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Gasoline Standards

EPA Comments on the Gasoline Additive MMT

General Information

Methylcyclopentadienyl manganese tricarbonyl (MMT) is a gasoline octane enhancer produced by the Afton Chemical Corporation (Afton), formerly known as the Ethyl Corporation. MMT is allowed in U.S. gasoline at a level equivalent to 1/32 grams per gallon manganese (gpg Mn).

Background: Clean Air Act Section 211(f)(4) Waiver

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.

Background: Clean Air Act Section 211(b) Health Effects Testing

Manganese can be a neurotoxicant at sufficiently high exposure levels, possibly causing irreversible neurotoxicity or other adverse health effects. However, ingested manganese is a required element of the diet at low levels. There is a concern that the use of manganese additives in gasoline could increase inhaled manganese exposures.

After completing a 1994 risk evaluation on the use of MMT in gasoline, EPA was unable, based on the available data, to determine if there 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 Afton Corporation to perform testing to help fill data gaps and provide information that potentially would result in a more definitive risk evaluation.

This testing included three health pharmacokinetic (PK) studies and one emission characterization study. Completed final reports for all of these studies have been submitted to EPA. These final reports can be found in the Federal Docket Management System (FDMS) at  identified by docket number EPA-HQ-OAR-2004-0074.

In addition to the already completed tests, the manufacturer developed physiologically-based pharmacokinetic (PBPK) models for manganese which were derived from data generated from the completed testing. Afton completed those PBPK models in 2010.

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

The Agency will study the results of all the Afton studies and other available information. The Agency may then be able to refine its risk evaluation and/or may ask for further testing.