12:00 AM CDT on Wednesday, April 7, 2010

MEXICO CITY – This megalopolis once had the world’s worst air, with skies so poisonous that birds dropped dead in flight. Today, efforts to clean the smog are showing visible progress, revealing views of snow-capped volcanoes – and offering a model for the developing world.

As Mexico prepares to host world leaders at a U.N. climate-change conference later this year, international experts are praising the country’s progress. Many say its determined efforts to control auto emissions and other environmental effects of rapid urbanization offer practical lessons to cities in China, India and other fast-growing countries.

International officials say steady improvement of Mexico City’s air could bolster President Felipe Calderon’s bid for a leadership role among developing countries addressing global warming.

“We have seen a lot of improvement. It is very clear,” said Luiz Augusto Cassanha Galvao, a senior environmental officer at the Pan-American Health Organization. “On a scale of one to 10, they were at 10, and now they’re at five.”

Mexican officials have attacked the root causes of pollution that plagues many large urban centers with spiraling growth.

They plan to reduce further vehicle emissions, which are the city’s greatest source of pollution. Pemex, the state oil monopoly, plans to build a $9.3 billion plant to produce low-sulfur fuel. Mexico City Mayor Marcelo Ebrard is expanding the low-emissions Metrobus system, which has eliminated 80,000 tons of carbon monoxide annually since 2005. Officials plan to add hybrid buses. A suburban train system is to replace hundreds of thousands of vehicles.

Mexico City appears to have cut most of its pollutants at least by half, said Miguel Naranjo, a Panama City-based official of the U.N. Environment Program, while recent studies show a number of cities in China and India recording higher levels of the most serious pollutants.

In 1992, the United Nations declared Mexico City the most polluted city on the planet. High ozone levels were thought to cause 1,000 deaths and 35,000 hospitalizations a year. Thermal inversions held a toxic blanket over a grimy city that seemed to embody the apocalyptic “Makesicko City” of the fiction of Mexican author Carlos Fuentes.

Mexico was forced to act. It replaced the city’s soot-belching old cars, removed lead from gasoline, embraced natural gas, expanded public transportation, and relocated refineries and factories. Experts say Mexico must do more, just to hold on to its progress in the face of uncontrolled growth.

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

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

The presence of lead in the air has dropped by 90 percent since 1990. Suspended particles – 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, though obviously, it is not 100 percent satisfactory,” said Raul Estrada, a spokesman for Greenpeacein Mexico.

“You shouldn’t even have one hour of high ozone levels in an entire year, and we have 530 hours,” said Humberto Bravo Alvarez, one of Mexico’s most respected air pollution experts.