PHOENIX (AP) -- An ozone health watch issued Thursday for metropolitan Phoenix has been extended through Friday.
Environmental regulators issue health watches when ozone levels are expected to approach federal health standards.
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.
Ozone is an invisible gas created when other pollutants in the air are heated by the sun.