MEXICO CITY, Aug 15 (Reuters) - Contaminated air that hangs over the Mexican capital, one of the world's biggest cities, does more damage to children than cigarette smoke and may cause chronic lung diseases when they are adults, a study showed on Wednesday.

In a study of 3,170 eight-year-old children at schools in the city, scientists found the pollution prevented young lungs from growing and working properly.

"Strikingly, the effect of pollutant exposure ... among the children in (the) study was slighter greater than the effect of exposure to maternal smoking among children in the United States," researchers at Mexico's National Public Health Institute wrote in the August issue of American Journal of Respiratory and Critical Care Medicine.

Perched at 7,300 feet (2,240 meters) in a bowl-shaped valley where the air is thin and vehicle fumes get trapped, Mexico City has tried to cut its smog levels by closing factories, hauling old cars off the roads, modernizing aging buses and promoting bicycle use.

But contamination in the metropolitan area, home to some 18 million people, is still a problem.

"Although we could not identify specific sources (of the pollutants), the effect is likely to be due to vehicular exhaust," the researchers who led the three-year study wrote.

Their research is considered a step beyond previous studies that documented reversible, short-term breathing problems that big polluted cities can cause children.

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