Risk assessment for combustion products of the gasoline additive MMT in Canada

Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used as an octane enhancer in Canadian gasoline since 1976. The main potential health concern is from manganese oxides produced on combustion (mainly Mn3O4), given the known neurotoxicity of chronic inhalation of manganese (Mn) dust from mining and industrial use. Relevant epidemiological studies of occupational exposure to respirable Mn are briefly reviewed; an ambient air reference value of 0.1 μg Mn/m3, and associated inhalation tolerable daily intake (TDI) and tolerable daily uptake (TDU) of 0.035 and 0.021 μg/kg b.w./day are derived. Ambient levels of PM2.5 (respirable) Mn in Canadian cities have remained unchanged or have decreased between 1986 and 1992, and do not reflect large changes in MMT usage during that time. Ambient levels of PM10 Mn in Canadian cities in 1992 were ≤0.025 μg Mn/m3. Mean, 90th and 98th percentiles of PM10 Mn inhalation uptake based on ambient monitoring data from high traffic areas and from estimates of personal exposure are below the inhalation uptake criterion. An assessment of exposure from air, food, water and soil revealed that <1% of total daily Mn uptake is derived from inhalation for all age groups. Therefore, based on current information, Mn derived from the combustion of MMT-containing gasoline is unlikely to represent a significant health risk to Canadians.