The Perchlorate Advisory Panel of EPA’s Science Advisory Board (SAB) met for two days in July to discuss recommendations for setting a Maximum Contaminant Level Goal (MCLG) for perchlorate. EPA made a determination to regulate perchlorate in February 2011 and is required, by statute, to release a proposed rulemaking within two years of that regulatory determination. Establishing an MCLG, defined under the Safe Drinking Water Act (SDWA) as the level at which no adverse health effects are expected from exposure to a given contaminant in drinking water, is a critical part of that rulemaking process. An MCLG is not an enforceable standard, but EPA is required to set the enforceable standard, the Maximum Contaminant Level, as close to the MCLG as feasible.
Within that context, the Perchlorate Advisory Panel members were charged with evaluating EPA’s latest approach in setting an MCLG. EPA’s current approach is documented in a white paper, “Life Stage Considerations and Interpretation of Recent Epidemiological Evidence to Develop a Maximum Contaminant Level Goal for Perchlorate,” developed for the Panel’s review. The EPA’s white paper approach is based on a reference dose (RfD) for perchlorate that was recommended in a comprehensive 2005 National Research Council (NRC) report. However, this approach differs from the NRC approach due to the direct application of the RfD to specific life stages in its analyses. The NRC report applied the RfD based on the characteristics of a healthy adult and then accounted for sensitive life stages by applying a safety factor to a perchlorate level at which no health effects are observed.
Though its final recommendations are still in development, by the end of the two-day meeting it was clear that the Perchlorate Advisory Panel was leaning toward a recommendation for MCLG development that differed significantly from both the NRC or EPA white paper approaches. The preliminary recommendations of the panel would not use the NRC-developed RfD at all, but would directly calculate an MCLG using physiologically based pharmacokinetic (PBPK) modeling. Part of the charge to the panel was to evaluate the potential of PBPK modeling in the analyses, and during its deliberations the Panel came to the conclusion that such modeling represented the best path forward to a comprehensive and defensible analysis of all the available data to set an MCLG.
The Perchlorate Advisory Panel is scheduled to hold a follow-up teleconference on September 25 and its final report is expected by the end of the year. At this point, it is unclear what effect moving to strictly PBPK model-based analyses to develop an MCLG would have on the regulatory endpoint. However, the development of such a model to inform the regulatory development process would appear to be incompatible with a February 2013 proposal date for a perchlorate regulation.
Additional information and background materials related to the Perchlorate Advisory Panel’s review are available at http://amwa.net/perchlorateSAB.