U S Environmental Protection Agency

Technical Manual for Basic Version of the Markov Chain Nest Productivity Model (Mcnest) U S Environmental Protection Agency

Publisher Marketing: In the United States Environmental Protection Agency's (USEPA) current pesticide risk assessment process, a pair of laboratory avian reproduction tests with mallards (Anas platyrhynchos) and northern bobwhites (Colinus virginianus) is conducted to evaluate how dietary pesticide exposure affects a standard suite of reproduction endpoints (USEPA 1996). The results of these tests are used in calculating risk quotients (RQ) by comparing the reported no-observed-adverse-effect concentration (NOAEC) for the most sensitive measured endpoint(s) with estimates of the maximum dietary exposure expected for a given application rate. As a screening tool, RQs are compared to an established regulatory level-of-concern to categorize the potential for unacceptable risk. Because of the high degree of uncertainty in these simple tools for characterizing risk, RQs typically incorporate conservative or worst-case assumptions about exposure and toxicity to reduce the chances of concluding a chemical has an acceptable level of risk when in fact it does not (i.e., false negative conclusion). Consequently, risk quotients can be used to identify the environmental concentration above which adverse effects to avian reproduction may occur, but they cannot determine the probability or magnitude of potential reproductive effects.