The use of ceramic particulate traps, in conjunction with manganese fuel additives, has been viewed as a way to reduce emissions of particulate matter from diesel-fueled vehicles. This report focuses on the potential health effects from increased public exposure to manganese emissions from such use.
Manganese is the 12th most abundant element on earth. Ambient levels of manganese rarely exceed 0.1 µg/m3. Manganese is classified as an essential element for physiological functions. Average daily intake of manganese, which is mostly from food, is about 3 to 4 mg/day (range 2 to 9 mg/day). Homeostatic mechanisms (primarily by regulation of intestinal absorption and biliary excretion) maintain the body burden of manganese between 12 and 20 mg.
The health effects of major concern from manganese exposure are neurotoxic and respiratory. Numerous studies, mostly in occupational groups exposed to very high levels of manganese (in the mg/m3 range), have demonstrated that prolonged exposure to relatively high levels of manganese leads to manganism (a Parkinson-disease like syndrome) and other symptoms of central nervous system toxicity, as well as to pneumonitis, bronchitis, and increased susceptibility to pulmonary infections. Pulmonary effects of manganese have been observed to occur at lower levels of exposure than neurotoxic effects.
The health effects of exposure to low levels of manganese have not been well characterized. An epidemiologic study (Saric et al. 1977) has reported neurotoxic effects of manganese at exposure levels that were between 300 and 5,000 µg Mn/m3. Another epidemiologic study (Nogawa et al. 1973), which focused on respiratory symptoms in children attending a school near a ferromanganese plant, has reported health effects at levels as low as 3 to 11 µg Mn/m3. However, serious problems with the study (such as poor characterization of exposure, failure to blind observers and to randomize students, and a lack of control for the confounding factors) suggest that the results of the study are not reliable. All other studies have reported adverse effects at levels that are one to two orders of magnitude greater.
A modeling study by Volkswagen (1984) for emissions of manganese suggests that the worst-case ambient concentration of manganese in an urban street canyon is likely to be less than 0.5 µg/m3 (the assumptions include 6.6 percent penetration of the manganese technology into the vehicle fleet, emissions of 100 percent the manganese added to the fuel in the emissions, high manganese consumption, and worst-case traffic conditions). HEI has independently verified the Volkswagen calculations. If an individual, such as a traffic policeman, worked outside all day in a street with the worst-case levels of manganese, his daily intake on manganese by inhalation from ambient sources would be 3.6 µg. This is a very small amount compared to the daily dietary intake of manganese (3 to 4 mg/day), and is not expected to tax the body's homeostatic mechanisms that regulate manganese.
On the basis of the information in the literature, and the exposure and dose of manganese estimated in this report and summarized above, it appears very unlikely that exposure to airborne manganese from mobile sources (worst-case level 0.5 µg/m3) would produce adverse neurologic effects. Regarding respiratory effects, the difference between estimated exposure (worst-case level 0.5 µg Mn/m3) and the lowest level at which health effects have been reported in the Nogawa et al. (1973) study (3 to 11 µg Mn/m3) is not very large. However, in view of the concerns regarding the Nogawa study, it appears unlikely that the increased ambient levels of manganese from trap-equipped diesel vehicles would produce adverse respiratory effects.
Given the paucity of information at the anticipated low levels of manganese from its use in diesel-powered vehicles, if manganese were to be used as a fuel additive, it is very important that the emissions of manganese be characterized in terms of their physical form and chemical species, the assumptions made in estimating the emissions, exposure, and dose of manganese be substantiated, and the results of the Nogawa study be reviewed and evaluated in greater depth than attempted in this report.