Air Pollution: America's Most Polluted Cities

Air pollution is a serious health threat, affecting children, older adults, and people with lung diseases. People of color and low-income communities are disproportionately impacted by the consequences of air pollution. While the Clean Air Act has improved air quality in the United States over the past 50 years, certain cities continue to experience poor air quality. This paragraph will explore which cities in the United States have the most air pollution and the factors contributing to this issue.

Los Angeles, California: Surrounded by mountains, pollution builds up and gets trapped

Los Angeles, California, is one of the most air-polluted cities in the United States. The Los Angeles basin is surrounded by mountains to the west and north, and a daily breeze blows pollutants from west to east toward the mountains. As a result, pollution builds up in the basin, creating episodes of smog until strong winds or a weak high-pressure system clears the air. Los Angeles is notorious for its smog, a combination of particle and ozone pollution. The prevalence of these pollutants results from various factors, including the burning of fossil fuels, vehicle emissions, manufacturing, and wildfires. The large population of 4 million in Los Angeles, with another 6 million in the surrounding county, contributes significantly to the city's air pollution due to heavy vehicular emissions and traffic congestion.

The city's air pollution has severe health implications for its residents. According to the County of Los Angeles Public Health Department, one in ten children have been diagnosed with asthma. The overall risk for cancer is increased by 900 for every million, as reported by the South Coast Air Quality Management District. Prolonged exposure to elevated levels of particulate pollution increases the risk of death from both cardiovascular and respiratory diseases. The California Air Resources Board estimates that particulate pollution causes over 9,000 deaths in California annually.

Ozone, a harmful ground-level pollutant, is formed when sunlight reacts with nitrogen oxides and organic substances in the air. Vehicle exhaust is a significant source of nitrogen oxides and reactive organic substances, making traffic a leading contributor to ozone pollution. Like particulate matter, ozone can cause respiratory infections, inflammation, and premature death. Together, particulate matter and ozone form the smog that Los Angeles is often known for. The summer months of June, July, and August tend to be more polluted than other months for both pollutants.

Efforts have been made to improve air quality in Los Angeles. The Air Quality Management District prepared the 2016 Air Quality Management Plan to set forth a comprehensive program to address the region's air pollution. Additionally, amendments to the Clean Air Act in 1990 have contributed to improvements in Los Angeles's air quality over the last three decades. Year-over-year trends have shown reductions in the city's air pollution, with an 11.8% decrease from 2018 to 2019. However, Los Angeles County still does not meet the U.S. EPA's national air quality standards for several pollutants, including particulate matter and ozone.

Bakersfield, California: Farms, forest fires, and road dust are the top sources of PM2.5 emissions

Bakersfield, California, has been described as having the "worst air in the United States". The city has seen little improvement in air quality over the last 20 years, despite advancements in fuel efficiency, emission control, and clean energy. Farms, forest fires, and road dust are the top sources of PM2.5 emissions in Bakersfield.

Agricultural activities such as crop burning, the use of pesticides, and wind-blown dust from farmland contribute significantly to the city's air pollution. Bakersfield is located in California's Central Valley, a major agricultural hub, and the emissions from these activities create a high concentration of airborne particulate matter.

In addition to agricultural sources, industrial activity, motor transport, and residential emissions also play a role in Bakersfield's poor air quality. The burning of wood in fireplaces and fire pits, as well as emissions from private motor vehicles, contribute to the high levels of PM2.5 in the area.

The geographic and climatic conditions of Bakersfield further exacerbate the problem. The city is surrounded by mountains, including the Sierra Nevada range, which can trap dry, polluted air over the southern section of the Central Valley.

While some improvements have been made through regulations discouraging crop burning and incentivizing the use of cleaner equipment, the city continues to struggle with meeting federal air quality guidelines. The challenge of reducing pollution is made more difficult by the presence of highly polluting industries and the influence of agricultural and oil industry interests in the region.

Overall, Bakersfield's air pollution is a complex issue influenced by a range of factors, and it continues to pose a significant threat to the health and well-being of its residents.

California's Central Valley: Pollution from coastal cities gets trapped in this valley

California's Central Valley is a vital agricultural hub for the state and country, consisting of the San Joaquin Valley and Sacramento Valley. The valley is responsible for a significant proportion of the country's agricultural output, producing a quarter of the nation's food, including 40% of its fruits, nuts, and other table foods.

However, the valley's unique geography also makes it susceptible to air pollution. Bounded by the Cascade Range to the north, the Sierra Nevada to the east, the Tehachapi Mountains to the south, and the Coast Ranges and San Francisco Bay to the west, the valley is surrounded by mountain ranges that trap the air moved easterly from sea breezes, facilitating the accumulation of pollution. This phenomenon is similar to the air pollution observed in Los Angeles, where a daily breeze blows pollutants from west to east toward the mountains, and the pollution builds up in the basin over several days due to a high-pressure system.

The transportation industry is a significant contributor to California's air pollution, with the state, particularly Los Angeles and its surrounding areas, having a high concentration of vehicles. More than half of the population owns a car, resulting in severe and declining air quality in the region. Additionally, the Central Valley's agricultural activities contribute to air pollution, with copious amounts of agricultural runoff from farms containing high levels of nitrates.

The impact of air pollution in the Central Valley is exacerbated by the region's vulnerability to wildfires. In 2019, 19 out of the 20 most polluted cities in the United States were located in California, which experienced severe fires that season. The smoke and particulate matter released during wildfires can linger over communities, posing serious health risks, especially to vulnerable individuals such as children, older adults, and people with lung diseases.

Salt Lake City, Utah: Pollution builds up and doesn't escape due to surrounding mountains

The American Lung Association's 2025 "State of the Air" report reveals that cities in California make up a significant proportion of the country's most polluted places in terms of air quality. Bakersfield, Visalia, and Fresno-Madera-Hanford ranked 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.

However, Salt Lake City, Utah, is also one of the most polluted cities in the United States. The city is surrounded by mountains, which means that pollutants build up and do not escape easily. The geography of the city, situated in the basin of the Wasatch Mountains, Oquirrh Mountains, and Traverse Mountains, can trap cold air in the Salt Lake Valley. This prevents the dispersal of polluted ground-level air and shields the city from winds that could otherwise clear out inversions. Seasonal fluctuations, particularly temperature inversions in the winter, further contribute to the accumulation of polluted air in the city.

The air quality in Salt Lake City has consistently failed to meet federal attainment levels for ozone and 24-hour PM2.5 pollution, resulting in "F" ratings. The excessive air pollution in the city poses significant health risks, especially to sensitive groups such as children, the elderly, and those with heart and lung diseases. Brigham Young University researchers found that Utah's air pollution reduces the average resident's life by 1.1 to 3.5 years. Additionally, an estimated 75% of Utah residents lose one year or more of their lives due to the state's air pollution levels.

While Salt Lake City has made improvements in air quality over the years, it continues to experience unhealthy ozone days and increased annual PM2.5 levels. The PM2.5 pollutant is particularly dangerous as it can bypass the body's defense systems, lodging deep within lung tissue and potentially entering the bloodstream. Residents of Salt Lake City are advised to stay informed about pollution levels and take necessary precautions to protect their health.

Dallas and Houston, Texas: High UV radiation and vehicle emissions increase ozone pollution

Dallas and Houston, two of Texas's largest cities, face significant air pollution challenges, particularly from high UV radiation and vehicle emissions, which contribute to increased ozone pollution.

Dallas, situated in North Texas, has struggled with air quality issues for years. In 2019, the city failed to meet national standards for 24-hour PM2.5 pollution, ozone days, and annual PM2.5 pollution. This poor air quality poses health risks to nearly 1.5 million residents, including children, the elderly, and those with heart and lung disease. Industrial and petroleum activities further exacerbate the problem, with facilities in North Texas emitting tens of thousands of pounds of unauthorised air pollution annually. The Dallas-Fort Worth area experienced 106 days of poor air quality, exceeding "good" AQI standards, in a single year.

Houston, Texas's largest city, also faces air pollution issues, especially regarding ozone pollution. In 2019, Houston ranked 9th nationally for worst ozone pollution, with an average of 22.3 days annually exceeding standards. Houston's high temperatures, abundant sunshine, large oil and gas industry, and dispersed population reliant on motor vehicles contribute to this problem. Despite meeting the EPA's annual PM2.5 exposure standards in 2019, Houston exceeded the World Health Organization's more stringent standard, indicating potential health risks for sensitive groups.

Both Dallas and Houston's lower latitudes and locations with fewer clouds expose them to heavier UV radiation than many other U.S. cities. This UV light interacts with vehicle emissions and other pollutants, cooking up ozone and contributing to the cities' air quality issues. While efforts are made to monitor and improve air quality, with organisations like the EPA playing a critical role, the health risks to sensitive groups in these cities remain a concern.

Frequently asked questions