PHOENIX (AP) -- Environmental regulators have issued an ozone health watch for metropolitan Phoenix on Monday.
Ozone is an invisible gas created when other pollutants in the air -- such as those created by combustion-- are heated by the sun.
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.