PHOENIX (AP) -- Environmental regulators are issuing a health advisory for high ozone for the Phoenix area for the second day in a row because searing heat and little wind is causing a buildup of the pollutant.
Ozone levels are already moderately high and Maricopa County air quality officials say they're expected to increase as it heats up on Wednesday.
The National Weather Service has issued an excessive heat warning for large areas of the state through Sunday, with temperatures in the Phoenix region above 100 degrees.
Ozone is an invisible gas created when other pollutants in the air such as those created by engines are heated by the sun.
Experts say ozone can irritate the lungs and aggravate existing health conditions such as asthma.