Catalytic converters, which are fitted to all cars to reduce harmful emissions, are increasing levels of other toxic gases that can be hazardous to human health, according to scientists.

They say that the benefits of the devices, which clean poisonous gases such as carbon monoxide from exhaust fumes, are being offset by the production of two other potentially dangerous compounds, hydrogen sulphide and carbon disulphide.

Since 1993 EU legislation has made it compulsory for all new vehicles to be fitted with three-way catalytic converters in an attempt to reduce carbon monoxide and nitrogen oxide emissions.

While the converters have achieved this goal, they have also changed the forms in which sulphur in petrol is released into the atmosphere: hydrogen sulphide and carbon disulphide are being generated instead of sulphur dioxide.

Levels of hydrogen sulphide measured in British cities are now up to 100 times higher than they were in preindustrial times, according to research by Simon Watts and Stephen Samuel, of Oxford Brookes University, who believe that catalytic converters are partly responsible.

“The amount of sulphur coming out of individual exhausts is lower than previously. However, it is no longer coming out as sulphur dioxide but these other sulphur compounds,” Dr Watts said. “Add to this the fact that there are now so many more exhaust pipes out there, and you see the problem.”

Hydrogen sulphide, a colourless gas that is responsible for the characteristic foul odour of rotten eggs and stink bombs can shut down several critical systems in the body. Carbon disulphide is a neurotoxin and a carcinogen. It can be dangerous to human health at high doses.

“The increasing levels of hydrogen sulphide in the air potentially pose a serious health concern,” Dr Watts said. According to his research, a typical reading in the 1500s would have been 70-100 parts-per-trillion. Recent measurements in Elephant & Castle, South London, found up to 7,000 parts-per-trillion.

Professor Ken Reid, head of the immunochemistry unit at the University of Oxford, said that long-term exposure to hydrogen sulphide could raise the risk of respiratory diseases and lung cancer.

“Hydrogen sulphide can modify antibodies in the lung, which compromises immunity and increases the likelihood of infection,” he said. “It’s now well recognised that this kind of repeated insult can set up a cancerous state.”

The gas may also be responsible for increases in asthma among city-dwellers as it can disrupt the way the lungs regulate the respiratory cycle.

Dr Watts added that lean-burn engines might have been a better solution to the problem. “Our alternative in the early Nineties was lean-burn engines, which produce much less hydrogen sulphide,” he said. “But catalytic converters were the easier option for car manufacturers. I think the EU was swayed by economic considerations.”

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