Catalytic convertors are increasingly being fitted to cars as a means of reducing some of the harmful emissions from car exhausts. But, though they eliminate one form of pollution, their action serves only to create another one. Ground-level ozone is kept at very low levels in the cities by some of the pollutants catalysts remove. So the more catalytic convertors that are fitted, the more such ozone can build up. Since ozone is a major cause of choking summer smogs, we can assume that people with breathing difficulties, such as asthmatics, will be much worse off the more that catalytic convertors are used. We should stop fitting cars with catalytic convertors.