The internal combustion engine contributes massively to global warming, kills around 1.2 million people a year in road accidents and, scientists now warn, is costing billions of US dollars in crop damage each year.
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 favor 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 US$5 billion in crops because of ozone, followed by China, with US$2.5 billion.
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 5 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 countries of the former Soviet Union more than US$7.5 billion 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 industrialization, annual mean ozone concentrations were between 10 parts per billion (ppb) and 15ppb.
In industrialized countries, levels have been rising annually by between 0.5 percent to 2.5 percent; in Britain, this average can reach 30ppb 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 40ppb or 50ppb to start affecting the plant.
The problem is that there can be high peaks, depending on the season, local topography and type of 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 grain belt, the average concentration is 64ppb, with occasional daily spikes as high as 120ppb.
The team grew a batch of soybeans in ozone of 62ppb and another at 75ppb, which corresponds to an expected 20 percent increase in ozone levels by 2030.
The plants that grew in 75ppb of ozone lost 20 percent of their harvest compared with the 62ppb 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.
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