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Perchlorate Peer Reviewers Meet

This week, a Peer Review Panel that is providing input to EPA on perchlorate met outside Washington, DC to review the latest modeling approach for determining the Maximum Contaminant Level Goal (MCLG).  At the recommendation of the Science Advisory Board a few years ago, EPA is taking a new approach to developing the MCLG by using a biologically-based dose-response model (BBDR) to predict the relationship between perchlorate exposure and thyroid hormone levels.  The model predictions can then be linked to neurodevelopmental effects and inform the development of the MCLG.  This is a much more complex process than that used to develop a reference dose (RfD) from health studies that traditionally has been used to determine an MCLG for contaminants with noncancer health effects.  The lengthy process to create and validate the model has caused EPA to fall behind a court-ordered timeline for a developing a final rule.

This second peer review panel is looking specifically at how the output from the BBDR model can be used to develop the MCLG.  In 2017, a different peer review panel examined the model itself.  Panelists commended EPA on improvements to the model based on comments from the first panel.  Generally, the panelists also believed the modeling approach was still superior to the traditional RfD method.  However, the panel did have some concerns about how the modeling would be applied when developing the MCLG.  EPA focused specifically on the first trimester of pregnancy in the model, but panelists thought that might be too simple an approach.  Later stages of the pregnancy, as well as the mother and breastfed infant, deserved more consideration.  The panel also suggested that additional health studies might need to be considered in the model.  All the panelists recognized that there was uncertainty with the model but many thought that the uncertainty could be accounted for, possibly better than in the RfD process.  EPA presented two approaches to utilizing the model to develop the MCLG – a two-stage approach and a three-stage approach.   There appeared to be no clear preference for either method, so EPA could use either approach in their MCLG determination.

The panel will produce a formal report to EPA in the next few weeks.  EPA will then take this input and use it as they continue to work to derive the MCLG and develop a final MCL for perchlorate.  Even fairly favorable comments by the panel will take time to be digested at EPA, with tweaks made to the model and the MCLG process, as appropriate.  As noted, EPA is already behind in their development schedule.  The final rule should be issued in late 2019, but that deadline is certainly in jeopardy, and a proposed rule may not be ready this year.

For more information see EPA’s perchlorate web page.