Smelly cats

Since January 1993, all new cars in the European Union have had to be fitted with a three-way catalytic converter. Although three-way converters will lower emissions of certain harmful chemicals, such as oxides of nitrogen, S.F. Watts and C.N. Roberts raise the question of whether they may result in a shift in the balance of sulphur species, especially in cities (Atmospheric Environment 33, 169-170; 1999). Before the introduction of three-way converters, sulphur in petrol was emitted mostly as sulphur dioxide (SO₂), but the authors estimate that in the future, emissions of hydrogen sulphide (H₂S) will increase. Both H₂S and SO₂are respiratory toxins, but only levels of SO₂are regularly monitored.

The data are both few and preliminary. Nonetheless, roadside measurements of H₂S in Britain and the United States seem to indicate that it reaches higher levels in Britain. Watts and Roberts speculate that this is due to different driving conditions, and point to previous studies that identified situations that favour higher emissions of H₂S instead of SO₂. These include driving in built-up city areas, notably in urban ‘stop-and-go’ traffic, and the first 10-20 km of a journey after cold starting, when the engine is rich in fuel. In Britain, as many as 40% of all car journeys are under 5 km and with increasing traffic congestion, the situation is likely to get worse.

Not only are emission data scarce, but it is hard to evaluate the levels of exposure that are harmful to humans. Concentration levels have been set for occupational hazards that are many times lower than have been recorded from traffic. But these limits appear to have been set simply on the basis of odour (that rotten egg smell) and are comparable with levels found around natural sulphur springs.