Mexican officials lifted a four-day air pollution alert in the nation's densely-populated capital after ozone levels dropped to acceptable levels.

Mexico City authorities issued the first air pollution alert in 14 years due to high ozone levels, restricting traffic, encouraging children to stay indoors and ordering factories to cut emissions.

The city government declared an "environmental contingency" following a surge in ozone concentration, which can cause respiratory and heart ailments.

Pollution was so severe that from Tuesday through Thursday officials temporarily banned some 1.1 million vehicles from the metropolis.

Environment minister Rafael Pacchiano said in a statement late Thursday that the restrictions were lifted "given the current conditions and thanks to the emergency measures taken."

The alert was issued when the ozone concentration surpassed the 190-point limit, surging to 194, which can cause respiratory and cardiovascular ailments. By Thursday, after the emergency measures were implemented, that level had dropped to 134, Pacchiano said.

Mexico City is one of the world's most populous cities, where 21 million residents live and a honking mess of more than 5.5 million vehicles chug along every day.

The four-day pollution alert marked a reversal from years of progress to improve air quality after the United Nations declared the Mexican capital the world's most polluted city in the 1990s.

In the 1990s, ozone levels reached 398 points. The environmental contingency was declared 12 times in 1993 alone, according to the Megalopolis Environmental Commission.

The last time the emergency was issued was in September 2002.