PHOENIX -- Environmental regulators are issuing a health advisory for high ozone for the Phoenix area for the fourth consecutive day.
They said searing heat and little wind is causing a buildup of the pollutant again on Friday. 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 levels are already moderately high and Maricopa County air quality officials said they're expected to increase as it heats up.
Ozone is an invisible gas created when other pollutants in the air such as those created by engines are heated by the sun.
According to experts, ozone can irritate the lungs and aggravate existing health conditions such as asthma.