A smoky haze settles over Chicago due to Canadian wildfires on June 27, 2023. (WTTW News)

Chicago’s air quality remains at unhealthy levels Thursday as the city and other portions of the Midwest have been blanketed by smoke from Canadian wildfires this week.

According to data from the Illinois EPA, Chicago’s Air Quality Indexwas at a 185 as of Thursday morning, placing it in the “unhealthy” range. That means older adults, people with heart or lung disease and children and teens should avoid strenuous outdoor activities.

Chicago remains under an Air Quality Alert issued by the Illinois EPA, which remains in effect until midnight Thursday.

The air quality has improved since earlier this week, when Chicago was in the “very unhealthy” category and had the worst air quality among major cities around the globe.

According to the Illinois EPA, pollution from wildfire smoke is made of fine particles that can cause serious health problems such as asthma attacks, heart attacks and strokes. Forecasts indicate Chicago’s air quality should continue improving in the coming days, and may move back into “moderate” levels by the weekend.

But officials believe the Canadian wildfires aren’t likely to die down anytime soon, with nothing in the forecast that looks likely to change.

Nearly every province in Canada has fires burning. A record 30,000 square miles have burned, an area nearly as large as South Carolina, according to the Canadian government.

And fire season usually doesn’t really get going until July in Canada.

Both high heat and smoky conditions are stressors on the body and can present potential challenges to human health, said Ed Avol, a professor emeritus at the Keck School of Medicine at University of Southern California.

But Avol added that while the haze of wildfire smoke provides a visual cue to stay inside, there can be hidden dangers of breathing in harmful pollutants such as ozone even when the sky looks clear. He also noted there are air chemistry changes that can happen downwind of wildfire smoke, which may have additional and less well-understood impacts on the body.

The National Weather Service Chicago on Thursday also issued a severe thunderstorm watch for parts of Illinois (including Chicago and Cook County), Indiana, Iowa and Missouri until 5 p.m.

Chicago officials have said those without access to safe indoor conditions should utilize the city’s public libraries, senior centers, Park District facilities and the Cultural Center to get away from unhealthy air.

Chicago also has six community service centers that operate from 9 a.m. to 5p.m. Those are located at:

Englewood Center — 1140 W. 79th StreetGarfield Center — 10 S. Kedzie Ave. (24 Hours)King Center — 4314 S. Cottage GroveNorth Area Center — 845 W. Wilson Ave.South Chicago Center — 8650 S. Commercial Ave.Trina Davila Center — 4312 W. North Ave.

Patty Wetli and the Associated Press contributed to this report.