Kiera Jefferson
Air pollution is a serious issue in the United States, causing an estimated 7 million deaths globally each year and contributing to health issues such as asthma, heart disease, and lung cancer. While the Clean Air Act has helped improve air quality over the decades, certain cities in the US continue to struggle with poor air quality. California, in particular, has consistently ranked as one of the most polluted states, with Bakersfield, Visalia, and Fresno-Madera-Hanford topping the list for worst year-round particle pollution in 2025. Other highly polluted cities outside of California include Eugene-Springfield in Oregon and Detroit-Warren-Ann Arbor in Michigan.
| Characteristics | Values |
|---|---|
| Date of Information | 2024-2025 |
| Source of Information | American Lung Association's State of the Air Report |
| Worst Pollutants | Ozone, short-term particle pollution, long-term particle pollution, road dust, diesel exhaust, vehicle emissions, industrial facilities, wildfires |
| Most Polluted Cities | Bakersfield, Visalia, Fresno-Madera-Hanford, Eugene-Springfield (Oregon), Detroit-Warren-Ann Arbor (Michigan) |
| Most Polluted States | California, Oregon, Michigan |
| Communities Most at Risk | Communities of color, lower-income communities |
| Health Risks | Asthma attacks, harm to lung development in children, stroke, ischemic heart disease, lung cancer, pneumonia, fetal and infant mortality, pregnancy complications |
| Population Impact | 156 million Americans (46% of the population) live in an area that received a failing grade for at least one measure of air pollution |
| Government Action | The Clean Air Act, U.S. Environmental Protection Agency |
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What You'll Learn
California cities dominate the list
The 2025 report revealed that almost half of all Americans (around 156 million people) are breathing unhealthy levels of air pollution. This is an increase of 25 million people compared to the previous year's report. The cities on the list are ranked based on their weighted average number of unhealthy air days, with the city with the highest number being ranked #1.
The top three cities with the worst air pollution in California are Bakersfield, Visalia, and Fresno, all located in California's Central Valley. A meteorological phenomenon known as an inversion layer often occurs in this region, creating a "lid" or "cap" on the atmosphere that traps pollutants underneath. The dust, dirt, and smoke particles get thicker, creating extremely unhealthy air quality. The high levels of agriculture and wood burning in the Central Valley contribute to the pollution, which gets trapped at the surface for people to breathe.
Other California cities that feature prominently on the list include Los Angeles, which ranked worst for ozone pollution, and the Sacramento-Roseville metro area, which ranked 10th for short-term particle pollution and 11th for high ozone days. Six of the top ten cities with the worst air pollution are in Southern California, including Ontario, Bloomington, Huntington Park, and San Bernardino.
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Vehicle emissions and wildfires are key causes
Vehicle emissions and wildfires are key contributors to air pollution, which affects nearly half of the US population. Air pollution is a complex mixture of gases, particles, and liquid droplets, with a variety of sources. It can trigger asthma attacks, harm lung development in children, and even be deadly.
Wildfires are a significant source of air pollution, releasing dangerous particle pollution that can spread over large areas. In the US, wildfires have become increasingly frequent and severe, with the number of acres burned each year increasing since the 1980s. While wildfire emissions of particulate matter are high, they are typically lower than vehicle emissions over a year. Vehicle emissions, particularly from passenger cars, contribute significantly to air pollution by releasing gases such as carbon dioxide and ozone.
California, for instance, has been ravaged by wildfires in recent years, with blazes emitting an estimated 4.4 megatonnes of CO2 equivalent in 2025. However, US cars produced an estimated 340-400 megatonnes of CO2 equivalent per year from 2018 to 2022, with passenger car emissions reaching 560 megatonnes of CO2 equivalent in 2005. California's wildfires in 2022 emitted around 9 megatonnes of CO2, with the highest estimate in the last 20 years being around 106 megatonnes in 2020.
While wildfires can produce more particulate matter emissions than vehicles, cars emit far more greenhouse gases, especially carbon dioxide. Clean combustion regulations have helped reduce particle emissions from vehicles, but the Trump administration's proposed rules to relax vehicle emissions standards could impact this progress.
To address air pollution, policymakers must take action. The Clean Air Act has successfully improved air quality over the past 50 years, but the US Environmental Protection Agency (EPA), which plays a critical role in monitoring and regulating air pollution, faces staffing and funding cuts that threaten its work.
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Air pollution is a serious health threat
Air pollution is a mix of hazardous substances from both human-made and natural sources. Vehicle emissions, fuel oils, and natural gas used to heat homes are all human-made sources of air pollution. Natural sources include smoke from wildfires, ash and gases from volcanic eruptions, and gases like methane.
Particulate matter (PM) is composed of chemicals such as sulfates, nitrates, carbon, or mineral dust. Vehicle and industrial emissions from fossil fuel combustion, cigarette smoke, and burning organic matter, such as wildfires, all contain PM. Fine particulate matter (PM2.5) can be inhaled into lung tissue and cause serious health problems. PM2.5 can penetrate the bloodstream, causing cardiovascular disease, lung cancer, and chronic pulmonary disease. It can also worsen conditions such as asthma and COPD, and trigger asthma attacks. According to the WHO, PM2.5 accounts for most health effects due to air pollution in the US.
The American Lung Association (ALA) has set an annual average guideline of 9 µg/m³ for PM2.5, while the WHO has set a much stricter limit of 5 µg/m³. Six of the top seven most polluted cities in the US are in California, according to the ALA's 2024 State of the Air Report. The U.S. Environmental Protection Agency (EPA) plays a critical role in cleaning up air pollution, but it faces significant staffing and funding cuts that endanger its work.
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The U.S. Environmental Protection Agency is critical for cleaning pollution
The U.S. Environmental Protection Agency (EPA) has played a critical role in cleaning up pollution and protecting people's health. The EPA's work is far-reaching and has helped improve the quality of the air that Americans breathe.
One of the EPA's key responsibilities is monitoring and regulating air pollution. The Clean Air Act, which the EPA is tasked with enforcing, has been successful in driving pollution reduction for over 50 years. Since 1970, the Clean Air Act has cut pollution as the U.S. economy has grown, and Americans now breathe less pollution and face lower risks of serious health issues and premature death. Between 1990 and 2020, national concentrations of several key air pollutants improved significantly, including carbon monoxide, lead, nitrogen dioxide, and sulfur dioxide. These improvements are a direct result of the EPA's efforts to enforce emissions standards and work with state, local, tribal, and federal governments to clean the air.
The EPA also recognizes the importance of voluntary participation in pollution reduction. The agency has implemented programs such as the "One Cleanup Program" and the "33/50 program," which encourage companies to find ways to prevent and reduce pollution in their operations. The Green Chemistry Program is another example of the EPA's work in this area, as it encourages the development of products and processes that follow green chemistry principles, reducing the use of hazardous chemicals and greenhouse gas emissions.
In addition to air pollution, the EPA also addresses water pollution. The Beaches Environmental Assessment and Coastal Health (BEACH) Act, authorized in a 2000 amendment to the Clean Water Act, focuses on coastal recreational waters. The EPA develops criteria to test and monitor water quality and notify the public of any concerns. This program enables better sampling, improved analytical methods, and enhanced communication with the public regarding water quality.
The EPA's work is constantly under threat from staffing and funding cuts, which endanger its ability to protect public health and the environment. Despite these challenges, the EPA remains committed to tackling pollution and improving the lives of Americans.
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Air quality was measured using three indicators
The Air Quality Index (AQI)
The AQI is a scale that runs from 0 to 500, with lower values indicating better air quality. An AQI of 50 or below is considered good, while readings of 100 and above are deemed unhealthy. The AQI is divided into six colour-coded categories, each corresponding to a different level of health concern. These categories are designed to help people quickly assess the air quality in their communities and take appropriate action if needed.
Particulate Matter (PM2.5)
PM2.5 refers to particulate matter with a diameter of 2.5 micrometres or less. These tiny particles can penetrate deep into the bloodstream when inhaled, posing significant health risks such as cardiovascular disease, lung cancer, and chronic pulmonary disease. The American Lung Association and the World Health Organization have set guidelines for PM2.5 concentrations, with the latter having a more stringent limit of 5 µg/m³.
Satellite Data
Instruments on the ground and satellites orbiting Earth, such as NOAA's GOES-R and JPSS series, collect information about the particles in our air. These satellites can measure particle pollution, ground-level ozone, and carbon monoxide levels. They provide frequent measurements, with GOES-R satellites offering particle pollution data approximately every five minutes during the day. This data is crucial for monitoring air quality and identifying areas of concern.
Using these three indicators, along with other data sources, organisations like the American Lung Association and IQAir compile rankings of the most polluted cities in the United States. These rankings help raise awareness about air pollution and guide efforts to improve air quality and protect public health.
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Frequently asked questions
Bakersfield in California has topped the annual list for the last six years. The city experiences a mix of dry climate, vehicle emissions, oil, gas and wildfires that contribute to its poor air quality.
Six of the top ten polluted cities in the US are in California, including Visalia, Fresno-Madera-Hanford, and the Bay Area. Outside of California, Eugene-Springfield in Oregon and Detroit-Warren-Ann Arbor in Michigan are highly polluted.
Air pollution in US cities is caused by a variety of sources, including vehicle emissions, industrial facilities, wildfires, wood-burning stoves, and large vehicles. Cars and trucks are reported to contribute to up to 90% of smog in Los Angeles.
