The alert automatically doubles from 20% to 40% the proportion of vehicles not allowed to circulate today.

It was declared yesterday afternoon after ozone levels rose above 150% of acceptable limits.

It also comes with other measures such as restricted activity at some gas stations and factories and a ban on burning solid or liquid materials.

Mexico City issued its first pollution alert in over a decade in March.

Normally one-fifth of the vehicles in the city and surrounding suburbs must stay at home on a weekday.

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