PHOENIX (AP) -- Environmental regulators have issued an ozone high pollution advisory for the Phoenix metropolitan area on Friday.
There have been three ozone health watches issued by the Arizona Department of Environmental Quality since Monday.
Ozone is an invisible gas created when other pollutants in the air are heated by the sun, such as those created by combustion.
Experts say ozone can irritate the lungs and aggravate existing health conditions such as asthma.
A health watch is issued when ozone levels are expected to approach federal health standards.
Officials say people who are unusually sensitive to air quality should consider limiting their time outdoors.
Read: Ozone season officially under way; invisible danger in the air