The villain is a molecule of oxygen called ozone.

Way up in the stratosphere, the wafer-thin ozone layer exists naturally as a protector of life, filtering out ultra-violet sunlight that would otherwise slice and corrupt human DNA.

At ground level, though, ozone can be dangerous. Formed by a reaction between nitrogen oxides and volatile organic compounds emitted by road traffic, ozone smog can be a life-shortening problem for people with bad respiratory problems.

Another fast-emerging picture is that this pollution is also inflicting a rising bill in damage to food plants, especially in regions where hot, sunny, windless conditions favour ozone formation.

Frank Raes, a Dutch scientist at the Joint Research Centre in Ispra, a unit funded by the EU's executive Commission, estimates that each year India loses five billion dollars in crops because of ozone, followed by China, with 2.5 billion dollars.

They are followed by Iran, Pakistan, Turkey and the east and west coasts of North America.

By 2030, says Raes, India will lose 20 percent of its crops through damage, compared with less than five percent through man-made global warming.

An assessment of 45 countries made in 2002 for the UN Economic Commission for Europe found that ozone is already costing farmers in Europe and the former Soviet Union more than six billion euros (7.5 billion dollars) a year.

Ozone enters plants through respiratory pores in the leaves. It then produces byproducts that crimp efficiency in photosynthesis, leaving a plant that is weak and undersized -- and with a crop size and quality to match.

Before industrialisation, annual mean ozone concentrations were between 10 and 15 parts per billion (ppb).

In an industrialised country, levels have been rising annually by between 0.5-2.5 percent; in Britain, this average can reach 30 ppb today on hot and sunny days, according to a 2004 study by University of York scientists in northern England. Such averages may not seem much, given that, depending on the crop, ozone levels need to hit 40 or 50 ppb to start affecting the plant.

The problem is that there can be big peaks, depending on the season, local topography and weather.

For one thing, ozone levels are highest in summer, when crops are growing.

And, ironically, they are often highest in the countryside. Ozone smog originates in the city but it takes several hours to form through local atmospheric chemical reactions, and then drifts to rural areas.

A 2003 study by University of Illinois researchers found that in Illinois, part of the US grainbelt, the average concentration is 64 ppb, with occasional daily spikes as high as 120 ppb.

The team grew a batch of soybeans in ozone of 62 ppb and another at 75 ppb, which corresponds to an expected 20-percent increase in ozone levels by 2030.

The plants that grew in 75 ppb of ozone lost 20 percent of their harvest compared with the 62 ppb group.

"What we found was surprising and a bit shocking -- with a 20-percent rise in ozone exposure, we also saw a 20-percent drop in yield," said Stephen Long, a professor at the university's departments of plant biology and crop sciences.

Raes, who is studying the effect of ozone on rice, corn, wheat and soybean, says that ozone smog's effect on agriculture may well be far worse than global warming.

Even so, ozone and global warming are linked, as both are caused by the fossil fuels and both are cross-border problems.

Countries such as the United States which complain that controls on carbon dioxide cost their economy too much should be told that by cleaning up fossil-generated CO2, they also help tackle ozone damage, says Raes.

"What's good for the climate is also good for the air," he says.

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