MEXICO CITY — Metropolitan authorities on Wednesday temporarily ordered all cars to remain idle one day a week in response to this notoriously smoggy capital’s worst air-quality crisis in over a decade.

Until now vehicles have been exempt from Mexico City’s “no circulation” rules if owners obtain a holographic sticker from a smog-check center certifying them as lower-emission.

But the Environmental Commission of the Megalopolis, a cross-government agency consisting of the capital and surrounding suburbs that together are home to more than 20 million people, said via Twitter that all cars must now comply, even if they have the exemption sticker.

Vehicles will also be forced off the roads one Saturday a month.

The measure will begin Tuesday and run until June 30, around the time that rains typically arrive and improve the air quality significantly.

Officials have been meeting to consider anti-pollution measures since a Phase 1 emergency because of high ozone levels – the first since 2005 – was declared two weeks ago, when warm temperatures and still air left pollution trapped in the volcano-ringed valley.