The alert will remain in effect through Thursday night.
As of 4:20 a.m. the North Side of Chicago and northern suburbs had unhealthy levels of air quality, according to the EPA. Areas near the border with Wisconsin and into southern Wisconsin was at very unhealthy levels.
The air quality alert comes as smoke from wildfires in Canada drifts into the area.
Smoke from Canadian wildfires in downtown Chicago
The NWS says air pollutants can cause headaches, irritated eyes and sinuses, fatigue, difficulty breathing, chest pains, asthma attacks, irritated throat and increased coughing.
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The Chicago Park District said it may move activities indoors on Thursday due to the air quality.
"Chicago Park District programs may be moved indoors, have limited time outdoors, or be canceled," an update read in part. "The Chicago Park District reminds visitors to take precautions while enjoying our parks, beaches, pools, and facilities during these conditions."
Experts say the combination of the heat, humidity, and pollution loaded with particulates could put those with respiratory problems at risk.
To stay safe people with heart or lung disease, older adults, children and teens are being advised to:
-Avoid strenuous outdoor activities
-Keep outdoor activities short
-Consider moving physical activities indoors or rescheduling them.
For everyone else:
-Choose less strenuous activities like walking instead of running so you don't breathe so hard (and)
-Be active outdoors when the air quality is better.
Health experts say under the current conditions, anyone suffering from a respiratory ailment or condition - would be advised to stay indoors.
ABC7 Accuweather Meteorologist Tracy Butler said there is a small chance for a few isolated storms Thursday afternoon. Butler said rain chances increase Friday afternoon and evening as well as Saturday evening.