Methylmercury (MeHg)

CASRN 22967-92-6

IRIS Assessment Plan for Methylmercury (Scoping and Problem Formulation Materials)

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In April 2019, EPA released the draft IRIS Assessment Plan (IAP) for Methylmercury for public review and comment. An IRIS Assessment Plan (IAP) communicates to the public the plan for assessing each individual chemical and includes summary information on the IRIS Program’s scoping and initial problem formulation; objectives and specific aims for the assessment; the PECO (Populations, Exposures, Comparators, and Outcomes) criteria that outlines the evidence considered most pertinent to the assessment; and identification of key areas of scientific complexity. The PECO provides the framework for developing literature search strategies and inclusion/exclusion criteria, particularly with respect to evidence stream (i.e., human, animal, mechanistic), exposure measures and outcome measures. The IAP serves to inform the subsequent development of the chemical specific systematic review protocol.


U.S. EPA. IRIS Assessment Plan for Methylmercury (Scoping and Problem Formulation Materials). U.S. EPA Office of Research and Development, Washington, DC, EPA/635/R-18/292, 2019.

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Gaseous elemental mercury is released into the atmosphere from natural (e.g., volcanoes) and manmade (e.g., fossil-fuel combustion) sources. Elemental mercury can be converted to inorganic mercury, which then can be transported and deposited to land or water. Methylmercury is formed from methylation of inorganic mercury by biota in water and soil. Once converted, methylmercury bioaccumulates in fish tissue and increases in concentration at successively higher levels of the food chain.

The general population is predominantly exposed to methylmercury through consumption of contaminated fish and other seafood. Subsistence fishing communities and other populations with high dietary intakes of predatory fish species could be exposed to higher-than-average levels of methylmercury. Contaminated rice and rice-based food products, such as infant cereals, also can be a source of methylmercury exposure.

Methylmercury readily crosses the placenta and concentrates in cord blood. It is also transferred from mothers to children via breastmilk. The developing nervous system is particularly sensitive to methylmercury, so gestational, lactational, and other postnatal exposures are of concern.


Date Description
01-Dec 1997EPA released the Mercury Study Report to Congress.
02-Oct 2000EPA released a draft document titled, Reference Dose for Methylmercury (NCEA-S-0930)" for a 30-day public review and comment. Following the review an external peer-review workshop was held to review the document.
03-Jan 2001EPA released the IRIS Summary for Methylmercury on the IRIS website and released the water quality criteria guidelines for states and authorized tribes.
04-Jan 2009EPA released the Methylmercury Implementation Guidance.
05-Apr 2010EPA released an update, referred to as the Methylmercury Implementation Guidance (Updated Apr 2010).
07-Apr 2019EPA released the IRIS Assessment Plan (IAP) for Methylmercury for public comment and review. [Federal Register Notice Apr 4, 2019]
08-May 2019EPA hosted a public science meeting to discuss preliminary meeting materials for methylmercury.


Following the public (webinar) meeting, the IRIS Program will use the IAP to inform the subsequent development of a chemical-specific systematic review protocol.

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