Chicago was the seventh-most polluted major city in the country in 2024, according to a global air quality report released Tuesday. The average annual concentration of small particulate matter in the air led to a 35% decrease in the air pollution levels that had placed the city second in the previous year’s ranking.
“The goal of the report is to raise awareness that eventually will translate into impact around global air pollution levels, and basically just continue to raise the alarm about PM2.5 in particular, and how damaging it is to people’s health,” said Christi Chester Schroeder, the lead author and an air quality science manager at Swiss air quality technology company IQAir.
On average last year, Chicago’s atmosphere had 8.4 micrograms of particulate matter per cubic meter of air, considerably lower than its 2023 average of 13 micrograms. But “that’s still a far cry” from the World Health Organization’s recommended guideline of 5 micrograms per cubic meter, Schroeder said.
Wildfires in the Canadian province of Quebec during the summer of 2023 contributed to higher levels of PM2.5 that year. Schroeder said other contributors to lower pollution levels could be that the winter of 2023-2024 was the warmest on record for the country and the fifth-warmest in Chicago — which meant people didn’t produce as much smoke from using wood-burning fireplaces to heat their homes.
The report comes almost two months into President Donald Trump’s new administration, which has tried pausing several Clean Air Act federal court cases, paving the way toward possible rollbacks or reversals. In February, the U.S. Court of Appeals for the District of Columbia Circuit granted a 60-day freeze on litigation over a U.S. Environmental Protection Agency decision last year that lowered annual air quality standards from 12 micrograms to 9 micrograms per cubic meter of air.
Fine particles, or PM2.5, are smaller than or equal to 2.5 micrometers — that’s about 30 times smaller than the width of a strand of human hair. Produced by vehicle exhaust, industry emissions and forest fires, PM2.5 can harm human health and sometimes be deadly. Initially, fine particulate matter may cause a burning sensation in the eyes and nose. But because of its small size, it can settle deep in the lungs and cross into the bloodstream.
The Biden-era rule for more stringent PM2.5 limits was a “big jump,” according to Brian Urbaszewski, director of environmental health at the Respiratory Health Association, a Chicago-based nonprofit. “I mean, we lowered it by 25%. It should have been lower, at least 8. But still, 9 was a good outcome.”
He said it would surprise him if standards were relaxed again, as it would “fly in the face of scientific evidence.”
“Under the first Trump administration,” he said, “they decided to not tighten the standard, despite overwhelming evidence that it needed to be, but they didn’t roll it back.”
The Illinois Environmental Protection Agency recommended Feb. 24 that its federal counterpart label the Chicago metropolitan area and the Metro-East St. Louis area as “nonattainment” areas, meaning they didn’t meet the latest federal air quality standards for PM2.5 from 2021 to 2023, the most recent three years of data.
The areas include Cook, DuPage, Kane, Will, Madison and St. Clair counties, as well as surrounding ones that contribute to pollution there such as Lake, McHenry, Kendall, Grundy, Monroe and Randolph counties. Should the U.S. EPA move forward with these designations, Illinois could have to set additional remediation measures for industries to ensure the nonattainment areas meet federal standards.
Air pollution accounted for 8.1 million deaths globally in 2021, becoming the second-leading risk factor for death after high blood pressure, according to a special report published by the Institute for Health Metrics and Evaluation, a global statistics research institute out of the University of Washington in Seattle, and Health Effects, a Boston-based independent research nonprofit. In children younger than 5, air pollution was the second-leading risk factor for death in 2021, after malnutrition.
According to IQAir, the most polluted major city in the country last year was Los Angeles. Wildfires in 2024 burned nearly 9 million acres across the United States, roughly equivalent to the size of Taiwan — of those, 1 million burned in California alone, according to the National Interagency Fire Center’s annual report.
After Los Angeles, the most polluted major U.S. cities included Fort Worth, Texas; San Antonio; Oklahoma City; Memphis, Tennessee, and Houston. Columbus, Ohio; El Paso, Texas, and Philadelphia rounded up the top 10.
Scientists at IQAir analyzed data from more than 40,000 air quality monitoring stations across 8,954 locations in 138 countries, territories and regions; 62 of those stations were in Chicago.
Besides using data from government stations — which can cost tens of thousands of dollars and require regular upkeep and maintenance, thus limiting coverage areas — IQAir also analyzes information from low-cost, low-maintenance sensors. In Chicago, the Illinois EPA owns only a handful of the monitoring stations used by the tech company. The rest are owned by individual contributors, which can be citizens, community groups and schools.
The scarce data from government-operated sensors — which has to be high quality given that it’s used for compliance and regulatory purposes — can be enhanced with more numerous data points from less complex sensors used by citizens, researchers and community advocates, which don’t have to meet the same standards but can prove useful in regular information-gathering.
“This sort of just confirms what federal reference monitors are already recording,” Urbaszewski said. “That we do have a problem.”
PM2.5 can have hyper-local variability; for instance, in Chicago, primarily Black and Latino neighborhoods with more traffic and heavy industry can have higher concentrations of particulate matter over time. That can be exacerbated by temporary factors such as fireworks on July Fourth.
Other sources like wildfires that produce giant migrating plumes also can settle particulate matter over large swaths of land.
“Levels can vary greatly depending on your geographical location, and also in terms of time; it varies from season to season, it can vary from day to day,” Schroeder said. “So expanding the inclusion of low-cost sensors gives you a much more comprehensive picture of trends in air quality, both in space and in time.”
When it comes to smog, Cook County is the worst neighbor in the country, EPA finds
In 2023, the U.S. EPA found that drifting air pollution from Chicago and the rest of Cook County contributed more to smog violations in other states than any other county. The Good Neighbor Plan — a rule that imposed stringent limits on industry to protect communities from smog pollution that blows across state lines — was blocked by the Supreme Court a year later.
The agency estimated that once the plan was fully implemented in 2026, it would have prevented roughly 1,000 premature deaths and avoided more than 2,000 hospital and emergency room visits, 1.3 million asthma attacks and 470,000 missed school days.
States also can remove really intense periods of pollution, like the Canadian wildfires in 2023, from legally binding scorecards that determine whether they’re doing enough to fight pollution and meet U.S. EPA standards. They can do so by seeking to invoke the so-called exceptional events exclusion for pollution humans don’t cause and can’t control, a request the U.S. EPA must approve.
“So it doesn’t really give a clear picture of the actual conditions that people were exposed to,” Schroeder said, making it harder to account for public health impacts. “This really goes back to: Why are you monitoring? “Are you monitoring for information? Are you monitoring for compliance?”
The areas in Illinois that the state EPA says don’t meet federal air quality standards are plagued by pollution beyond the so-called exceptional events, Urbaszewski said.
“The wildfires made it worse than it would have been,” he said. “But even if you took the wildfires out of the equation, we’d still have a problem.”
The U.S. EPA began allowing the exclusions in 1977. As wildfires fueled by climate change have become more frequent, more states have sought to classify them as exceptional.
“The fact of the matter is that, as climate change continues to evolve, we are going to have more and more extreme weather events. We’re going to have more dry events in some places, and that is fuel for the fire, in terms of wildfires, and so this is not something that’s going to go away,” Schroeder said.
Wildfires shouldn’t be treated as exceptional events but rather as the norm, she added. And air quality should become an everyday consideration.
“Most people don’t consider air quality in their everyday lives, particularly in the United States, because we are lucky enough to be in a place in the world where we have pretty good air quality compared to a lot of other places,” Schroeder said. “But I think it’s really important that — even us as Americans — everybody around the world should know that air quality is not a local issue. What happens on the other side of the world impacts us here.”
Freelancer John Lippert and Chicago Tribune’s Michael Hawthorne contributed.
