American Cities: Pollution's Worst Offenders

Air pollution is a serious issue in the United States, with the American Lung Association's 2025 State of the Air report finding that nearly half of Americans live in areas with failing air quality. The report ranks cities based on their weighted average number of unhealthy air days, with California cities like Bakersfield, Visalia, and Fresno-Madera-Hanford topping the list for year-round particle pollution. Other highly polluted cities outside of California include Eugene-Springfield in Oregon and Detroit-Warren-Ann Arbor in Michigan. Factors contributing to air pollution vary from wildfires and cookstoves in rural areas to road dust, vehicle emissions, and industrial facilities in urban centers. The effects of air pollution are far-reaching, with communities of color and low-income areas disproportionately impacted by health risks such as asthma, respiratory issues, and other illnesses.

California cities are among the most polluted

The high levels of air pollution in California cities can be attributed to various factors. One significant contributor is vehicle emissions, as the microscopic pollutants from cars and industrial facilities can become trapped in the state's valleys due to geographical features such as mountains. For example, Los Angeles, one of the top hotspots for fine particle air pollution, is surrounded by mountains, and prevailing winds blow pollutants towards these mountains, trapping them in the basin. Additionally, California's large and growing population, increased vehicle ownership, and industrial activities contribute to the release of many contaminants into the air through combustion processes.

Wildfires also play a significant role in California's air pollution. The state has experienced devastating wildfires, particularly in 2020, which blanketed cities in orange-coloured smoke. The tiny particles of soot produced by these fires can pose a greater hazard to human health than the fires themselves. The impact of wildfires on air quality is further exacerbated by climate change, as the increased frequency and intensity of fires contribute to higher levels of air pollution.

It is important to recognize that air pollution disproportionately affects certain communities. According to analyses, Black neighbourhoods often experience worse air quality than white areas located just a few miles away. Historically, industrial facilities and other polluting industries have been located in areas with lower property values and where residents have less political power to push back. This has resulted in a disproportionate burden of air pollution on Black communities, exacerbating existing racial disparities in health and environmental justice.

The health impacts of air pollution are significant and wide-ranging. Fine particles of pollution can infiltrate the lungs and bloodstream, leading to increased death rates from respiratory diseases, heart attacks, strokes, and other health issues. It can also trigger asthma attacks, harm lung development in children, and increase the risk of lung cancer. Additionally, air pollution has been linked to premature births and lower birth weights in newborns. These health risks disproportionately affect vulnerable populations, including children, older adults, and individuals with lung diseases.

Racial disparities in air quality

The United States obtained an overall ranking of 84 out of 106 world cities in terms of air quality in 2020. The cleanest city was Waimea, Hawaii, while the most polluted was Yosemite Lakes, California. Los Angeles, California; Gary, Indiana; Bakersfield, California; and Chicago, Illinois, are among the top five hotspots for fine particle air pollution in America.

Despite this, it is important to note that air quality and its impacts are not experienced evenly across racial groups in the United States. People of colour breathe more particulate air pollution on average, a finding that holds across income levels and regions of the US, according to a study by researchers at the EPA-funded Center for Air, Climate, and Energy Solutions. The study found racial-ethnic disparities for nearly all major emission categories. White people are exposed to lower than average concentrations from emission source types, while people of colour experience greater than average exposures from source types causing 75% of overall exposure.

This disparity is also seen in the placement of polluting industries. In Birmingham, Alabama, for example, the dirtiest industries are located where property values are the lowest and where people have the least political power to push back against them. These areas tend to have higher populations of colour.

The EPA has acknowledged the spatial sparseness of regulatory monitors in the US, which could result in misclassification of exposures and compound known environmental injustices. Studies have shown that racial and ethnic disparities exist in the spatial distribution of these monitors, with some racial groups more likely to be affected than others.

Overall, the evidence suggests that racial disparities in air quality are a significant issue in the United States, with people of colour being disproportionately affected by air pollution and its associated health risks.

The impact on vulnerable groups

Outdoor air pollution is unhealthy for everyone, but some groups are more vulnerable than others. In the United States, people of colour are found to breathe more particulate air pollution on average, regardless of income levels and regions. This includes African Americans, Hispanics, Asians, and other people of colour. They are disproportionately exposed to a regulated air pollutant called fine particulate matter (PM2.5). This is due to decades of residential segregation, with people of colour tending to live in areas with greater exposure to air pollution.

Socioeconomic status also appears to be tied to greater harm from air pollution. Multiple studies show that low socioeconomic status increases the risk of premature death from fine particle pollution. People with lower incomes are more likely to live near sources of pollution and have fewer resources to relocate. They may also have poorer job opportunities, dirtier workplaces, and higher traffic exposure, increasing the risk of harm.

Children are another vulnerable group, as their airways are small and still developing, and they breathe more rapidly and inhale more air relative to their size than adults. Growing up breathing high levels of air pollution can affect lung development, putting them at greater risk of lung disease as they age. Children also tend to spend more time being active outdoors, increasing their exposure on bad air days.

Pregnant people are also at higher risk, as exposure to air pollution during pregnancy is strongly associated with premature birth, low birth weight, and stillbirth. Additionally, people living with asthma, COPD, cardiovascular disease, and diabetes are more vulnerable to the effects of air pollution, as it can trigger asthma attacks and put additional stress on compromised organs.

Overall, the impact of air pollution on vulnerable groups is a serious health concern, and it is important to address disparities and environmental injustices to protect the health of these populations.

The role of transportation

Transportation plays a significant role in air pollution in America's most polluted cities. Motor vehicles, including cars, SUVs, and pickup trucks, have historically been a major contributor to air pollution. While new vehicles are significantly cleaner than older models, with almost 99% less tailpipe pollutants, the increasing vehicle ownership and miles travelled have led to more contaminants being released into the air. California, for instance, has a car culture, with a majority of residents opting to drive rather than use public transportation or walk. This has resulted in Los Angeles, Bakersfield, and other cities in the state being among the top hotspots for fine particle air pollution.

Vehicle emissions, particularly from nitrogen dioxide and sulfur dioxide, are major sources of outdoor air pollutants. These emissions have been linked to various health risks, including respiratory problems, asthma, and other lung diseases. People who live or work near busy highways or traffic areas are at a higher risk of experiencing the harmful effects of air pollution. Additionally, certain communities, such as those with lower political power, are disproportionately affected by the placement of polluting industries, leading to higher levels of air pollution in their neighbourhoods.

The transportation sector is one of the largest sources of carbon pollution in the United States. Efforts to reduce transportation-related air pollution have been cost-effective, with benefits to public health and the environment outweighing the costs of implementing emissions reduction programs. Since the 1970s, the U.S. Environmental Protection Agency (EPA) has played a crucial role in setting and enforcing emissions standards for various types of vehicles and equipment, including passenger cars, heavy-duty trucks, and buses, and construction machinery. These standards have sparked technological innovations, such as the automotive catalytic converter, and have led to significant improvements in air quality in American cities, reducing smog and improving the health of citizens.

Despite these efforts, there is still much to be done to address local air quality issues in many places across the United States. The EPA is now focusing on the critical challenge of climate change, as carbon pollution from burning fossil fuels is rapidly altering the Earth's climate. The agency is working to develop standards for aircraft emissions and continue its work in reducing pollution from other transportation sources.

The health consequences

Air pollution is a major threat to global health and prosperity, causing more than 6.5 million deaths each year worldwide. It is a mix of hazardous substances from both human-made and natural sources, including vehicle emissions, fuel oils, natural gas, manufacturing by-products, coal-fueled power plants, and chemical fumes.

In the United States, cities with high levels of air pollution include Los Angeles, California; Gary, Indiana; Bakersfield, California; Chicago, Illinois; and Yosemite Lakes, California. These cities are hotspots for fine particle air pollution, also known as PM2.5, which can have significant health consequences.

PM2.5 refers to particulate matter that is 30 times thinner than a human hair and can be inhaled deeply into the lung tissue. These fine particles can also enter the bloodstream, leading to a variety of health issues. Exposure to PM2.5 has been linked to increased mortality, with research finding that exposure to PM2.5 from coal doubled the mortality risk compared to PM2.5 from all sources.

Additionally, air pollution has been associated with adverse pregnancy outcomes, including an increased risk of premature birth, lower birth weight, and even miscarriage. It can also trigger asthma attacks, harm lung development in children, and worsen existing respiratory conditions such as asthma and COPD.

Furthermore, air pollution disproportionately affects people of color and low-income communities. People living in poverty are more likely to reside near sources of pollution and have limited resources to relocate. They also face higher rates of psychosocial distress and chronic stress, which can increase their vulnerability to pollution-related health effects.

Frequently asked questions