Aug 20, 2010 6:01 AM
PHOENIX (AP) The Arizona Department of Environmental Quality has issued an ozone health watch for metropolitan Phoenix on Friday.
Ozone production is expected to increase as daytime temperatures rise and little cloud cover is expected.
Ozone is an invisible gas created when other pollutants in the air are heated. It can aggravate existing respiratory conditions such as asthma.
An ozone health watch is issued when the levels may reach federal standards.