Mexico City residents breathed a sigh of relief Saturday when authorities lifted a 4-day-old pollution alert after ozone levels declined to only 1 1/2 times acceptable limits.

But they're not out of the smog yet. Even as the alert was suspended, smoke was seen rising from stubborn forest fires in mountains surrounding the city of 8.5 million.This week's alert - the longest since 1997 - was triggered by smoke from forest fires here and elsewhere, mixed with car and industrial emissions. Under emergency measures, 40 percent of the city's 4.5 million vehicles are banned from the streets during alerts.

Alerts are declared when ozone reaches 250 points on a scale where 100 is considered an acceptable level, and are officially discontinued when ozone remains under 180 points. Smog above those levels began to collect Tuesday.

Unusually bad pollution in recent weeks led authorities to decide Friday to impose stricter emissions controls on taxis and buses, and ban 50 percent of cars - especially older models - during future alerts.