Due to wildfires burning in Arizona, the Santa Catalina Mountains are almost invisible beyond a ridge on Freeman Road near Old Spanish Trail in Tucson.

 Kelly Presnell / Arizona Daily Star

Wildfire smoke this month triggered Tucsonโ€™s worst air pollution in nine years.

On six of the days between Aug. 19 through Thursday, at least one of eight county air monitors around the metro area showed levels of the invisible but toxic ozone that exceeded federal EPA standards.

All this year, the county has endured eight days of excessive ozone. Thatโ€™s the most since 2011, the Pima County Department of Environmental Quality says.

On a more positive note, a big change in the weather predicted to start Saturday, Aug. 29, is likely to bring more winds and cloudier, cooler skies. That should bring the air back to more healthy levels as winds blow out much of the wildfire smoke, DEQ officials said Friday.

The effect of this weather shift will be โ€œlike when you open up a window and let the air out,โ€ said Beth Gorman, a DEQ program manager.

DEQ officials and the National Weather Service blame the high ozone levels largely on smoke that has invaded this area from wildfires burning as close as the Rincon Mountains and as far away as California and Colorado.

Environmental Protection Agency limits for the lung-tightening compound were also topped on June 11, as the Bighorn Fire in the Catalina Mountains was expanding toward a final total of nearly 120,000 acres.

In June 2011, when more bad air days occurred, wildfires were also burning near Tucson, in the Sonoita area and near Sierra Vista, Gorman said.

On this yearโ€™s other day that exceeded the ozone standard, May 5, wildfires were not a factor, county officials say. That dayโ€™s bad air was due in part to hot weather โ€” also a factor in this monthโ€™s elevated ozone levels โ€” and to polluted air bearing down from Phoenix, officials said at the time.

Ozone, which can trigger or aggravate respiratory problems such as asthma, is formed by the reaction of volatile organic compounds and nitrogen oxides in sunlight.

Both those ozone ingredients are found in wildfire smoke.

At high enough levels, ozone can trigger chest pains and coughing, irritate the throat and inflame the airways. It also can reduce lung function, harm lung tissue and worsen bronchitis and emphysema, the EPA says.

On Friday, ozone levels edged near the standard again, but started dropping at around 3 p.m. Officials didnโ€™t yet know whether the standard was exceeded again for the day.

Starting Saturday, Tucsonโ€™s rash of bad ozone days should end. The high-pressure air mass that has hung over this region for many weeks is about to be replaced by a trough of low-pressure air that will bring cooler temperatures and a greater chance of rain, the weather service says.

Saturdayโ€™s high temperatures are forecast to range from 95 to 100. That compares to temperatures that exceeded 106 degrees in Tucson for six of the seven previous days, including Friday.

โ€œThe low pressure will increase winds. That helps ventilate the area and get out the smoke lingering from wildfires,โ€ said Gorman.

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Contact reporter Tony Davis at tdavis@tucson.com or 806-7746.

On Twitter @tonydavis987