If marathon training or fixing the roof were on the to-do list for the next day or two, hold off. The Minnesota Pollution Control Agency (PCA) has issued a health advisory in the Twin Cities for Monday and Tuesday due to a potential spike in ozone pollution.

The advisory is effective each day from noon until midnight.

Officials expect the air quality index, a daily indicator of air and contaminants, to reach 97. Above 101, the air is considered unhealthy for asthma patients, children, the elderly and people who engage in vigorous outdoor activities. Above 150, it's considered unhealthy for everyone.

Ozone, known commonly as smog, can have harmful respiratory health effects.

It "is primarily a respiratory irritant that acts almost like a sunburn [for the lungs]," said Cassie McMahon, a PCA air quality specialist. Ozone can also harm crops and forests.

The agency advises people to avoid or reduce outdoor activities during the day, especially between around 11 a.m. and sundown, when temperatures could hit the mid-90s.

In the air, the amount of ozone, which is made up of three oxygen atoms, varies according to temperature. It is produced when pollutants released by vehicles, factories, household appliances, burning wood and even paints and solvents chemically react with molecular oxygen, which is has two oxygen atoms.

On a hot, sunny day, that chemical reaction speeds up, generating more man-made ozone.

This situation is different from what happens in the stratosphere's ozone layer, which naturally contains small amounts of ozone produced when ultraviolet waves break up molecular oxygen into two single oxygen atoms. These loners sometimes collide with intact molecular oxygen and form ozone. This "good" ozone helps shield Earth from harmful ultraviolet radiation.

The PCA advisory was originally announced for Monday only, but the agency extended it due to high forecasted temperatures and smoke making its way to Minnesota from wildfires raging in Colorado. Possible showers on Wednesday might quench the heat and help reduce the amount of ozone in the air.

"We monitor air quality conditions every day, so if conditions do degrade on Wednesday, we will issue an advisory or alert as appropriate," McMahon said. "We want everyone to have a safe holiday."

Daniela Hernandez • 612-673-4088