Authorities in greater Mexico City extended an air pollution alert for a fourth day, as smog levels improved slightly but pollution remained at almost 1.5 times acceptable limits in some areas.

The city's first air pollution alert in 11 years resulted in a driving ban that kept hundreds of thousands of cars from the roads on Wednesday. A metropolitan commission said that had helped, but hot, dry, windless conditions were expected to last through midday on Thursday.

In a rotating scheme, a different but equivalent set of cars were to be told to stay off the streets on Thursday.