Kiera Jefferson
Chicago has a long history of unhealthy air pollution, dating back to the city's industrialization in the late nineteenth century. In recent times, the city's air quality has been affected by haze from Canadian wildfires, which has resulted in particle pollution and impacted the health of vulnerable residents. While the haze over Chicago is a visible sign of poor air quality, the city also has a broader issue with pollution, particularly from transport emissions and ozone. With a worsening trend in recent years, Chicago's air pollution is a cause for concern for all residents, not just those in sensitive groups.
| Characteristics | Values |
|---|---|
| Air quality | Ranked 9th worst in the world for cities with a population of over 4 million |
| Air quality | Ranked among the top three most polluted cities worldwide |
| Cause of haze | Smoke from wildfires in central Canada |
| Air quality index (AQI) | 100 is the level where air is labelled unhealthy |
| AQI in 2019 | 52 ("moderate") |
| PM2.5 pollution | Ranked worst in Illinois and 79th nationally |
| Ozone | Chicago deemed "nonattainment" by US EPA for exceeding 3.4 calendar days of unhealthy air pollution by more than 5 times |
| Number of unhealthy ozone days per year | 19.2 |
| Number of unhealthy ozone days in 2019 | 14 |
| Number of unhealthy ozone days in 2018 | 9.8 |
| Particle pollution in 2017 | 6.7 μg/m3 |
| Particle pollution in 2018 | 9.4 μg/m3 |
| Particle pollution in 2019 | 12.8 μg/m3 |
| Air quality | Generally acceptable for most individuals, but sensitive groups may experience symptoms |
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What You'll Learn
Chicago's air quality is among the worst in the world
Chicago has a long history of unhealthy air pollution, dating back to the city's industrialization in the late nineteenth century. The city relied on dirty coal, which produced soot and shrouded Chicago in a dense toxic cloud. This led to an increase in instances of pneumonia, asthma, and heart and lung diseases. While there has been a long-term trend of improving air quality, recent years have shown a worsening trend due to factors such as suburbanization and increasing transport emissions.
The Weather Company ranked Chicago as having the ninth-worst air quality among cities with a population of over four million. IQ Air, a Swiss air-technology company, placed Chicago among the top three most polluted cities globally. The city's PM2.5 pollution, or fine particle pollution, ranks the worst in the state of Illinois and 79th nationally. These fine particles can be embedded deep in the human lungs and pose health risks, especially over extended periods.
The poor air quality in Chicago has led to health concerns, particularly for sensitive groups such as people with asthma, heart disease, and other illnesses. Symptoms to look out for include wheezing, coughing, a fast heartbeat, fatigue, chest pain, and shortness of breath. Healthy individuals are also advised to limit strenuous activities and be cautious of potential health effects from prolonged exposure to polluted air.
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Wildfire smoke from Canada
Chicago's air quality has been ranked among the worst in the world due to Canadian wildfire smoke. Northern winds have blown smoke from wildfires in central Canada down to Chicago, causing poor air quality and a hazy sky. The smoke contains microscopic contaminants known as particle pollution that can embed deep in human lungs and be hazardous over a long period. People with conditions such as asthma and heart disease have been warned to be careful, and even healthy individuals have been advised to refrain from strenuous activities.
The smoke from the wildfires in Canada has affected several cities in the United States. The Weather Company ranked Chicago as the ninth-worst city in the world for air quality among cities with a population of over 4 million. IQ Air, a Swiss air-technology company, ranked Chicago among the top three most polluted cities globally. The Illinois Environmental Protection Agency issued an air pollution "action day" for the Chicago area, which was in effect until midnight Friday due to elevated particle pollution.
The Canadian government has determined that hot and dry conditions caused by climate change have contributed to the intense fires. Experts have predicted that these climate-driven fires may become the new normal. Canadian wildfire smoke has also impacted other parts of the United States, with authorities in Minnesota issuing an air quality alert, warning that fine particle levels could reach unhealthy levels. The Michigan Department of Environment, Great Lakes, and Energy (EGLE) also warned that pollutants from the smoke could be unhealthy for sensitive groups.
The National Weather Service urged Americans in the affected states to keep windows closed overnight to prevent smoke from entering their homes and to run air conditioning if possible. While officials have stated that the situation is less hazardous than in 2023, when Canadian wildfires turned the skies across the Northeast hazy, the smoke and poor air quality issues are expected to temporarily alleviate as winds change direction.
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Particle pollution and ozone
Chicago has a long history of unhealthy air pollution, dating back to the city's industrialization in the late nineteenth century. During this period, the soot produced by burning dirty coal to heat buildings, run motor engines, and power steel mills shrouded the city in a dense toxic cloud. This increased instances of pneumonia, asthma, and heart and lung diseases.
Particle pollution, or particulate matter, refers to a mixture of tiny solid and liquid particles in the air. These particles are emitted directly from power plants, factories, and vehicles, or they form as a result of chemical reactions between gases from burning fuels, sunlight, and water vapour. Particle pollution is extremely harmful to human health, as the particles can enter deep into the lungs and even the bloodstream. Exposure to particle pollution can cause respiratory issues, worsen heart and lung diseases, and has been linked to premature death.
Ozone is a harmful gas pollutant and a key component of smog. It is formed when precursor pollutants, nitrogen oxides (NOx), and volatile organic compounds (VOCs) react in warmer temperatures and sunlight. Unlike particle pollution, ozone is a ground-level pollutant, meaning it is not emitted directly into the air but is instead formed through chemical reactions. Ozone is dangerous as it can reduce lung function by 20% and trigger adverse health reactions in vulnerable groups, including children, older adults, and those with respiratory or pulmonary conditions.
In Chicago, both particle pollution and ozone levels have contributed to the city's poor air quality. In 2019, Chicago's air quality index (AQI) averaged 52 ("moderate"), with fine particulate matter (PM2.5) and ozone being the most common main pollutants. Chicago has struggled to comply with federal ozone standards, and in 2019, the city experienced an increase in the number of unhealthy ozone days.
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Transport emissions
Chicago has a long history of poor air quality, dating back to the city's industrialization in the late nineteenth century. During this period, the burning of dirty coal to heat buildings, power engines, and run steel mills caused a significant increase in air pollution. As a result, instances of pneumonia, asthma, and heart and lung diseases became more prevalent.
In recent years, Chicago has faced new challenges with air quality, particularly with particle pollution, which has nearly doubled since 2017. Transport emissions have been a significant contributor to this issue. The suburbanization of the city and its heavy reliance on motor vehicles have led to an increase in transport emissions, affecting the city's air quality.
According to the Regional Transportation Authority (RTA), improving public transit is crucial to reducing transportation emissions. The RTA has acknowledged the efforts of the Chicago Transit Authority (CTA), Metra, and Pace in transitioning to more sustainable practices, such as converting to electric fleets. For example, Pace has committed to a 5-year capital plan that includes purchasing electric buses, with a focus on serving minority and lower-income communities first.
Additionally, the RTA has recognized that buses and passenger trains produce significantly lower emissions per passenger mile than cars, with buses emitting 33% less and passenger trains 76% less. As a result, initiatives to encourage the use of public transit, walking, and biking are essential to reducing transportation emissions and improving air quality in Chicago.
While Chicago has made strides towards improving air quality, the city continues to face challenges, especially with the increasing particle pollution levels. By focusing on reducing transport emissions and transitioning to more sustainable practices, Chicago aims to improve the health and well-being of its residents.
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Chicago's history of unhealthy air pollution
Chicago has a long history of unhealthy air pollution, dating back to the city's industrialization in the late nineteenth century. During this period, the city relied on dirty coal to heat buildings, run motor engines, and power steel mills. The soot produced by burning this coal covered Chicago in a dense toxic cloud, leading to increased instances of pneumonia, asthma, and heart and lung diseases.
As early as 1881, regulations were implemented to combat the city's air pollution problem, making Chicago one of the earliest cities to do so. However, the issue has persisted and evolved over time. In 2019, Chicago averaged an air quality index (AQI) of 52, deemed "moderate" by the US Environmental Protection Agency (EPA). Despite this relatively moderate rating, the EPA notes that such air quality can be potentially unhealthy for sensitive groups, including children, the elderly, and people with pre-existing cardiovascular or respiratory health conditions.
Ozone, a harmful gas pollutant and key component of smog, has been a significant contributor to Chicago's air pollution. In 2019, the city exceeded the allowed number of calendar days with unhealthy air pollution by a significant margin, averaging 19.2 unhealthy ozone days per year. This issue is expected to worsen with rising temperatures due to global warming, as higher temperatures facilitate the formation of ozone.
In addition to ozone, particle pollution, particularly PM2.5, has been a concern for Chicago. The city ranked 79th nationally out of 1517 metropolitan areas in the US for PM2.5 pollution. Transport emissions from the increasing use of motor vehicles have contributed to this issue, with gas-powered transport emitting not only PM2.5 but also nitrogen oxides and reactive organic substances, which are precursors for ozone.
More recently, in 2021 and 2025, Chicago's air quality was significantly impacted by smoke from wildfires in Canada, leading to hazy skies and poor air quality. These incidents serve as a reminder that Chicago continues to face challenges in maintaining healthy air quality, and that external factors can also play a significant role in the city's air pollution levels.
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Frequently asked questions
Yes, the haze over Chicago is due to pollution. In 2021, smoke from wildfires in the west caused hazy skies over Chicago, impacting air quality. In 2025, Chicago's air quality was ranked among the worst in the world due to Canadian wildfire smoke.
The haze in Chicago is primarily caused by particle pollution, specifically PM2.5 pollution, which includes microscopic contaminants that can be embedded deep in the lungs and cause respiratory issues.
The haze can lead to serious health effects for anyone, even with brief exposure. Sensitive groups, including children, the elderly, and people with pre-existing health conditions, are especially vulnerable and may experience symptoms such as coughing, wheezing, and chest pain.
To mitigate the effects of the haze, it is recommended to limit outdoor activities and stay indoors as much as possible. Sensitive groups should be particularly cautious and avoid strenuous activities. Monitoring real-time air quality data and forecasts can help individuals make informed decisions about their activities.
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