Methylcyclopentadienyl Manganese Tricarbonyl (MMT) has been shown to be an effective octane enhancer in gasoline. It is presently used in the U.S. in leaded gasoline and in Canada in both leaded and unleaded gasoline. Because MMT has a low vapor pressure and a short half-life in sunlight, it is unlikely that significant concentrations of MMT could occur in the environment as a result of its use as a gasoline additive. Greater than 99.9% of the manganese from MMT is converted into inorganic oxides of manganese during the combustion of MMT-containing gasoline. Authors of several health assessments on MMT concluded the increase in environmental levels of manganese oxides would be slight compared to the natural background levels of manganese and would present no health hazards. Studies showed that MMT usage had no effect on CO emissions, caused a slight decrease in NOx emissions, and a slight increase in HC emissions. MMT usage caused a decrease in NOx + HC emissions in one test and had no effect in another.