An ozone health watch for the Valley has been extended into Thursday by the Arizona Department of Environmental Quality.
People with existing respiratory conditions should limit time outdoors, especially in the late afternoon when ozone production is at its peak.
Ozone is an invisible gas that is created when other pollutants in the air are heated by the sun. Temperatures Thursday will remain in the mid to high 90s.
Ozone levels shot up quicker than expected, with a near exceedance occurring at the Cave Creek monitor on Tuesday. Similar concentrations were expected Wednesday and Thursday afternoon.