The Illinois Environmental Protection Agency is warning Chicago-area residents about high levels of air pollution.
The EPA says air quality on Tuesday is expected to be unhealthy for people who have breathing problems, active children and the elderly.
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Officials expect elevated levels of ground-level ozone because of hot temperatures and sunny weather.
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 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.