Mexico City residents are packing buses and subway cars, and many are walking or biking to work because authorities have barred millions of vehicles from the streets due to a pollution alert.

New regulations were imposed recently after the capital experienced its worst air-quality crisis in over a decade. A Phase 1 alert was declared when smog levels hit 1½ times acceptable limits Tuesday.

Pollution levels on Wednesday are down somewhat, but a dense haze is obscuring the volcanic mountains that ring the city.

Martin Gutierrez is executive coordinator for the Environmental Commission of the Megalopolis. He says the alert will remain in place until at least 5 p.m. Wednesday.