This component of the overall ERA methodology develops the functional relationships between the stressors and the ecological responses of concern. The exposure-response functions are central to ERA. Fundamentally, ERA can be described as the development and application of uncertain exposure-response functions in assessing ecological impacts. For a given stressor, these functions estimate the severity of the expected ecological response in relation to the magnitude, frequency, and duration of the exposure. The derivation of exposure-response functions depends on the quantity and quality of available data.
Sources of data that might be used in the construction of exposure-response functions include: the results of toxicity tests (acute, chronic) performed under controlled laboratory conditions, direct measures of exposure and response in controlled field experiments, and the application of statistical relationships that estimate the biological effects of chemicals based on physical or chemical properties of specific toxicants. The order of preference among these sources of data identifies field observations as the most valuable, followed by laboratory toxicity tests, and finally by the use of empirical relationships. In the absence of directly relevant data, the development of exposure-response functions may require extrapolations among similar stressors or ecological effects for which data are available. For example, effects might have to be extrapolated from an available test species to an untested species of concern. Similarly, toxi-city data might be available only for a chemical similar to the specific chemical stressor of concern. An extrapolation from the known chemical to the unknown would be required to perform the assessment.
Exposure-response functions generally increase mono-tonically and are nonlinear. Some evidence exists for certain stressors (e.g., ionizing radiation) that actually result in a positive response for very low magnitudes of exposure. This uncommon phenomenon of hormesis remains the exception to the usual sigmoid-shaped function. Depending on the nature of the stressor, the functions can exhibit a threshold magnitude of exposure required before any ecological response is observed. This value is termed a lowest observed effects concentration or LOEC. The LOEC can be used as an endpoint for comparison with exposure estimates in assessing risk. Another exposure value often used as an endpoint is the concentration (or dose) that produces 50% of the maximum response. For example, the concentration that results in 50% mortality during a prescribed period of exposure (e.g., 48, 96 h) defines the LC50 (lethal concentration that produces 50% mortality). An EC50 defines an exposure that results in a 50% decrease in an endpoint other than mortality, for example, growth.
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