The Mexico City government has declared its first air pollution alert in 11 years, after ozone levels reached almost twice the acceptable limit.

The measure 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 in 2005.