An ozone health watch for the Phoenix area has been extended through Friday, according to the Arizona Department of Environmental Quality.
An ADEQ official said the area's pollution levels could exceed the federal health standard for air quality.
Ozone is an invisible gas created when other pollutants in the air - such as those created by combustion - are heated by the sun.
Experts said ozone can irritate the lungs and aggravate existing health conditions such as asthma.
Officials say people who are unusually sensitive to air quality should consider limiting their time outdoors.
They also recommended that people avoid waiting in their cars at drive-thru restaurants, unplug electronic devices that aren't being used and fuel their cars after dark or during cooler evening hours.
Copyright 2013 CBS 5 (KPHO Broadcasting Corporation). All rights reserved. The Associated Press contributed to this report.