MEXICO CITY (AP) — 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 1½ 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 Mexico City’s typical spring weather: hot, dry weather, a lack of wind and sunny days that favor the creation of ozone.  Source