Air pollution in the fast-growing cities of the third world may be stunting their food crops by 40 per cent, researchers at Imperial College have found.

This poses serious problems because the culprit pollutant, ozone, is increasing in concentration as car congestion and heavy industry increase, says Nigel Bell, of the college's Centre for Environmental Technology.

He and colleagues have produced the first substantial evidence that ozone is such a danger in the third world, although it is known to affect agricultural production in North America and Europe.

Ozone is produced in a photochemical reaction when sunlight shines on nitrogen oxides and volatile organic compounds. It is most common in conditions of still air, high temperatures and sunshine. It is produced in cities but it can waft over large areas and often exerts its worst effects in rural areas.

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The work was done in conjunction with local universities in Egypt, on radish and turnip crops, and in Pakistan, on wheat and rice. In Egypt, some plants were coated with a chemical that prevents ozone getting into them - uncoated crops ended up 30 per cent smaller than coated ones. In Pakistan the crops were grown in open containers, half with charcoal-filtered air blown upwards at them. "We got a large reduction of 30-40 per cent in yield of wheat and rice in the ambient air compared to the charcoal-filtered air," said Professor Bell, who presented the work at an Environment Day held at Imperial College.

"We were surprised," he said. "It was so large an effect. It's possible that these crops from hotter countries may be more sensitive than others. We think it is a widespread effect.

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"We've felt for a long time that the biggest impact of air pollution was going to be in the third world," said Professor Bell. The third world is where most cities of more than eight million inhabitants are."

Now Professor Bell and Mike Ashmore, senior lecturer at Imperial College, have a contract with the Overseas Development Administration to do further studies and to look at ways of coping with the problem. This will probably involve altering crops, for example by finding varieties that can resist pollution, rather than trying to reduce the causes of pollution. The work will be done with Jules Pretty, of the International Institute for Environment and Development.