Commuters ride on one of Mexico City's new smog-free double-decker buses.
Commuters ride on one of Mexico City's new smog-free double-decker buses, part of plan to tackle traffic and pollution in the megacity. Carlos Jasso/Reuters

The city has made strides in clearing up its notorious smog, but progress has stalled in recent years.

MEXICO CITY—With more than 20 million people living in its metropolitan area, a geographical bowl ringed by mountains, Mexico City has long struggled with air pollution. At times during the past two years, ozone concentration levels in the city reached such extreme levels that officials issued environmental risk alerts, urging people to stay indoors.

If the past is any indication of the future, 2018 could again be grim.

“The ozone season is just beginning,” said Luis Gerardo Ruiz, of the Center for Atmospheric Sciences at Mexico’s National Autonomous University, in early April. The period from March to June is when pollution tends to worsen, due to hotter temperatures and lower rainfall.

Over the past two decades, ozone levels gradually fell below government limits as authorities moved factories out of the capital and tightened regulations on fuel and cars. Beginning in 1989, a system called Hoy No Circula, or “No-Drive Days,” prohibited drivers from using their vehicles one weekday per week, with a schedule based on license-plate numbers. In 2008, this system was expanded to include Saturdays. Mexico City’s government also opened bus rapid transit lines and launched a large bike-share system to promote alternatives to driving.

The megacity, once known as the dirtiest in the world, got cleaner.

But in recent years, the rhythm of improvement has slowed, Ruiz said. Despite Hoy No Circula, the enormous number of cars and trucks—and the nitrogen oxide they emit—remain problems. (A scientific study of the Saturday driving rule found that it hasn’t reduced pollution levels or increased use of public transportation.)

Other measures may help. Last year, the country’s environmental secretary implemented a national strategy for air quality to coordinate the response of various government agencies to the pollution. New emissions standards for engines used in heavy-duty vehicles were adopted by the Mexican government in what has been called the single most critical policy to reduce black carbon emissions. Soot-free buses started operating this year on Mexico City’s historic Reforma Avenue.

Kate Blumberg, a fellow at the International Council on Clean Transportation, said making further progress comes down to political will. “I’d say if there were short-term things that have been done, it’s not going to keep getting better until they take some ... long-term actions,” she said. “It doesn’t make sense for the trucks and buses to be cleaner than the passenger cars.” If light-vehicle emission standards are adopted, she said, the step after that could be electrifying vehicle fleets.

Presidential frontrunner Andres Manuel Lopez Obrador, who served as Mexico City’s mayor from 2000 to 2005, has burnished his credentials as a fierce environmentalist, which could bode well for the city’s air quality. The potential impact on public health is hard to overstate: Recently, researchers identified a higher risk of Alzheimer’s disease and suicide in people exposed daily to high concentrations of ozone and fine particulate matter.

For now, Roberto Lopez, a city resident who leads a group of elite runners on weekly 55-to-90-mile runs, has to factor the dirty air into his plans. “When we train in the city, our throats are very irritated and breathing is hard,” he said, noting that some runners wear masks, train at far-flung parks, or keep an inhaler in their pocket if breathing becomes particularly difficult.

Lopez and his group have another coping strategy: They leave. “On the weekends, we go to the mountains so we can practice without the pollution problems,” he said.

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