IRIS Summary Update (2001) and EPA Guidelines for Methylmercury (2009 & 2010)

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In January 2001, U.S. EPA finalized the guidance for methylmercury in the Water Quality Criterion for the Protection of Human Health: Methylmercury, Final for states and authorized tribes.

Then eight years later, EPA released new guidance under the Methylmercury Implementation Guidance, but the following year the previous Obama Administration asked EPA to confirm the continued appropriateness and applicability of several agency actions, including this guidance. Therefore, based on the outcome of this review, in April 2010, EPA improved the document's clarity, and as a result, the ability of states and tribes to protect public health from methylmercury impacts by issuing a 2010 update, but this did not change the fundamental policy recommendations from the original January 2009 document.

Methylmercury is a highly toxic substance; there are a number of adverse health effects associated with methylmercury exposure. Most extensive are the data for neurotoxicity, particularly in developing organisms. Therefore the brain is considered to be the most sensitive target organ for which there are data suitable for derivation of an RfD.

The National Research Council (NRC) considered three epidemiological longitudinal developmental studies suitable for quantitative risk assessment: the Seychelles Islands, the Faroe Islands, and New Zealand. The Seychelles study has yielded no evidence of impairment related to methylmercury exposure, while both the other studies have found dose-related adverse effects on a number of neuropsychological endpoints. The Faroe Islands study, the larger of the latter two studies, has been extensively peer-reviewed and was used for the derivation of the RfD. The NRC's major finding was that the results of the Faroe Islands study provide scientifically credible basis on which to base EPA's RfD value.


This final Guidance for Implementing the January 2001 Methylmercury Water Quality Criterion provides technical guidance to states and authorized tribes on how they may want to use the January 2001 fish tissue-based recommended water quality criterion for methylmercury in surface water protection programs (e.g., TMDLs, NPDES permitting). The guidance addresses questions related to water quality standards adoption (e.g., site-specific criteria, variances), assessments, monitoring, TMDLs, and NPDES permitting. The guidance consolidates existing EPA guidance where relevant to mercury.

This document has been reviewed in accordance with U.S. Environmental Protection Agency policy and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.