Inhalation health risks of manganese: an EPA perspective

In 1994, the U.S. Environmental Protection Agency (EPA) denied a petition by Ethyl Corporation to allow the use of methylcyclopentadienyl manganese tricarbonyl (MMT) in unleaded gasoline, because of health concerns related to the inhalation of manganese (Mn) particulate emissions from combusted MMT. Although Ethyl successfully challenged EPA's denial of the petition on legal grounds, issues raised in EPA's health risk assessment have not been resolved to date. This paper summarizes features of the EPA health risk characterization, which included the use of various statistical techniques to derive several estimates of inhalation reference concentration (RfC) values for Mn as alternatives to the established value of 0.05 microgram Mn/m3. An exposure assessment projected distributions of personal exposure levels to particulate Mn if MMT were used in all unleaded gasoline. It was estimated that exposure levels of 5-10% of the modeled population might exceed a possible alternative RfC value of 0.1 microgram Mn/m3. However, due to data limitations, the risk characterization for Mn/MMT could raise only qualitative concerns about potential public health impacts and was unable to provide a quantitative estimate of risk. To improve the risk characterization, better information on Mn/MMT population exposures and health effects is needed. Much of this information is expected to be obtained under provisions of Section 211 of the Clean Air Act. Among the specific issues that remain to be resolved are the form or forms of Mn emitted from the combustion of MMT in gasoline and the potentially different toxic properties of Mn in different forms.