The alert automatically doubles from 20 per cent to 40 per cent the proportion of vehicles not allowed to circulate today.
It was declared yesterday afternoon after ozone levels rose above 150 per cent 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.
PTI