The Americas

Mexico City declares 3rd day of traffic cuts over high smog

  • FILE - In this March 18, 2016, file photo, view shows Mexico City covered in smog a day after a pollution alert was lifted. Mexico City authorities have declared a pollution alert on Monday, May 2, 2016, after smog rose to 1½ times acceptable limits for second time in less than a month. (AP Photo/Rebecca Blackwell, File)

    FILE - In this March 18, 2016, file photo, view shows Mexico City covered in smog a day after a pollution alert was lifted. Mexico City authorities have declared a pollution alert on Monday, May 2, 2016, after smog rose to 1½ times acceptable limits for second time in less than a month. (AP Photo/Rebecca Blackwell, File)  (The Associated Press)

  • Air pollution hangs over Mexico City, Tuesday, May 3, 2016. Mexico City authorities have declared a new pollution alert and will require 40 percent of cars to keep off the streets. (AP Photo/Eduardo Verdugo)

    Air pollution hangs over Mexico City, Tuesday, May 3, 2016. Mexico City authorities have declared a new pollution alert and will require 40 percent of cars to keep off the streets. (AP Photo/Eduardo Verdugo)  (The Associated Press)

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 almost 1.8 times acceptable limits.

Before the rule was implemented last month, newer or cleaner cars were exempt from the one-day driving ban.

The pollution spike is due to seasonal weather patterns and the increasing number of cars in the city.