An ozone health watch has been issued for Friday by the Arizona Department of Environmental Quality.
Local ozone levels were elevated for the third day Thursday, with only five Valley recording sites in the "good range."
Ozone is an invisible gas that is created when other pollutants in the air are heated - such as vehicle combustion. Ozone can aggravate the lungs and can create health problems for people with existing respiratory conditions, such as asthma.
Local ozone levels for this time of year are running well ahead of climatology for reasons not clearly identified, the department said in a press release.
People with health issues, as well as the very young and the elderly, should limit time outdoors Friday. Ozone production tends to be at its highest in the late afternoon.