Comments on the Gasoline Additive MMT (methylcyclopentadienyl manganese tricarbonyl)

MMT is a gasoline octane enhancer produced by the Ethyl Corporation. MMT is allowed in U.S. gasoline at a level equivalent to 1/32 grams per gallon manganese (gpg Mn).

In its decision on the use of MMT in the U.S., the Agency determined that MMT added at 1/32 gpg MN will not cause or contribute to regulated emissions failures of vehicles. Some have expressed concerns that the use of MMT may harm on-board diagnostic equipment (OBD) which monitors the performance of emissions control devices in the vehicle. As of this time, the Agency believes the data collected is inconclusive with regard to OBD.

Manganese is a neurotoxin and can cause irreversible neurological disease at high levels of inhalation. However, ingested manganese is a required element of the diet at very low levels. There is a concern that the use of manganese additives in gasoline could increase inhalation manganese exposures.

After completing a risk evaluation in 1994 on the use of MMT in gasoline, EPA was unable, based on the available data, to determine if these is a risk to the public health from exposure to emissions of MMT gasoline. The Agency stated "Although it is not possible based on the present information to conclude whether specific adverse health effects will be associated with manganese exposures in the vicinity of or exceeding the [estimated safe level over a lifetime of exposure], neither is it possible to conclude that adverse health effects will not be associated with such exposures." This assessment was based upon the level of MMT allowed in U.S. gasoline.

More specifically, modeling indicates that, as a result of MMT use in unleaded gasoline, certain portions of the population may be exposed to levels in the same range as the Reference Concentration (RfC or safe level for a lifetime exposure with an order of magnitude uncertainty). Because the expected exposure is not much higher or much lower than the RfC, a definitive conclusion about risk is impossible to reach. Long-term animal testing and exposure research are needed to more accurately define the risk.

Under Clean Air Act authority to require testing of motor fuels and additives, EPA has required the Ethyl Corporation to perform testing to help fill in data gaps and potentially provide information that would result in a more definitive risk evaluation. After submission of this additional information late in 2004, the Agency will study the results. The Agency may then be able to refine its risk evaluation or may ask for further testing based upon the results of the testing now being performed.

With funding from Ethyl Corporation, Research Triangle Institute (RTI) has also completed a study of manganese exposures in Toronto, Canada where MMT is used. The Agency is evaluating this study to determine what impact it might have on any evaluation of risk associated with use of the additive.