CHICAGO—Parks, beaches and restaurants offering outdoor dining are typically booming here in the summer months after a long, frigid winter, but smoky skies have kept many residents indoors since Tuesday when Chicago’s air quality was briefly the worst in the world.

Smoke from wildfires in Canada has blanketed parts of the Midwest and Great Lakes this week, with the highest particulate matter levels in parts of Illinois, Wisconsin, Michigan, Pennsylvania and Iowa, according to air quality tracking by the Environmental Protection Agency. PM2.5, particulate matter smaller than 2.5 microns—about 30 times smaller than the diameter of a human hair—is a mixture of solid and liquid particles in the air that can harm human health when inhaled.

Climate experts say it is hard to tell if this will be something Midwesterners will have to deal with frequently, but this will likely not be the last time.

“It’s something that we don’t see very often [in Chicago],” said Illinois state climatologist Trent Ford.

Rising temperatures due to climate change have made wildfires more common and more intense worldwide, temporarily deteriorating air quality in areas surrounding the blazes and, occasionally, in regions far from the flames. 

This trend is evident in the western U.S., where wildfires have been burning more acres of land in recent years, but the circumstances that led to wildfire smoke from the Northeast into the Midwest are not typical. 

“In the past years, it was mainly the western part of Canada that has seen a lot of wildfires, now it looks like it can happen in the eastern part as well,” said Nicole Riemer, professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. “The conditions may not be like this every year, but it’s probably going to happen again.”

The dry atmospheric conditions that led to wildfires in Northeastern Canada are the same kind of conditions that blew the smoke into the U.S. Midwest and Northeast, Ford said.

Air quality advisories were in effect across the region since Tuesday, with air quality index levels reaching “very unhealthy” in some areas. The haze is slowly flowing into the mid-Atlantic and Northeast, according to the EPA’s tracking of the smoke.

The event follows smoke that enveloped the Northeast earlier this month. At the time, New York City briefly had the worst air quality of any city in the world, and it recorded the highest number of emergency room visits for asthma this year, with the Bronx especially hard hit by the haze, Inside Climate News reported.

The swaths of smoke exacerbate health risks for people already burdened by air pollution from transportation and industrial sources. Respiratory health risks are already higher in neighborhoods with larger Black and brown populations, which are more likely to live closer to sources of pollution.

People breathing in the smoke may experience a range of issues, including scratchy throats, eye irritation, coughing, wheezing and difficulty breathing. Particulate matter can get into the circulatory system through the lungs and is associated with outcomes from stroke and heart attacks, to premature births and premature deaths, according to Brian Urbaszewski of the Respiratory Health Association.

The smoke worsens already poor air quality in communities like Chicago’s Southwest and Southeast Side, where ozone and particulate pollution levels are already higher than in the rest of the city. These communities already deal with some of the worst air pollution in the U.S., according to an analysis by the Guardian in March.

“Marginalized communities are suffering additional harm right now, but next week, when the rest of the city is breathing clean air again, we will still be breathing pollution from very active industrial operations that are in close proximity to our built environment,” said Alfredo Romo, executive director of Neighbors for Environmental Justice in Chicago. “For us, the problem doesn’t end when the wind shifts.”

The Canadian wildfires have forced federal, state and local officials to raise awareness about the poor air quality and how it negatively affects the environment and human health, said Romo. He hopes this created a larger sense of urgency to address the cumulative impacts of industrial pollution and transportation that disproportionately impacts the Southwest and Southeast side communities.

“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,” said Chicago Mayor Brandon Johnson in a public statement on Wednesday. 

The City of Chicago–like other affected cities–recommended that Chicagoans stay indoors and use a KN95 or N95 mask while outside. They offered public libraries, senior centers, park facilities and the city cultural center as places for people without properly ventilated and safe indoor spaces. Exposure to the smoke can’t be completely avoided indoors but can be limited by closing windows and doors and using air filters, said Urbaszewski.

“If it’s really hot, smoky and you don’t have an AC, keeping windows shut and succumbing to heat is a risk, so people in that situation should look elsewhere for a temporary safe, cool place to stay,” said Urbaszewski.