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By Kate Nalepinski
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Air pollution is not just a nuisance—it's a growing public health concern that affects millions of Americans daily. From respiratory distress to long-term cardiovascular risks, the impact of poor air quality can be profound, particularly for vulnerable populations.
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Why Is Air Quality Bad Wednesday?
During the early morning on Wednesday, several U.S. cities were experiencing heightened air pollution, with parts of Chicago among the affected areas, due to elevated air pollutants, including ozone.
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Air Quality in Chicago
From around 2 a.m. to 10 a.m., the air in Chicago had reached a level of pollution deemed unhealthy for sensitive groups, according to AccuWeather.
According to data from the Environmental Protection Agency's (EPA) AirNow interactive map, as of 3 p.m. ET, air quality in Chicago had improved, though conditions in Newton County, in north Indiana, were classified as "moderate." The area surrounding nearby Milwaukee County, Wisconsin, was also labeled "moderate."
The American Lung Association (ALA) explains that ozone pollution worsens in warmer weather when vehicle and industrial emissions react with sunlight. "Breathing ozone irritates the lungs, resulting in inflammation—as if your lungs had a bad sunburn," the group says. These conditions often emerge during spring and summer when heat and stagnant air prevent pollutants from dispersing.
Chicago's heavy traffic and industrial activity contribute to the local ozone load. On days like Wednesday, when wind and air circulation are limited, these emissions accumulate, leading to an elevated air quality index (AQI) and increased health risks for sensitive populations.
Others Regions With Poor Air on Wednesday
- El Paso County, Texas: Unhealthy air quality for sensitive groups
- Pinal County, Arizona: Unhealthy air quality for sensitive groups
- Riverside County, California: Unhealthy air quality for sensitive groups
Most Polluted Cities by Ozone in 2025
According to the ALA's 2025 State of the Air report, the top metropolitan areas in the United States most affected by ozone pollution this year have been largely concentrated in the West and Southwest. The top five, based on the weighted average number of unhealthy ozone days:
- Los Angeles-Long Beach, California
- Visalia, California
- Bakersfield-Delano, California
- Phoenix-Mesa, Arizona
- Fresno-Hanford-Corcoran, California
These rankings reflect the persistent challenge in areas where geography and climate conditions trap pollutants. The report notes that more than 42 million Americans live in counties that received failing grades for all three types of pollution monitored—ozone, year-round particle pollution and short-term particle pollution.
How Does Ozone Affect Air Quality?
Per the EPA, ozone is a gas composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level.
Ground-level ozone, a key component of smog, is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). It occurs when pollutants emitted by cars, power and chemical plants and other sources react in the presence of sunlight.
Exposure to ozone can cause health problems, particularly for children, those who are pregnant, older adults and people with preexisting conditions like lung disease, according to the ALA.
In 2021, 2022 and 2023, approximately 37 percent of people were exposed to levels of ozone that put their health at risk.
High ozone levels have been tied to immediate breathing problems, like asthma attacks, risk of respiratory infections, shortness of breath and coughing, and increased need for people with preexisting conditions to seek medical treatment, per the ALA. Long-term exposure is linked to increased respiratory illnesses, metabolic disorders, nervous system issues and reproductive issues.
Federal and local agencies recommend limiting outdoor exertion during high ozone alerts, especially during midday and early evening when levels are typically at their highest. Monitoring of local AQI reports is advised for vulnerable populations.
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