Mexico City - The Mexico City government declared its first air pollution alert in 11 years on Monday after ozone levels reached almost twice the acceptable limit.The measure requires older and more heavily polluting vehicles to stay off the road on Tuesday, in an attempt to improve air quality.The city's environment office attributed the conditions to a high-pressure system and intense sunlight.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 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 2 240m above sea level.The alert declared on Monday also limits highly polluting industrial processes, and officials recommend that people stay indoors and not perform vigorous exercise outdoors.