exposure assessment include remote sensing (habitat, productivity, and albedo) and aerial and ground-based mapping. Those methods are especially appropriate for such assessments as in the spotted owl study, in which habitat change, rather than a contaminant, is the stressor. Some participants suggested that ecosystem characteristics (measures of structure and function) can be used to quantify exposure.
