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Mexico City drastically reduced air pollutants since 1990s

The presence of lead in the air has dropped by 90 percent since 1990. Suspended particles -- pieces of dust, soot or chemicals that lodge in lungs and cause asthma, emphysema or cancer -- have been cut 70 percent. Carbon monoxide and other pollutants also have been drastically reduced.

"It is no longer an emergency situation," said Raul Estrada, a spokesman for Greenpeace in Mexico, "though obviously, it is not 100 percent satisfactory."

In March, the Mexico City Atmospheric Monitoring System, which has pollution sensors all over the city, began to measure additional chemicals, such as the highly toxic benzene, that are suspected to contribute to Mexico City's ozone problem.

Ozone levels have dropped 75 percent since 1992, but they still exceeded international standards for a total of 530 hours last year.

"You shouldn't even have one hour of high ozone levels in an entire year, and we have 530 hours," said Humberto Bravo Álvarez, one of Mexico's most respected air pollution experts. Bravo said corruption, such as people paying off emissions inspectors, has reduced the impact of the new controls.

Had Mexico not taken action, "it would be living hell," said Exequiel Ezcurra, a former head of Mexico City's National Institute of Ecology who is now a professor at the University of California, Riverside.

Ezcurra attributes much of the improvement to a requirement that Mexico-based auto manufacturers put catalytic converters on cars produced for the Mexican market. Now Mexico must require all diesel vehicles to be retrofitted with a filter that is the equivalent of a catalytic converter, he said.

"If the government decides to do something about it, it can be done," said Nobel Prize-winning air quality expert Mario Molina. "There's really no excuse not to do more."

Experts say Mexico must do more, simply to hold on to its progress in the face of uncontrolled growth.

Cars have doubled to more than 4.2 million. New suburbs are endemic.

Mexico City's geography adds to the problem; the city of more than 20 million is cradled in a 7,300-foot-high bowl, surrounded by peaks higher than 17,000 feet that trap pollutants.

But experts say many places overcame similar challenges. European cities, for example, halved pollution in recent decades by dramatically reducing coal fuel.

"Simple measures that enormously reduce pollution are feasible, and they are not expensive," said Michal Krzyzanowski, an air quality adviser for the World Health Organization.

"It is not the destiny of mankind to live in polluted cities."

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