MEXICO CITY -- The Mexico City government declared its first air pollution alert since 2005 Monday after ozone levels reached almost twice the acceptable limit.
According to the city's environment office, a high-pressure weather system and intense sunlight may have caused the high ozone conditions.
The air alert requires older and more heavily polluting vehicles to stay off the road Tuesday, in an attempt to improve air quality.
Mexico City used to regularly reach high smog levels, before a rule was introduced to discourage cars more than 8 years old. That rule was recently relaxed by a court order, and environmental activists and officials say that has led to more cars on city streets.
Ozone is a component of smog that can cause respiratory problems. Mexico City's last city alert for ozone was in 2002. The last pollution alert for air particles was 11 years ago in 2005.
Mexico City sits in a high mountain valley, where the surrounding mountains can trap pollutants and prevent them from dispersing. The city is 7,350 feet (2,240 meters) above sea level.
The alert declared Monday also limits highly polluting industrial processes, and officials recommend that people stay indoors and not perform vigorous exercise outdoors.