Environmental regulators have issued an ozone health watch for metropolitan Phoenix on Thursday.
A health watch is issued 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 — such as those created by combustion — are heated by the sun.
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