Catalytic Converters Disperse Their Metals

Elevated levels of transition metals emitted from automobile catalytic converters have been measured at busy street intersections in Boston in the first major U.S. study of catalyst metals in urban aerosols (Environ. Sci. Technol. 2005, 39, 9464). Reported concentrations of platinum, palladium, rhodium, and osmium are not considered high enough to pose a serious health risk, but they point to an emerging global environmental problem, given that the worldwide number of vehicles is expected to more than double in the next few decades.

Sebastien Rauch of Chalmers University of Technology, in Sweden, trapped micrometer-sized particles containing the metals with air filters and analyzed the samples by inductively coupled plasma mass spectrometry in collaboration with Bernhard Peucker-Ehrenbrink of Woods Hole Oceanographic Institution, in Massachusetts, and Harold F. Hemond of Massachusetts Institute of Technology. Concentration ratios of the metals and osmium isotope ratios allowed the team to peg catalytic converters as the source of the metals, rather than natural or industrial sources.

Catalytic converters have been in use in the U.S. for 30 years. They reduce vehicle emissions of carbon monoxide, unburned hydrocarbons, and nitrogen oxides by converting them to more benign carbon dioxide, water, and nitrogen. The devices have greatly reduced air pollution, but their metals have been detected in air, soil, plants, and waterway sediments around the world.

Average metal concentrations in Boston were in the range of 1–8 pg/m³ for platinum, palladium, and rhodium. Osmium, present in catalytic converters only as an impurity, was detected at 0.07 pg/m³. These concentrations are considerably less than established occupational exposure limits of 1–2 µg/m³.

Not much is known about the toxicity of the airborne metals, Rauch says, but finding ways to better stabilize them to prevent their loss from catalytic converters "should be a priority to limit their potential impact."