Updating the U.S. EPA IRIS Assessment of Methylmercury
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As a result of the hundreds of studies published in the last 20 years that assess the health effects of methylmercury exposure, the U.S. EPA determined that a re-evaluation of the dose-response of methylmercury was warranted. The existing methylmercury assessment, published by the EPA Integrated Risk Information System (IRIS) program in 2001, was based on an analysis by the National Academy of Sciences and considered three cohorts of children prenatally exposed to methylmercury. This assessment derived a reference dose (RfD)--an estimate of a daily oral exposure to the human population, including sensitive subgroups such as children, that is not likely to cause harmful effects during a lifetime—of 0.1 µg/kg-day based on neurodevelopmental effects. Specifically, 7-year-olds exposed prenatally to methylmercury demonstrated cognitive impairment as evidenced by their performance on the Boston Naming Test. Since the publishing of the 2001 IRIS Methylmercury Assessment, the number of cohorts and corresponding epidemiological publications has increased significantly. For example, a recent IRIS literature search identified more than 200 epidemiological publications with dose-response information on neurodevelopmental effects of methylmercury exposure pre- and/or post-natally, based on dozens of epidemiological cohorts. This presentation will describe the initial steps in the development of an updated IRIS Assessment of Methylmercury. Specifically, it will focus on the approach to assessment development, including the decision to evaluate developmental neurotoxicology first; the literature search strategy; specific aims of the assessment; identification of key scientific issues; and study quality evaluation. This presentation will also discuss considerations in evaluation of the new data available for an updated quantitative methylmercury assessment based on the literature search results.