US EPA

In May 2020, EPA released the Systematic Review Protocol for the Methylmercury IRIS Assessment (Preliminary Assessment Materials). As part of developing a draft IRIS assessment, EPA presents a methods document, referred to as the protocol, for conducting a chemical-specific systematic review of the available scientific literature. Protocols communicate the rationale for conducting the assessment of methylmercury, describe screening criteria to identify relevant literature, outline the approach for evaluating study quality, and describe the dose-response methods.

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.

The overall purpose of this report is to characterize the dose-response relationship between methylmercury exposure and developmental neurotoxicity outcomes and to use this information to update the reference dose (RfD) value.

The IRIS Program will use the methods for conducting the systematic review as described in the protocol to develop the draft assessment.

U.S. EPA. Systematic Review Protocol for the Methylmercury (MeHg) IRIS Assessment (Preliminary Assessment Materials). U.S. Environmental Protection Agency, Washington, DC, EPA/635/R-19/243, 2020.