Heavy trucks belch black smoke and lines of buses battle through a virtually gridlocked sea of cars inching beneath a haze of exhaust fumes.

Welcome to Mexico City in 2007. With car ownership more than doubling over the last decade, the megalopolis once dubbed the world's most polluted city should by now be almost uninhabitable, its residents gasping through oxygen masks.

The air doesn't exactly smell sweet. But look up beyond the tops of office buildings these days and the sky is blue.

Over the past decade Mexico City has rid its streets of the most polluting cars and bounced back from the dark days of the 1980s and 1990s, when children painted the sky as black or brown and opaque air sent choking birds plummeting to the ground.

Now, for a couple of days most months, the snow-capped volcanoes that serve as a dramatic backdrop to the city are visible - after decades of being blanketed by yellow smog.

"Things have changed," said Jose Luis Perez, 70, who has spent 50 years selling newspapers in the ever-more congested city center. "With the new cars and regulations, Mexicans don't pollute like they used to."

Air pollution earned Mexico City a place on a list of the world's 35 worst polluted places last year, but it escaped the top 10.

Nobel prize-winning Mexican chemist Mario Molina points out that the city's ozone levels are still higher than Los Angeles and Houston, the smoggiest US cities.

But gone are the regular smog emergencies of the 1990s when cars were temporarily banned from the streets and children kept inside until the air cleared.

"Pollution is much better than in the 1990s and much better than Los Angeles in the 1960s. There you were choking, your eyes would get watery and you could smell it," said Molina.

Tight emissions rules and better fuel have helped, scientists say, but perhaps the biggest change came through a credit boom during the 6-year term of former President Vicente Fox, who left office in December.

With unprecedented access to cheaper cash, Mexico City residents rushed out and bought new cars with fuel injection systems and catalytic converters, which emit up to 20 times less pollutants than older models.

Around 4 million cars are known to circulate in central Mexico City, up from about 1.5 million in 1996, according to industry statistics.

"Even though there are more vehicles circulating, the percentage of old, rundown cars has gradually dropped," said Jose Agustin Garcia, a pollution researcher at Mexico's UNAM university.

But Garcia, who still develops a hacking cough in the months he takes to the streets to do field work, warns that with larger, gas-guzzling models helping clog the city, the positive impact of cleaner motors could be outweighed.

"Our studies say things are going to get worse in the future," he said. "The trend toward 4x4 cars used to take kids to school counteracts the benefit of catalytic converters."

Both Molina and Garcia say more needs to be spent on public transport - the metro is efficient but does not reach new areas of the sprawling city and the buses are chaotic.

One area worrying the scientists is the level of fine particulate matter in the air - suspended particles of solids like dirt, soil, dust, pollens, molds, ashes and soot.

(China Daily via Agencies January 31, 2007)