Chicago's Air Pollution: What's Behind The Smog?

Chicago, the third most populous city in the United States, has been ranked as one of the most polluted major cities in the country. In 2024, it was ranked 7th, down from 2nd the previous year. The city's air pollution is driven by transportation and daily emissions from residents, with transport emissions from planes, trains, boats, automobiles, and locomotives being the largest emission source. Wildfire smoke, temperature inversions, and the increase in extreme weather events due to climate change have also been identified as significant contributors to Chicago's air pollution.

Chicago's high population and transportation hub status

Chicago is the third most populous city in the United States, with a population of approximately 2.6 million people as of 2025. The city has a high concentration of adults aged 18-64, making up 66.77% of the population. The largest age group is 25-44-year-olds, indicating a dynamic urban population of professionals, students, and young families. Chicago's population is also diverse, with many international migrants seeking opportunities in the city.

As a major transportation hub, Chicago is a crossroads for rail, road, and air traffic. The city has the most federal highways in the country and is the second most visited city, with O'Hare International Airport being one of the busiest airports globally. Chicago is also a significant freight hub, with access to the Great Lakes and Mississippi River maritime systems. The region's freight network includes road, rail, and air transport, supporting industries such as manufacturing, construction, and trade.

The high population and transportation activity contribute significantly to Chicago's air pollution. Emissions from planes, trains, boats, automobiles, and trucks have become the city's largest emission source, surpassing highly regulated coal. Congestion on roads and rails leads to increased emissions, affecting local air quality. Additionally, temperature inversions caused by warm air trapping cooler air from Lake Michigan can exacerbate pollution levels.

While Chicago has made efforts to regulate emissions and improve air quality, the balance between economic activities and environmental concerns remains a challenge. The city's unique confluence of geography, history, and industry has cemented its status as a transportation hub, impacting its air pollution levels.

Diesel truck and train activity

Chicago is a major transport hub, with rail, road, and air traffic all contributing to its air pollution problem. The city has the most federal highways in the country and is the second-most visited city, with O'Hare International Airport being one of the busiest airports in the world. In recent years, transport emissions have overtaken highly regulated coal as the city's largest emission source.

Diesel transport is a major contributor to Chicago's PM2.5 emissions, which are fine particulate matter that can be harmful to human health, especially for sensitive groups such as children, the elderly, and people with pre-existing health conditions. While diesel trucks and trains remained consistent during the COVID-19 lockdown, they pollute more on average than passenger cars.

Chicago's location in the Midwest on the shore of Lake Michigan also makes the city prone to temperature inversions, which can further worsen air quality. During the summer, cool air from Lake Michigan can be trapped by a warmer air layer above, preventing normal pollution dispersion and causing pollution to accumulate. This is exacerbated by high emissions from diesel transport.

To address the issue of diesel transport emissions, the City of Chicago has implemented initiatives such as the Diesel Retrofit Program, which includes new clean diesel requirements, and idling reduction measures for diesel vehicles. These efforts aim to reduce the impact of diesel transport on the city's air quality and improve the health of its residents.

In addition to the city's initiatives, community members are also actively pushing back against polluting industries and projects on Chicago's Southeast Side, which has historically been a hub for industrial activity. Residents are concerned about the potential impact of diesel-driven truck traffic on air pollution in the area, and their efforts have led to some successes in blocking projects that could exacerbate the problem.

Temperature inversions

Chicago's air pollution is largely the result of diesel transport, followed by passenger vehicles, industry, and winter wood burning. The city's location on the shore of Lake Michigan makes it prone to temperature inversions, specifically cool air inversions and marine inversions. Temperature inversions occur when a layer of warm air sits above a layer of cooler air, trapping the cooler air and preventing normal pollution dispersion.

In 2019, Chicago experienced 14 days of unhealthy ozone levels, an increase from 9.8 unhealthy days in 2018. The COVID-19 lockdown measures did not significantly improve air quality, with only a 1% improvement in April 2020 compared to April 2019. Diesel truck and train activity, which remained consistent during the lockdown, may be to blame.

Transport emissions have overtaken highly regulated coal as Chicago's largest emission source. The city is a major national hub for transport, with the most federal highways and one of the busiest airports in the world. In recent years, emissions from planes, trains, boats, automobiles, and locomotives have contributed significantly to Chicago's air pollution.

While Chicago's air quality has shown a long-term trend of improvement, recent years have seen a worsening trend. Particle pollution nearly doubled between 2017 and 2019, and the city ranked as the second-most polluted major city in the US in 2023 and the seventh-most polluted in 2024.

Wildfire smoke

Wildfires have become more frequent due to climate change, and smoke from these fires has contributed to poor air quality in Chicago. While Illinois has not experienced local wildfires, smoke from wildfires in Canada has travelled long distances and affected the state. In 2024, Chicago was ranked the seventh-most polluted major city in the US, and wildfires were a contributing factor.

The Illinois Environmental Protection Agency (Illinois EPA) provides daily air quality forecasts through the Air Quality Index (AQI). It is a colour-coded system that ranges from Good (Green) to Hazardous (Maroon). During periods of poor air quality due to wildfire smoke, it is recommended to reduce outdoor activities and wear an "N95" or "N100" respirator if outdoor activities are necessary. Running HVAC systems with the fresh-air intake closed or set on recirculate can help prevent outdoor smoke from entering indoor spaces.

The increase in smoke exposure since 2020 is not limited to a small number of highly exposed counties but is pervasive across most counties in the country. In 2023, the average American resident experienced nearly 150 days of complete wildfire smoke coverage in their county, a significant increase compared to the levels from 2006 to 2020. This highlights the need for a coordinated national or international response to address the impacts of wildfire smoke and ensure the implementation of effective fire control and suppression policies.

Power plants, manufacturing, and fossil fuel burning

Chicago's air pollution is a result of various factors, including power plants, manufacturing, and fossil fuel burning. With nearly 10 million residents, Chicago is the third most populous city in the United States, and its transportation network is a major national hub for road, rail, and air traffic. The city's high population and transport activity contribute significantly to its air pollution levels.

Power plants that burn fossil fuels, such as coal, natural gas, and oil, release emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), carbon dioxide (CO2), mercury (Hg), and other pollutants. These emissions can have serious health and environmental impacts, and while the power sector has made significant reductions in recent decades, concerns remain. The impact of power plant emissions extends beyond the immediate vicinity, affecting communities located miles away.

Manufacturing industries also contribute to Chicago's air pollution. The city's industrial activities, including the production and use of fossil fuels, release pollutants into the atmosphere. Additionally, the burning of fossil fuels for energy and transportation further exacerbates the issue. Local, state, and federal governments have recognized the issue and are working towards reducing fossil fuel use.

The impact of these emissions on Chicago's air quality is evident. In 2019, the city experienced 14 days of unhealthy ozone levels, an increase from the previous year. The fine particulate matter (PM2.5) and ozone are two of the most common main pollutants responsible for the city's Air Quality Index (AQI). While Chicago's air pollution levels have shown a long-term improving trend, recent years have seen a worsening trend, with particle pollution nearly doubling since 2017.

To address these issues, Chicago established the Department of Air Pollution Control in 1959 to investigate and regulate emission sources. Subsequent regulations, such as the federal Clean Air Act of 1970 and recent city and state legislation, have helped mitigate emissions. Additionally, initiatives like the Argonne National Laboratory project aim to identify climate-related impacts on neighborhoods and help communities adapt to and address poor air quality.

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