The Illinois Environmental Protection Agency has declared an Air Pollution Action Day in the metro Chicago area for Friday because of elevated ground-level ozone.
The unstable gas is created when volatile chemicals and exhaust from smokestacks and tailpipes "cook" in the heat and sun.
It can be harmful to people with heart and lung problems and to those who exercise outdoors. Health officials recommend that everyone limit outdoor activities on Friday 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.
Friday's high temperature in Chicago is projected to reach 103 degrees.
Copyright 2012 The Associated Press.