MEXICO CITY —
Mexico City is ordering 40 percent of cars and trucks to stay off the streets Thursday, extending for a third day a traffic cutback aimed at lessening pollution.
Under a rule in effect through June, one-fifth of the city's vehicles normally must stay at home on a weekday, with the day determined by license plate numbers. But on Wednesday, smog stayed above 1.5 times acceptable limits for a third straight day, meaning an additional 20 percent of vehicles can't be used Thursday.
Ozone, a key component of smog, reached 1.9 times acceptable limits. The metropolitan environmental commission blamed the hot, dry weather, a lack of wind, and sunny days that favor the creation of ozone.
Before the traffic rule was implemented last month, newer or cleaner cars were exempt from the one-day driving ban.
The emergency measures are in effect until June, when the arrival of the rainy season tends to cleanse the city's air.
A smog alert declared last month lasted two days, but the current alert showed no sign of abating in its third day.
"Several institutions have been measuring air quality in recent days with this double driving ban, and there has not been any significant or substantive change detected,'' said Jose Ordonez Diaz, a biology and climate professor at Mexico's National University.
He said the city has underlying problems of poor public transportation, polluting industry and sprawling suburbs. "It is basically the lack of public planning and unplanned development, both residential and industrial,'' Ordonez Diaz said.
Mexico City sits in a high valley, where the surrounding mountains can trap pollutants and prevent them from dispersing. The city is 7,350 feet (2,240 meters) above sea level.
It had about 4.7 million registered vehicles in 2014, the most recent year for which figures are available. City authorities have pledged to come up with a more effective and stringent system for testing vehicle emissions by the end of June.
But Mexico City's notoriously corrupt and lax law enforcement can't even handle current standards. While many relatively new, clean private cars were kept off the streets Wednesday, diesel trucks blasting plumes of black soot continued to roam the city's streets, with no attention from police.