CHICAGO -- Chicago-area residents are being warned about high levels of air pollution Saturday.
The Illinois Environmental Protection Agency says another day of hot weather prompted an Air Pollution Action Day in the metro Chicago because it expects elevated levels of ground-level ozone.
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Ozone is an unstable gas created when volatile chemicals and exhaust from smokestacks and tailpipes "cook" in the heat and sun.
The pollutant can be harmful to people with heart and lung problems and to those who exercise outdoors.
Health officials recommend that everyone limit outdoor activities Saturday and try to stay in a cool place. They also discourage activities that can contribute to ground-level ozone, including driving vehicles and mowing lawns.
Saturday's high temperature in Chicago is projected to be in the low to mid 90s.