Methylcyclopentadienyl manganese tricarbonyl (MMT) is an octane enhancer that forms manganese particles when burned as a gasoline additive. These particles can be emitted into the atmosphere or deposited on engine and vehicle components, causing concern in either case.
There is substantial controversy regarding MMT: automakers are widely opposed to its use; many public health advocates and regulatory agencies are concerned about the potential health impacts; and manufacturers vigorously defend it as safe and effective.
Recent studies on the human health and vehicle and emissions impacts of MMT bring new information to the debate. A study published by the Health Effects Institute (HEI) explored the mechanisms for transporting manganese into and out of the brain. Other studies have shown that fine particles containing manganese can be absorbed into the blood through the lungs and ferried directly into the central nervous system and brain. Manganese associated with fine particles also enters the brain directly via the nasal passages, which contain nerves that have been shown to transport manganese into the brain. The impact of low-level, chronic exposures is unclear, especially for sensitive populations.
A coalition of the Alliance of Automobile Manufacturers (AAM) and two other automobile associations recently completed a comprehensive MMT test program. And in 2004, Ford completed a post-mortem analysis of the Escorts used in the AAM study. In focused testing on Low Emission Vehicles (LEV), the AAM study found that MMT increased emissions of hydrocarbons (HC) over the entire 100,000 miles of testing, causing seven of the eight light-duty vehicles tested to exceed LEV certification standards. Nitrogen oxide (NOx) emissions were initially lower for MMT-fueled vehicles, but increased over time, as did emissions of carbon monoxide (CO), to become much higher than clear-fueled vehicles at 100,000 miles. MMT increased emissions of all three pollutants at the end of the study by an average of 31-37% and reduced fuel economy over the life of the program by 2% or 0.6 mpg (Benson and Dana 2002). The emissions impact of MMT was especially dramatic in Ford Escorts designed to meet LEV standards.
These findings begin to illuminate the mechanisms by which manganese additives to gasoline could cause adverse health effects and damage to pollution control systems. Environment Canada is commencing a third-party review to consider new vehicle-related findings and Health Canada is reviewing health-related research. In its 1994 MMT risk evaluation, the U.S. EPA stated: “Although it is impossible to state whether a health risk would definitely exist at projected exposure levels, neither can the possibility of such a risk be ruled out… Given the information that is available at present and the uncertainties discussed here, a reasonable basis exists for concern regarding potential pubic health risks, especially for sensitive subpopulations, if MMT were to be widely used in unleaded gasoline.” If MMT were widely used as a gasoline additive, it could take decades, as occurred with lead additives, before the full health consequences were understood and agreed upon.