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The Independent Online
THE CATALYTIC converter is the main weapon in the battle against smog.

Severe air pollution caused by the effect of strong sunlight on vehicle exhaust fumes can be tackled effectively by the use of catalysts - devices fitted into the exhaust system which are able to break down the more noxious by-products of burning gasoline, such as carbon monoxide and various sulphur compounds. As more and more cars are equipped with emission- cleaning exhausts, smog levels should drop.

But cat-equipped cars may have a big disadvantage. It has always been accepted that a catalytic converter acts to reduce the engine's efficiency, thus increasing fuel consumption and therefore increasing carbon dioxide emissions - allegedly one of the main causes of reported global warming.

But a report by the United States government green watchdog, the Environmental Protection Agency (EPA), has claimed that gases produced by car catalysts are 300 times more potent than carbon dioxide - the most common man-made "greenhouse gas". The offending chemical is nitrous oxide, otherwise known as laughing gas.

The EPA study earlier this year, of gases thought to cause global warming, concluded that from 1990 to 1996, production of nitrous oxide from cars in the US rose by nearly 50 per cent. The suspected cause is the growth in the number of vehicles with catalytic converters.

Nitrous oxide now makes up about 7.2 per cent of the gases cited in global warming, the EPA said. Vehicles fitted with catalytic converters produced nearly half of that nitrous oxide.

This just goes to show that using technology to solve one environmental problem quite often creates another.