Automakers and environmentalists are hoping a new Health Effects Institute (HEI) report will help convince the US Environmental Protection Agency to more strictly regulate, or possibly even ban, the octane enhancer methylcyclopentadienyl manganese tricarbonyl (MMT).
EPA, automakers, and oil companies jointly sponsor HEI; the Boston-based group was created in 1980 to study the health impacts of motor vehicle pollution.
MMT is a manganese-based fuel additive used in Canada but rarely found in the US. EPA allows it in conventional gasoline at a level of 1/32 g/gal manganese, but the product is now in only 0.3% of the US gasoline pool.
MMT's sole manufacturer, Richmond, Va.-based Ethyl Corp., is expected later this year to submit emissions and toxicity tests the agency will use to review the additive's future use in conventional and clean fuels.
Some refiners outside of North America also are mulling MMT. But given that methyl tertiary butyl ether—also touted to be a clean fuel product and octane enhancer—has now been banned in some US states, MMT may wind up competing with MTBE for international markets.
Convincing evidence
HEI's research report found "convincing evidence" that manganese, like lead, has the potential to accumulate in the brain under prolonged exposures, even at relatively low levels.
University of Kentucky researchers Robert Yokel and Janelle Crossgrove investigated the mechanism by which manganese enters and exits the brain across its protective blood-brain barrier. The two scientists said their work with laboratory rats suggests the brain may not efficiently protect itself from unhealthy buildups of the metal.
"This finding has important implications for neurotoxicity resulting from chronic manganese exposure," HEI said. "Although Yokel and Crossgrove studied manganese transport rates in rats, their observations may be relevant to humans because transport mechanisms at the blood-brain barrier are similar in rodents and humans."
HEI said their results support the current understanding that the potential for manganese accumulation in the brain should be considered when assessing risk from exposure to manganese in the environment. They said future studies and risk assessments also should consider susceptible populations, such as people with iron deficiencies or liver disease, who may be at greater risk from increased manganese uptake.
Next steps
Complying with a 1996 court order, EPA allowed MMT back into US conventional gasoline after a 2 decade absence. But the agency will revisit the issue after Ethyl submits its own toxicity and pollution data sometime this year; neither EPA nor Ethyl will say when exactly that will be.
Automakers have never liked MMT, and it's unlikely Ethyl's data will change their minds. An automaker 2002 study found the additive can increase smog, reduce fuel economy, and cause low-emission vehicles to fail hydrocarbon emission standards (OGJ, Aug 12, 2002, p. 26).
Ethyl said at that time that the automaker studies were seriously flawed and there has never been convincing evidence to suggest MMT harms emission control systems.