A cure worse than the disease.

Catalytic converters have cleaned up the exhaust from cars. But, according to the Environmental Protection Agency in the U.S., the converters contribute to global warming.

Catalytic converters have been reducing smog for 20 years; they break down compounds of nitrogen and oxygen from car exhaust that can combine with hydrocarbons, also from cars, and turn into smog.

But, researchers suspect that the converters sometimes rearrange the nitrogen-oxygen compounds to form nitrous oxide, a potent greenhouse gas; more than 300 times more potent than carbon dioxide, the most common of the greenhouse gases, according to experts.

Some say the answer is to drive electric cars. Others suggest hybrid cars with small, efficient engines running on gasoline to help turn the wheels and to charge batteries for electric motors that also run the wheels. The result would he higher fuel economy and lower greenhouse gas emissions. The car industry would prefer simply to adjust catalytic converters to reduce their production of nitrous oxide.