Nicolas Bukowiecki, from the Swiss Federal Laboratory for Materials Testing and Research and his colleagues used an automatic sampling device to collect hourly air samples from a busy city centre street in Zurich, and from a freeway location near Zurich, for more than a month. They then used synchrotron radiation X-ray fluorescence spectrometry to analyse the size and type of particles in the air. By carrying out vehicle counts at the same time as the sampling they were able to estimate the relative contribution of each pollutant from light and heavy vehicles.

In particular they were interested in antimony – a rare metallic element that is potentially carcinogenic and has similar poisoning effects to arsenic. In the city centre with stop-start traffic the average antimony emission levels were 11 μg per kilometre for light vehicles and 86 μg per kilometre for heavy vehicles. By contrast the levels recorded next to the freeway with freely flowing traffic were more than ten times lower.

Until now antimony has not really been considered as an air pollutant and the only guidelines relate to water pollution, where the World Health Organisation recommends that drinking water should contain less than 20 μg of antimony per litre. Meanwhile it is known that the minimum lethal dose for arsenic is between 70 and 200 mg per day. "If we compare it to arsenic I think we can say that the levels of antimony are acceptable, but we need to keep an eye on them," says Urs Baltensperger, head of the Laboratory of Atmospheric Chemistry at the Paul Scherrer Institut in Switzerland and one of the study authors.

Other elements that the scientists measured included iron, copper, zinc, molybdenum, tin and barium. In all cases the pattern of emissions was similar to antimony, with far higher levels being recorded in the busy city-centre environment than the freeway, and a greater proportion of the emissions coming from heavy vehicles over light vehicles. Their findings are published in the journal Environmental Science and Technology.

By analysing road dust Bukowiecki and his colleagues showed that the predominant source of the metals was vehicle-brake wear, either by direct emission or through re-suspension after deposition in the street canyon.

At first glance the figures suggest that heavy vehicles produce significantly more pollution than light vehicles and could be considered more of a public health hazard. However, particle size is also a factor. The scientists discovered that the majority of the particles thrown out by heavy vehicles were larger in size (between 2.5 and 10 μm diameter), whereas a substantial fraction of the particles produced by light vehicles were of medium size (between 1 and 2.5 μm diameter).

"The smaller particles more easily penetrate into the lungs, whereas the larger particles are often stopped in the nose and mouth and then swallowed," Baltensperger told environmentalresearchweb. "The uptake of these particles from the lung to the blood is faster than from the stomach, and so potentially more damaging."

Comparing the results with lower-resolution studies carried out in other cities, Bukowiecki concludes that brake-pad wear pollution will be significant in any city that suffers from traffic congestion. However, meteorology can make a big difference.

The team found that the pollution diminished rapidly in wet weather because the road dust was washed away by rain. "Washing the streets every day would really help to damp down this pollution, but is of course neither economic nor ecological" says Baltensperger. Other measures that would help to reduce the particulate load include more frequent road sweeping and reducing traffic congestion.