Risk Assessment for the Combustion Products of Methylcyclopentadienyl Manganese Tricarbonyl (MMT) In Gasoline
MMT, or methylcyclopentadienyl manganese tricarbonyl, has been used in Canada since 1977 as an octane enhancer in unleaded gasoline, and particularly since the removal of alkyl lead additives. MMT was banned from use in unleaded fuel in the U.S. in 1977 because it was found to increase hydrocarbon emissions and to block the type of catalytic converter then in use for unleaded fuel: MMT remains in use in the U.S. in leaded gasoline. U.S. octane needs have been met by changes in refinery production to increase aromatic content and/or branched chain hydrocarbon percentage, and also by the use of oxygenated fuels (ethanol, methanol, MTBE).
The known neurotoxicity of manganese at high (occupational) exposure levels has led to concern regarding the use of MMT as a gasoline additive, particularly given the recent experience with lead. In 1978 Health and Welfare Canada (1978) provided a review of the human health implications of the expected increase in MMT use with emphasis on increases in ambient manganese levels, and concluded that there was "no evidence at present to indicate that expected ambient manganese concentrations would constitute a hazard to human health". A similar conclusion was reached by the Royal Commission on Lead in the Environment (Royal Society of Canada 1986), as well as by two independent studies commissioned by Health and Welfare (Midwest Research Institute 1987; Hill 1988).
Ethyl Corp., the principal North American manufacturer of MMT, applied to the U.S. EPA for a waiver of the prohibition on inclusion of new additives (including MMT) in American unleaded gasoline three times, and were repeatedly denied (1977, 1981 and 1992) on the basis that MMT increased hydrocarbon emissions and blocked catalytic converters (U.S. EPA 1992). After the U.S. Court of Appeals accepted a petition for review of the 1992 denial decision, and after submission by Ethyl Corp. of extensive emission data, the U.S. EPA concluded in November 1993 that MMT did not contribute significantly to increases in hydrocarbon emissions or failure of catalysts after extended service based on available data. However, the U.S. EPA required an additional 6 months to determine whether MMT use poses a health risk to Americans, particularly given that the Reference Concentration for manganese had just been lowered to 0.05 mg Mn/m3 from 0.4 mg Mn/m3. On July 13th, 1994, the EPA announced that it would deny Ethyl Corp.'s waiver application on the grounds that there remain unresolved concerns regarding the health impact of manganese emissions produced by MMT use.
Health Canada undertook an independent risk assessment for the Canadian situation, focusing on the new epidemiological studies and Canadian exposure data.
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