Manganese in the U.S. gasoline supply.

Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organic manganese compound recently approved for use in the United States as a gasoline additive. MMT use is expected to increase. This Commentary analyzes the impact of MMT use on population exposure to manganese, the health effects associated with manganese exposure, and the possibility that MMT use will lead to toxicity in the population. Although MMT use would result in only a small increment in most people's manganese exposure, certain populations will be disproportionately exposed. Although manganese is an essential nutrient at low levels, high-level manganese exposure leads to a characteristic severe nervous system toxicity. Pulmonary toxicity also occurs at high levels of exposure, and developmental toxicity to fetuses is an important concern based on more limited data. Selected subpopulations may be especially susceptible to the toxic effects of manganese. The critical question is whether the additional population exposure to manganese that would result from widespread MMT use would lead to toxic effects. Currently available evidence does not permit firm conclusions. Common sense and prudence therefore dictate that MMT not be used until further data are available and its safety is confirmed. Several measures are recommended to address the impending use of MMT in the U.S. gasoline supply.