Four straight days of unhealthy air quality since Saturday have taxed people with asthma and breathing problems and set up Minnesotans for a potentially harsh summer.

Poor air quality isn’t unusual in May, when occasional spikes in temperature combine with airborne pollutants to increase ozone levels, but a run of four days hasn’t occurred in the Twin Cities since the hot summer of 2012, said Daniel Dix, a meteorologist for the Minnesota Pollution Control Agency.

“It’s tying hand in hand with our temperature, our heat wave,” he said. “Very unusual.”

The agency predicts and monitors daily ozone and air quality index levels, because high levels can exacerbate breathing problems for those with asthma, allergies and other respiratory disorders.

When the forecast projects an index above 100 for the following day, the agency issues a statewide alert so vulnerable people can take steps to protect themselves, such as staying indoors in the afternoon.

Minnesota’s average levels were 101 Saturday, 133 Sunday, and 115 Monday. The forecast for Tuesday was above 100 as well.

Levels above 150 are considered unhealthy to all — not just those with respiratory problems — but those days remain rare in Minnesota. They are sometimes linked to large environmental events such as wildfires.

Even in summer 2012, the Twin Cities had only six days with unhealthy ozone levels, Dix said. This year, “we’ve already had four,” if Tuesday’s forecast proves true, he said.

Ozone is produced in the lower atmosphere on hot, dry, sunny days by a chemical reaction between volatile organic compounds and oxides of nitrogen. High levels are more common in arid cities such as Phoenix and Los Angeles.

June is peak month

June is usually the peak month for poor air quality days in Minnesota — before the high humidities of late summer reduce the odds of ozone formation. But poor air quality in May offers the added misery of high pollen counts, Dix noted. “The heat, the air quality and the pollen are going to get to you.”

Tree and grass pollen levels were both in the moderate to high range this weekend.

Emergency room visits didn’t increase noticeably at Regions Hospital in St. Paul, or at Methodist Hospital in St. Louis Park, according to hospital spokespeople. But Dr. Andrew Nickels, an allergy and immunology specialist with Park Nicollet, said the number of patients reporting severe symptoms has surged.

Nickels said he can’t sort out which ones suffered ill effects from poor air quality vs. those simply sniffling and wheezing due to seasonal pollen.

Both are probably playing a role along with the rapid arrival of summer temperatures and high pollen levels before people had time to adjust and start taking allergy medications, he said. “I suspect that it is because of the kind of zero-to-60 season we’ve had, where it was very cold, very cold, very cold and then, bam!”

While ozone plays a crucial role in Earth’s upper atmosphere, absorbing ultraviolet radiation, its formation at ground level is problematic, Dix said. He described its role as, “Good up high, bad down low.”

Minnesotans can take steps to help reduce ozone levels on high-alert days, such as avoiding driving or mowing lawns, he said.

Filling up gas tanks in the evening, after ozone levels have peaked, can help as well.