Gabriela Escalante stalks the rumbling streets alongside newspaper, peanut and candy vendors, wading deep into traffic at red lights across town.
Her eyes are fixed on tailpipes.
A member of Mexico City’s ecoguarda, or environmental police, she and some 50 colleagues are on the lookout for white clouds of toxic exhaust, stopping hundreds of offending motorists each day, issuing 1,262 peso fines (US$95) fines and confiscating license plates — a small but urgent army fighting the capital’s infamous air pollution.
“We detect, we detain and we fine,” said Escalante, 27. “This is the air we all breathe.”
Not long ago, air in this throbbing capital was so bad that cyclists wore surgical masks. Birds fell dead in mid-flight and children used brown crayons to draw the sky. Ozone exceeded safe levels on 97 percent of days in the year.
But the metropolis ranked the world’s most polluted by a 1992 UN report has since slashed some of its worst emissions by more than three-quarters and has become a model for improving urban air quality.
Capitals such as Beijing, Cairo, New Delhi and Lima are now more contaminated, the World Bank says, while air in at least 30 other cities contains more toxic particles, including Barcelona and Prague.
When Latin American leaders met here last month to discuss the environment, many looked to Mexico as an example of progress, said Sergio Jellinek, a World Bank spokesman who attended the forum.
Still, a nagging cloud of ozone has been harder to reduce — a sign of the secondary air pollution problems that cities can expect even after cutting their most visible contaminants.
With the onset of winter, the worst time of year for pollution, Mexico City said it planned to spend 40 billion pesos (US$3 billion) by 2012 to expand public transit and further slash emissions.
“There has been a large improvement, and it’s important to show it could be done,” said Mario Molina, a Nobel Prize-winning Mexican chemist now advising US president-elect Barack Obama’s transition team on environmental issues. “But there’s still a long way to go to get really satisfactory air.”
Ringed by volcanoes and nearly 1km higher than Denver, the city’s geography and population make it a “perfect factory” for pollution, said Adrian Fernandez, head of the National Institute of Ecology, Mexico’s version of the EPA.
In thin air at over 2,240m fuel burns less efficiently, releasing more unused particles. Breathing deeper to fill their lungs, people inhale more toxins.
High-altitude sunshine speeds the chemical reactions that transform emissions into a lethal stew of smog. That brown cloud blankets the city, lowering temperatures and trapping pollutants on the ground.
“What you have is a casserole dish with a lid on top,” said Armando Retama, a chemist at the city’s Environment Department.
Mexico City and its sprawling suburbs swelled from 3 million people in 1950 to more than 20 million today, making it the world’s second-biggest urban area after Tokyo. Economic growth kept pace, boosting energy consumption and flooding the roads with more than 4 million vehicles.
Traffic is so clogged that average speeds have dipped to 21kph, the Environment Department says.
Even with today’s cleaner cars, experts agree that 70 percent to 80 percent of emissions are vehicle-related.