Become an air pollution activist. Identify local groups working to combat air pollution, and find a way to get involved. By educating yourself and others about the problem, you can have a greater impact than you'd have by simply making lifestyle changes.
Join a group that plants trees to help clean the air.
Become a bike activist. Work to have safe paths built in your city.
Contact your local representatives to speak up about issues particular to your region. If there's a factory spewing pollutants into the air, for example, get politically active to put a stop to it.