Brian Barton
Air pollution is a serious issue in the United States, with nearly half of the population living in areas with failing air quality as of 2025. The American Lung Association's (ALA) 2025 State of the Air report ranks cities based on their weighted average number of unhealthy air days. According to the report, California cities make up a significant proportion of the country's most polluted places, with Bakersfield, Visalia, and Fresno-Madera-Hanford ranking as the top three metropolitan areas with the worst year-round particle pollution. Other highly polluted cities outside of California include Eugene-Springfield in Oregon and Detroit-Warren-Ann Arbor in Michigan. Los Angeles-Long Beach, California, is also often listed as one of the most polluted areas in the United States due to its high population density and resulting motor vehicle emissions.
| Characteristics | Values |
|---|---|
| Ranking Based On | Weighted average number of unhealthy air days |
| Number of People Impacted | More than 42 million people live in counties that received an F for all three air pollution measures |
| Top Polluted Cities | Bakersfield, Visalia, Fresno, Los Angeles |
| Causes of Pollution | Vehicle emissions, biomass burning, farms, forest management/agricultural waste burning, road dust, wildfires, cookstoves, diesel exhaust |
| Health Risks | Asthma attacks, harm to lung development in children, respiratory issues, cancer, cardiovascular disease, lung cancer, chronic pulmonary disease |
| Economic Impact | Low air quality negatively affects worker productivity, increases absenteeism, and adds healthcare costs |
| Improvement Strategies | Clean Air Act, improvements in pollution control technology, reductions in vehicle emissions and biomass burning |
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What You'll Learn
California's Central Valley
The valley's topography contributes to the air pollution issue, as the surrounding mountain ranges trap air pollutants. Additionally, pollution sources in the valley include heavy truck traffic on I-5 and Highway 99, diesel-burning locomotives, tractors, irrigation pumps, wood-burning stoves, and fireplaces. The top three local sources of PM2.5 emissions in 2023 were farms (20%), forest management/agricultural waste burning (20%), and road dust (14%). The health impacts of air pollution are well understood and include asthma, cardiovascular disease, lung cancer, and chronic pulmonary disease.
Clean air activists argue that the Valley’s air district has not done enough to meet air standards due to the influence of polluting industries in shaping policies around air quality. Air pollution is a serious health threat and can trigger asthma attacks, harm lung development in children, and even be deadly. It is essential for policymakers at all levels of government to take steps to improve the air quality in their communities.
Despite the efforts of the EPA and other organizations, California's Central Valley continues to struggle with air pollution and its negative impacts on the health and well-being of residents. It remains to be seen whether the implemented measures will successfully reduce pollution levels and improve air quality in the region.
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Bakersfield, California
The city's high air pollution levels are attributed to a combination of factors, including its high-emission industries, growing residential emissions, and unfavourable geographic and climatic conditions. Bakersfield is a hub for energy production, agriculture, and distribution in California. The oil and gas industry is a significant contributor to the city's air pollution, with Kern Oil, the primary oil refinery in the area, facing numerous Clean Air Act violations due to over-polluting. The city's agricultural activities, including farm land machinery, crop burning, and pesticides, also play a role in the poor air quality.
The geographic location of Bakersfield further exacerbates the problem. The city is surrounded by mountains, including the Sierra Nevada range to the east and the California Coast Ranges to the west, which trap polluted air in the valley. This leads to an accumulation of toxic farming chemicals, dust, vehicle and train fumes, and oil-drilling exhaust. The impact of these emissions is felt more acutely in certain areas of Bakersfield, with locations closer to major petroleum extraction or refinement facilities experiencing higher pollution levels.
The consequences of Bakersfield's poor air quality extend beyond the environment and public health. Low air quality negatively impacts worker productivity, increases absenteeism, and contributes to direct and indirect healthcare costs. It also affects children's lung development and can trigger asthma attacks in those who suffer from the condition. Despite the Clean Air Act's success in reducing pollution over the past 50 years, the EPA faces significant staffing and funding cuts that endanger its ability to continue protecting people's health from air pollution.
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Vehicle emissions
The unique geography of certain cities also plays a role in trapping vehicle emissions and contributing to poor air quality. For example, Salt Lake City is surrounded by mountains, which prevents pollutants from escaping easily. Similarly, Bakersfield is surrounded by mountains that trap truck and train fumes, as well as oil-drilling exhaust. Additionally, the Central Valley in California acts as a basin where pollution from coastal population centers gets trapped, leading to higher concentrations of air pollution.
The lack of emissions testing for vehicles and underdeveloped public transit systems in certain states, such as Indiana, further exacerbates the problem of vehicle emissions. This is particularly concerning in areas with high populations and dense road networks, such as Indianapolis, where people of color are disproportionately affected by air pollution. The situation in Bakersfield has also been attributed in part to lax enforcement against industrial polluters, allowing vehicle emissions to worsen the air quality.
To combat vehicle emissions and improve air quality, the Biden administration has taken several climate actions. These include the signing of the Inflation Reduction Act, which aims to reduce greenhouse gas emissions, and an executive order to make half of all new US cars and trucks electric by 2030. Additionally, the Clean Air Act and the National Ambient Air Quality Standards have played a crucial role in regulating hazardous air pollutant emissions and protecting public health.
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Climate change
The sources and impacts of air pollution are varied and complex. Wildfires, cookstoves in rural areas, road dust, and diesel exhaust from vehicles in cities all contribute to the problem. Climate change exacerbates the issue by creating conditions that increase the risk of wildfires and make it more challenging to reduce ozone pollution. The economic costs of air pollution are also significant, with low air quality impacting worker productivity, increasing absenteeism, and driving up healthcare costs.
The World Health Organization has set a guideline recommending that annual average concentrations of PM2.5, or fine particulate matter smaller than 2.5 microns, should not exceed 5 µg/m³. Exposure to PM2.5 can lead to severe health issues as these particles can enter the bloodstream and cause cardiovascular disease, lung cancer, and chronic pulmonary disease. In 2024, Los Angeles had the highest PM2.5 annual average of the 25 most populated U.S. cities, at 10.1 µg/m³.
Despite advancements in pollution control technology, emissions from vehicles and biomass burning remain significant contributors to air pollution. The American Lung Association's 2024 State of the Air Report identified Bakersfield, Visalia, and Fresno as the top three most polluted cities in the U.S. by PM2.5 concentrations. Six of the top seven cities were in California, with four in the state's Central Valley, where pollution from coastal population centers gets trapped.
Addressing air pollution requires action from policymakers and individuals alike. The Clean Air Act has played a crucial role in improving air quality over the past 50 years. However, the U.S. Environmental Protection Agency (EPA), responsible for protecting people from air pollution, faces significant staffing and funding challenges that threaten its ability to continue this work effectively. Individuals can also take proactive steps, such as checking air quality forecasts and avoiding outdoor activities when unhealthy air quality is expected.
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People of colour and low-income communities
People of colour are disproportionately affected by air pollution in the United States. They are 61% more likely to live in a county or city with air pollution than Caucasians and are three times more likely to live in a place that has failed all three air quality thresholds established by the American Lung Association. More than 20 million people, approximately 6.4% of the US population, live in one of the 15 countries where year-round particle pollution levels are worse than the national air quality limit.
Low-income communities are also particularly vulnerable to the effects of air pollution. For example, the Low-Income Home Energy Assistance Program (LIHEAP) helps vulnerable families with energy costs, recognising that access to energy is a health and safety issue. Despite the critical role of the US Environmental Protection Agency in protecting people's health from air pollution, significant staffing and funding cuts are endangering their work.
California is home to some of the most polluted cities in the US, with Bakersfield, Visalia, Fresno-Madera-Hanford, and San Jose-San Francisco-Oakland ranking as the top four most polluted cities in the country according to particulate matter (PM 2.5) levels. Nearly half of the people in the US live where the air they breathe is of poor quality, and more than 42 million people live in counties that received an F for all three air pollution measures in "State of the Air" 2025.
Addressing air pollution is critical to protecting the health of vulnerable communities. Air pollution can trigger asthma attacks, harm lung development in children, and even be deadly. Climate change exacerbates the problem by enhancing conditions for ozone pollution and making it harder to clean up communities with high ozone levels. At the 2021 Climate Leaders' Summit, President Joe Biden pledged to cut US emissions by half by 2030 and achieve zero carbon emissions by 2050. The Inflation Reduction Act, signed into law by the Biden administration, aims to reduce greenhouse gas emissions and improve air quality, benefiting the health of all Americans, especially those in low-income and communities of colour.
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Frequently asked questions
Bakersfield, California, is the most polluted city in America, according to the American Lung Association's 2024 State of the Air Report.
Six of the top seven polluted cities in the US are in California, with four in the state's Central Valley, a 450-mile valley parallel to the Pacific coast. When pollution from coastal cities is carried inland by the wind, it gets trapped in the valley. Bakersfield, at the southern end of the valley, has a drier and hotter climate, which is worse for air quality.
Air pollution can trigger asthma attacks, harm lung development in children, and can even be deadly. Fine particulate matter smaller than 2.5 microns (PM2.5) can penetrate deep into the respiratory tract, reaching the lungs and entering the bloodstream, leading to respiratory issues and cancer.
