Kiera Jefferson
Air pollution is a serious public health issue in the United States, causing at least seven million premature deaths globally each year, according to the World Health Organization. In the US, air pollution disproportionately affects people of colour and people with lower incomes, putting them at higher risk for illnesses such as asthma, lung cancer, and heart attacks. While the Clean Air Act has driven pollution reduction over the past 50 years, certain areas in the US continue to experience high levels of air pollution. This introduction will explore the sources and impacts of air pollution in the US, focusing on the most polluted areas and the efforts to improve air quality.
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What You'll Learn
California's wildfire season
California is facing increasingly severe wildfire seasons, which are a major source of air pollution in the state. Wildfires create a complex mixture of air pollutants, including cancer-causing substances and tiny particles that can aggravate existing health problems and increase the risk of heart attack or stroke. These particles can be very small, with diameters of 2.5 micrometres or less, allowing them to penetrate deep into the lungs, and even enter the bloodstream. The health impacts of wildfire pollution include increased hospital admissions, premature mortality, asthma attacks, and adverse effects on lung development in children.
The 2020 California wildfire season was particularly devastating, with clusters of wildfires in Northern California surrounding the Bay Area and raising air pollutant concentrations to hazardous levels. The highest mortality was estimated to occur in Los Angeles County, which houses a quarter of the state's population. The American Lung Association's State of the Air 2024 report also ranked Los Angeles as the city with the most contaminated air in the country. The city's basin-like geography, surrounded by mountains, helps trap pollutants.
Climate change is a significant contributor to the increasing frequency and intensity of California's wildfires. The state's 2010 Rangeland Assessment noted that the fire season has been starting earlier and ending later, with a rise in the severity of acreage burned in recent years. These trends are expected to continue, posing a growing threat to public safety and air quality. California has recognised the wildfire threat in its 2009 Climate Adaptation Strategy and is developing adaptation actions, including protecting forests and increasing public awareness of proper land management strategies.
During wildfire season, people are advised to avoid outdoor activities, especially when the Air Quality Index (AQI) is considered 'Unhealthy for Sensitive Groups' or higher. Sensitive groups, including children, the elderly, pregnant women, and people with heart or respiratory conditions, are at higher risk from the health effects of wildfire smoke. In the event of an evacuation, it is recommended to keep vehicle windows closed and run the air conditioner with the recirculate setting turned on. Additionally, individuals can create a cleaner air space within their homes using DIY air cleaners made with box fans and MERV 13 air filters, which have been found effective in reducing smoke particles.
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Los Angeles' air pollution
Los Angeles, California, is known for its poor air quality, which has severe implications for the climate and the health of residents. LA has the most contaminated air in the US, with a population of over 10 million people. The city is surrounded by mountains, which form a natural trap for pollutants.
Vehicle emissions and traffic congestion are significant contributors to LA's poor air quality. The city has a large population of 4 million, with another 6 million in the surrounding county, and is the third most economically powerful city globally, with a booming shipping industry. Nearly 40% of the country's imports pass through the LA area, transported by fossil-fuelled vehicles. During the early weeks of the COVID-19 pandemic, traffic reductions in LA resulted in a 30% decrease in nitrogen oxides, according to a study by UCLA professor Yifang Zhu. However, as restrictions loosened, the short-term gains in air quality were quickly lost.
The Port of Los Angeles and the Port of Long Beach are the two busiest container ports in the US, and the city's economic structure relies heavily on these ports. The burning of fossil fuels contributes to climate change, trapping more heat energy and raising average temperatures. This, in turn, creates ideal conditions for wildfires, which further degrade air quality. Climate change enhances the conditions for ozone pollution and makes it more challenging to clean up communities with high ozone levels. Wildfires driven by dry winds, low humidity, and high temperatures release smoke and dangerous particle pollution.
The health impacts of air pollution are severe and include triggering asthma attacks, harming lung development in children, and increasing the risk of lung cancer and heart attacks. People of colour and low-income populations are disproportionately affected by air pollution, putting them at higher risk for illness and premature death.
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Fracking near schools
Air pollution is a serious health threat in the United States. It can trigger asthma attacks, harm lung development in children, and even be deadly. In 2025, nearly half of the people in the U.S. lived in areas where the air quality received a failing grade. More than 156 million people resided in counties that received a failing grade for either ozone or particle pollution, and over 42 million lived in counties that failed all three air pollution measures.
California, in particular, has seen an increase in pollution due to its endless wildfire season. Los Angeles, for instance, has the most contaminated air in the country. The city is surrounded by mountains, which form a natural trap for pollutants in the basin.
Moreover, fracking near schools has emerged as a pressing issue in California and other states. A 2021 study revealed that over 650,000 children in nine states attend school within one mile of a fracking well, including Arkansas, California, Colorado, New Mexico, North Dakota, Ohio, Pennsylvania, Texas, and West Virginia. This proximity to fracking sites puts these children at increased risk of exposure to dangerous chemicals and air pollution, with potential negative health consequences.
In California, protests against fracking and oil drilling near schools and homes in minority and low-income communities have gained momentum. Studies show that oil drilling disproportionately occurs in these areas. For example, in the town of Shafter, California, there are 45 fracking sites within 1.5 miles of a junior high school. The predominantly Latino community has witnessed severe health issues among its children, including asthma and epilepsy, which may be linked to air toxins from nearby wells.
The issue of fracking near schools highlights the vulnerability of specific populations and the need for stricter regulations and a transition to renewable energy sources. It underscores the importance of prioritizing the health and safety of children and other at-risk groups, as well as addressing the disproportionate impact of pollution on minority and low-income communities.
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Indoor air pollution
Outdoor air pollutants can also enter indoor spaces through open doors, windows, ventilation systems, and structural cracks. This includes harmful smoke from chimneys, radon from the ground, and volatile chemicals from contaminated soil or water supplies. Additionally, natural sources like mold, pet dander, and dust mites can contribute to indoor air pollution, affecting the respiratory health of occupants.
Climate change further exacerbates indoor air pollution. It enhances conditions for ozone pollution and increases the risk of wildfires, which release dangerous particle pollution that can spread indoors. The Clean Air Act has helped improve air quality over the past five decades, but recent cuts to the U.S. Environmental Protection Agency's funding and staff endanger its ability to protect people's health from air pollution.
The health impacts of indoor air pollution are significant. It can trigger asthma attacks, harm lung development in children, and increase the risk of cardiovascular diseases and lung cancer. People of color and lower-income populations are disproportionately affected by indoor air pollution, facing higher risks of illness and mortality.
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Mobile sources of pollution
Motor vehicles, in particular, are a significant contributor to air pollution in America. They release a range of harmful pollutants, including air toxics, volatile organic compounds, nitrogen oxides, and carbon monoxide. These emissions have adverse effects on both human health and the environment. People living, working, or studying near major roads or traffic areas are at an increased risk of experiencing health issues related to air pollution, with vulnerable groups including children, older adults, and individuals with pre-existing cardiopulmonary disease being especially susceptible.
The health impacts of mobile source pollution are significant. Fine particles, known as particulate matter (PM2.5), and ground-level ozone (O3) are among the most concerning pollutants emitted by mobile sources. These pollutants have been linked to a range of adverse health effects, including respiratory issues, aggravated asthma, and even premature death. Risk assessments have estimated tens of thousands of early deaths and hundreds of thousands of cases of morbidity associated with population exposure to these pollutants.
Additionally, mobile sources contribute to the deposition of toxic air pollutants on soils and water bodies. These pollutants can be taken up by plants and ingested by animals, eventually making their way up the food chain. This leads to potential health risks not only for animals but also for humans who consume these contaminated plants and animals.
To address the issue of mobile source pollution, the EPA has implemented regulations and voluntary programs aimed at reducing emissions from these sources. The Mobile Sources Section of the Department of Environmental Conservation also manages strategies for controlling on-road and non-road mobile sources of air pollution. Educational initiatives, such as the ECO-Driving program, aim to raise awareness among students about the environmental impact of transportation choices and encourage the adoption of cleaner alternatives.
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Frequently asked questions
Los Angeles has the most contaminated air in the country. The city is a large basin with the Pacific Ocean to its west and mountains to its north and east. These mountains help form a natural trap, confining pollutants in the Los Angeles basin.
California retains its position of being the state with the most metro areas on the list, with 10 of the 25 most polluted cities.
There are four main types of air pollution sources: mobile sources (cars, buses, planes, trucks, and trains), stationary sources (power plants, oil refineries, industrial facilities, and factories), area sources (agricultural areas, cities, and wood-burning fireplaces), and natural sources (wind-blown dust, wildfires, and volcanoes). Mobile sources account for more than half of all air pollution in the United States.
