For Mexico City, though, it's an especially sad backslide. While it was the most polluted city in the world back in 1992, efforts to clean up its cars and improve air quality made a big difference. Thanks to smog checks and policies to decommission older, more polluting vehicles, officials didn't need to issue pollution alerts in Mexico City for 11 years. Last year, though, the government relaxed those restrictions.
This week, as the surrounding mountains trap dirty air in the valley containing the country's capital, Mexico City banned over a million vehicles from the roads as ozone levels have nearly doubled the acceptable limits. (Ironically, this comes just shortly after the city hosted a Formula E race.) The government even considered shutting down industry until the air quality improved. To help convince people to give up their cars, the city offered free bus and subway rides to citizens. Sadly, only about 800,000 of the cars banned followed the rules according to Mayor Miguel Ángel Mancera, and the pollution alert lasted four days before finally being lifted on Thursday (despite continued excessive levels of ozone).
Moving forward, Mexico City will revisit its pollution control measures in hopes of avoiding another crisis, according to environment secretary Alejandro Pacchiano. It's estimated that the recent relaxations of pollution rules has added an extra 1.4 million cars to the roads.