The wildfire smoke that arrived in Chicago on Tuesday took many by surprise, including weather experts. When it will leave remains unclear.

Forecasters had predicted improved conditions for Wednesday, but air quality and visibility remained the same or worsened, weather officials said. That’s because dynamics like location and weather patterns make the smoke’s path difficult to predict.

For the second day in a row, air quality was considered “very unhealthy” in the Chicago area, according to AirNow, a website combining county, state and federal air quality data. The Illinois Environmental Protection Agency forecast that air quality could improve Thursday to “unhealthy for sensitive groups” like children and adults with respiratory conditions.

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Gino Izzi, senior meteorologist with the National Weather Service in Romeoville, said it’s difficult to forecast when the smoke will completely clear up, as this is “uncharted territory.”

Satellite data showed on Wednesday that “there’s smoke pretty much around us everywhere,” Izzi said. He said there were not a lot of strong winds besides one blowing from the south, which could bring Tuesday’s smoke back into the area.

According to the monitoring site IQAir, Chicago had the worst air quality in the United States and ranked second out of 95 cities worldwide Wednesday. It ranked No. 1 on the global site Tuesday.

Ten air quality alert days have occurred in the Chicago Metropolitan area in 2023, with eight of those coming in the month of June, the weather service said.

The main pollutant behind the lingering smoke is PM2.5, tiny particles of solids and liquids that are small enough to enter the lungs and bloodstream, creating a plethora of health problems. It’s not just in wildfire smoke, either. PM2.5 can be emitted from factories, power plants, diesel and gasoline vehicles, residential fireplaces and wildfires.

“These PM2.5 are highly, highly dynamic, you know, meaning they keep on evolving with time in the atmosphere, and their composition in one region could be vastly different than in another region,” said Vishal Verma, an assistant professor of environmental engineering at the University of Illinois at Urbana-Champaign.

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In the case of wildfire smoke, there are two dangerous compounds at play when trees burn. One is volatile organic compounds, which include the cancer-causing benzene. The other is transition metals like iron or copper, which are found in tree soil because of industrial activities or natural rock formations.

“The two major classes of compounds in these particles, which are most responsible for the health effects, are organic compounds and transition metals, both of which are actually rich in wildfire emissions,” Verma said.

Once those particles are in the air, atmospheric processes can alter the chemical makeup, making them more hazardous to human health. Ozone levels, the amount of sunlight and wind patterns all affect wildfire smoke.

For example, high ozone levels occur when there’s more sunlight. This makes good conditions for a process called oxidation, which is when a substance changes because of the addition of oxygen. Verma said particles are then more water-soluble, making it a lot easier for them to enter the lungs.

“Yes, the concentration of these particles will get diluted as they travel in the atmosphere,” Verma said. But this lower concentration of particles in Chicago compared with Canada is a trade-off. There are fewer of them, but they are more harmful by the time they get here.

David Martell, a professor emeritus at the University of Toronto and expert in forest fire management systems, said Canada’s wildfire season typically peaks June to August, with things quieting down in September.

According to the Canadian Interagency Forest Fire Centre, the 10-year average for area burned is about 1.2 million acres. This season as of Wednesday, about 19 million hectares have burned — nearly 16 times the national average. There are currently more than 480 active fires.

Martell said this wildfire season can be attributed to climate change, but it’s hard to pinpoint the exact factors that have worsened. He anticipates wildfires will only become more challenging in the future.

“Climate change has made our fire regimes worse than they would have been without climate change,” Martell said.

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At the “very unhealthy” PM2.5 levels, everyone is at risk for health effects. But it’s particularly dangerous for people with underlying respiratory conditions like asthma or chronic obstructive pulmonary disease, experts said. These are the same groups that are considered “sensitive” for Thursday’s forecast.

Emergency rooms citywide have reported more patients coming in complaining of symptoms related to the wildfire smoke.

“In my pulmonary clinic, all of my lung patients have been complaining as to how their breathing has exacerbated through this,” said Phillip Cozzi, a pulmonary disease specialist with Edward-Elmhurst Health. “So it’s really the people who are at risk that are suffering most.”

Cozzi said some people aren’t taking the air quality alert very seriously. He’s had to pull up the index for patients to get them to take precautions.

“When they look at it, they recognize hey, I’m not making this up, this is a legit phenomenon and ‘I need to be mindful of the exposure,’” Cozzi said.

He said he doesn’t anticipate a lot of long-term health effects for people with healthy lungs exposed to short-term pollution, which is supported by the World Health Organization. It’s more of an issue for people living in frequently polluted environments. But that could change as wildfires in Canada become more frequent.

In a statement from the mayor’s office issued Wednesday, the city encouraged Chicagoans, particularly sensitive populations, to take precautions. Those include:

• Avoid spending time outdoors. If you must go outside, keep outdoor activity short and wear a KN95 or N95 mask while outside.
• Stay indoors and keep your indoor air as clean as possible by not smoking, using candles, or vacuuming.
• Delay or reduce, and/or halt outdoor activities and events wherever possible.
• Follow your doctor’s advice if you have asthma, lung or cardiovascular disease. Some symptoms of breathing smoke include wheezing, chest pain, shortness of breath, and trouble breathing. If your symptoms worsen, call your physician or 911.
• Keep windows and doors closed and turn on your air conditioner if you have one.
• Seek shelter elsewhere if you do not have an air conditioner and your home is too warm.
• Use an air purifier if you have one.
• Limit driving a vehicle if possible.
• Stay tuned to local news media advisories.

“The alarming air quality readings over the course of the last two days prove that all Chicagoans must keep their guard up,” said Mayor Brandon Johnson in the statement. “My administration and I are keenly aware of the climate crisis’s impact in this moment; vulnerable communities in Chicago bear a continuously heavier burden from climate exacerbated extreme weather. We must continue driving equitable and just solutions, reducing our emissions and building a more resilient city for all.”

For the air quality forecast and tips on how the public can reduce pollution go to www.airnow.gov.

Chicago Tribune’s Deanese Williams-Harris contributed.