It's easy to assume that cars pollute our cities only via their exhaust pipes. But Swedish chemists have shown that wear and tear on tyres and brakes contribute to significant emission of toxic metals into urban air.

David Hjortenkrans and Bo Bergbäck have updated an earlier case study of the Swedish capital Stockholm, in order to determine how metal emissions have been affected by rapid developments in automotive materials. 

The Swedish team took samples of a variety of brake linings and tyres from a range of manufacturers and used atomic absorption spectrophotometry to determine their copper, cadmium, lead, antimony and zinc contents. They then used this data and other factors such as traffic volume, particle emission, vehicle type, sales data and material lifetime to calculate the emissions of each metal in the Stockholm area.

'Brake linings remain one of the main emission sources for copper and zinc, and probably antimony, in urban areas. But there has been a significant decrease in lead emissions from brakes over the last decade,' said Hjortenkrans. 'For zinc, tyres are still a major source,' he added. This is due to the use of zinc oxide as a filler in tyre rubber, at roughly 2 per cent by weight.

Rising source

The UK's National Atmospheric Emissions Inventory (NAEI) compiles estimates of emissions from transport and industry. A spokesperson for NAEI pointed to the fact that emissions from tyres and brakes are not regulated. Since emissions from many more tightly controlled sources are generally in decline, the contribution to total emissions from tyres and brakes is on the rise. NAEI data, published on their website, indicates that around 18 per cent of UK zinc emission in 2004 was caused by road transport. This is almost entirely due to tyre wear, say the NAEI. Tyre and brake wear was also the largest single cause of copper emission in 2004. 

Roy Harrison an atmospheric chemist at the University of Birmingham, UK and advisor to the UK government on air quality, told Chemistry World that the research was encouraging. 'Non-exhaust sources are very important,' he said. 'They would appear to be a similar magnitude as exhaust emissions at the sites we measure.' 

'Our measurements suggest total non-exhaust particulate emission is increasing in magnitude,' he added.

Tom Westgate