Air pollution is expected to worsen Monday, contributing to unhealthy conditions for people with respiratory illnesses and other health issues.
Government forecasters say ozone pollution is expected to reach levels "unhealthy for sensitive groups" as the heat index is also expected to climb to dangerous temperatures.
Ozone pollution is caused when emissions from cars, trucks, factories and other sources interact with sunlight. That causes chemicals to cook in the air and create a dangerous type of pollution that is most common in the summer.
The pollution levels will be particularly bad for people with asthma and other lung conditions. Those who work outdoors should shorten their time in the heat. Seniors and the very young may also be vulnerable to the poor air quality.
Chicago Public Schools canceled outdoor activities Monday, the first day of school.
Some forest fire smoke from Canada may also be contributing to "moderate" particle pollution, though the fires from the north are nowhere near the size and intensity as those seen last year.
"The air quality problem this week will not be nearly as intense as last year during the Canadian wildfires, but it will be important for folks to take precautions," said Illinois State Climatologist Trent Ford. "This week's problem is mainly ozone, which is more effectively created during heat waves."
With this week's temperatures climbing into the upper 90s, it's possible that this summer will be as much as 2 degrees higher than the normal average over the past 30 years, Ford said.
That heat and sun make conditions ripe for ozone air pollution, he added.
"It's not unusual to have concurrent extreme heat and air quality problems, especially in cities," Ford said.
As for temperatures, late August has historically not been as hot as this week.
The hottest August 27 on record at O'Hare Airport is 97 degrees in 1973, Ford said. There's a chance that record will fall Tuesday.
Heat and pollution can also vary greatly on a given day depending on area of the city.
