Mexican authorities have issued a new smog alert for the capital after ozone levels rose to almost twice acceptable limits.

It's the fifth time this year that Mexico City has seen such an alert, which triggers additional restrictions on automobile usage.

On Saturday, 40 percent of vehicles were barred from the streets of the capital and the surrounding suburbs.

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

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.

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