Methylcyclopentadienyl Manganese Tricarbonyl (MMT) is a manganese-based fuel additive
used to increase gasoline octane. It was widely introduced in the United States in the 1970s
by the Ethyl Corporation when regulations restricting the use of tetraethyl lead first came into
force. Today health scientists and automobile manufacturers are strongly opposed to its use.
Combustion of MMT releases manganese, a potent neurotoxin when inhaled. In vehicles it
can leave deposits that degrade the performance of pollution control systems. Similar concerns regarding tetraethyl lead have led to a near global ban on its use. Alternatives to MMT are in widespread use and can cost-effectively achieve similar octane levels without the associated health risk.
Link: Methylcyclopentadienyl Manganese Tricarbonyl (MMT): A Science and Policy Review