PHOENIX (AP) -- The Phoenix metropolitan area is again under an ozone high pollution advisory.
The Arizona Department of Environmental Quality says a health watch is issued when ozone levels are expected to approach the federal health standard.
The Phoenix metro area has been under an ozone health watch since Tuesday.
A high pollution advisory is issued when ozone levels are expected to exceed federal health standards.
Ozone is an invisible gas created when other pollutants in the air are heated by the sun.
Friday's temperatures are expected to be near 100 degrees. ADEQ officials say that'll create optimal conditions for ozone production and accumulation for the next few days.
Residents with existing health conditions like asthma should limit time outdoors, especially in the early evening when ozone production is at its peak.