Metal spews from tires and brake pads

As well as tailpipe emissions, cars and other vehicles throw off metal pollutants from wear on various parts. Despite European regulations requiring cleaner materials in vehicles, a study in Stockholm shows that tires are a significant source of cadmium, while brake pads emit a variety of other metals.

Bo Bergbäck and his colleagues at the University of Kalmar in Sweden analyzed metals in tires and brake-pad linings. They used average wear and replacement patterns for the parts, along with national traffic data for 2005, to estimate the quantities of various metals dispersed into the environment. The team compared its findings with data obtained in the 1990s.

Besides being a well-studied city in terms of environmental impacts, Bergbäck says, Stockholm represents “an average city in many respects,” making the data relevant to other urban areas.

Tires contain zinc because of the zinc oxide used in the vulcanization process, which often has small but significant amounts of cadmium associated with it, Bergbäck says. Tires remain a major source of emissions of those two metals in Stockholm. Brake linings shed a significant amount of zinc and copper, he adds.

In general, lead and cadmium admissions decreased 90 percent between 1998 and 2005, while emissions for zinc and copper have declined more modestly, the researchers report in the Aug. 1 Environmental Science & Technology.

Another group of researchers in Sweden is studying the bioavailability and biological impact of these metals. “We know that there are increased concentrations of these metals,” Bergbäck says, “[but] we can’t say anything about biological effects.”

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