Yes, just as president Ronald Reagan said in 1981. "Trees cause more pollution than automobiles do," he opined. A little later, environmental scientists ruefully confirmed he was partially right. In hot weather, trees release volatile organic hydrocarbons including terpenes and isoprenes - two molecules linked to photochemical smog. In very hot weather, the production of these begins to accelerate.

America's Great Smoky Mountains are supposed to take their name from the photochemical smog released by millions of hectares of hardwoods.

This week Natural Environment Research Council scientists warned that as summer temperatures rise in the UK, the isoprene output from trees could make a small but noticeable contribution to human discomfort. Isoprene serves as a catalyst, driving the rate at which sunlight breaks down oxides of nitrogen - mostly from agriculture and cars - to produce atmospheric ozone.

Ozone is a triple molecule of oxygen. High in the stratosphere it is a godsend, screening out cancer-causing ultraviolet radiation. But in the lower atmosphere it is a toxin: it causes stinging eyes, prickling nostrils and aggravates severe respiratory problems. Statisticians calculate that in August 2003 - the long hot summer that caused an estimated 20,000 deaths in western Europe - more than 500 British deaths could be attributed to ozone pollution.

But the experts say the trees alone are not the problem. The real villain is the motor car. Trees soak up carbon dioxide, and respire oxygen, doing far more good than harm. And finally, as one forester observed: why worry about a few harmful natural chemicals? In a truly antiseptic world we would all be dead.