Brian Barton
Air quality is a significant concern in the United States, with nearly half of the population living in areas with poor air quality. Various factors contribute to air pollution, including industrial emissions, transportation, agriculture, and wildfires. While the Clean Air Act and the efforts of the Environmental Protection Agency have improved air quality over the years, significant challenges remain. Recent rankings based on EPA data from 2019 and 2021 indicate that Vermont is the least polluted state, with the lowest levels of industrial toxins and pollution-related health risks. Hawaii, Alaska, and Maine also stand out for having the lowest air quality index (AQI) values, indicating superior air quality. However, it is important to acknowledge that air quality can vary significantly within states and that the data may not capture all pollution sources.
| Characteristics | Values |
|---|---|
| State with the least pollution | Vermont |
| How pollution is measured | Toxic chemical pollution and long-term chronic health risks |
| Air quality index (AQI) | Less than 50 is considered "good" |
| States with the best air quality | Hawaii, Alaska, Maine |
| States with the worst air quality | Louisiana, Nevada, Indiana, Delaware, Utah |
| Cities with the cleanest air | Burlington-South metro area |
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What You'll Learn
Hawaii has the best air quality in the US
Hawaii has some of the cleanest air in the US, according to the American Lung Association's 2019 "State of the Air" report. The report, which assesses the two most common outdoor air pollutants, ozone pollution (smog) and particle pollution (soot), found that Honolulu and Kahului-Wailuku-Lahaina were among the cleanest areas in the country.
Honolulu County recorded a new low for annual average particle pollution, with less than half the highest level ever recorded in 2009-2011. The city's air quality has continued to be excellent in more recent times, with readings from mid-2021 showing very good levels of air quality. In 2020, Honolulu had a PM2.5 reading of only 3.8 μg/m³, an extremely low reading that placed it well within the WHO's target goal for the best quality of air.
However, it is important to note that Hawaii is not entirely free of air pollution. There are still sources of pollution present, such as vehicle emissions, which can cause temporary rises in air pollution. Additionally, the state's high levels of solar radiation can convert primary pollutants into ozone, particularly during the hotter months. Despite this, Hawaii's air quality remains among the best in the US and the world.
In contrast, states like California, Arizona, and Louisiana struggle with poor air quality. California, despite ranking sixth among the least polluted states, has several cities that rank among America's most polluted, including Los Angeles, Bakersfield, and Fresno-Madera-Hanford. Arizona's Maricopa County, which has the nation's fastest-growing population, had good air quality on just 4% of days in 2021. Louisiana, meanwhile, ranks as the most polluted state in the US, with a Tulane University study finding very high incidences of cancer in the state.
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Vermont has the second-best air quality
The state of Vermont is known for its good air quality. The Vermont Agency of Natural Resources writes, "Vermont's air quality is considered to be among the best in the nation." The state's verdant valleys and mountains, bordered by Canada to the north, New York to the west, Massachusetts to the south, and New Hampshire to the east, contribute to its clean air.
Vermont's air quality is not without its challenges, however. Wildfires in Canada have been known to impact the air quality in Vermont, and the state is not entirely pollutant-free. The American Lung Association has ranked cities by ozone pollution, year-round particle pollution, and short-term particle pollution, with some California cities ranking among the most polluted in the nation.
Despite these challenges, Vermont consistently ranks highly for its air quality. On August 2, 2025, Vermont's Air Quality Index (AQI) level was 29, which is considered "good." The AQI levels in the state have been fluctuating, with the highest level in the last 24 hours being 33 (still considered good) and the lowest being 21. The air quality in Vermont is generally considered satisfactory, and the state's residents can enjoy outdoor activities without worrying about the risks associated with air pollution.
Vermont's good air quality can be attributed to various factors, including its geographical location and the state's efforts to maintain a clean environment. The state's natural beauty, with its valleys and mountains, likely plays a role in maintaining good air quality. Additionally, the Vermont Agency of Natural Resources works to protect the state's natural resources and promote a healthy environment for its residents.
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California ranks sixth for least polluted states
California ranks sixth among the least polluted states in the US. However, this statistic is surprising given that several of California's cities are among the most polluted in the country. Los Angeles-Long Beach, Bakersfield, Fresno-Madera-Hanford, and Sacramento-Roseville are all listed in the American Lung Association's top 10 most polluted cities in terms of ozone pollution, year-round particle pollution, and short-term particle pollution. The state's national parks also seem to be suffering. A report from the California State Parks Foundation highlights that severe wildfires, drought, and sea-level rise are ravaging state parks, which make up almost a quarter of California's coastline. Four of the most polluted national parks in the US are in California: Sequoia and Kings Canyon National Parks, Joshua Tree National Park, Mojave National Preserve, and Yosemite National Park.
Despite these issues, California is doing relatively well in terms of pollution, especially when compared to its neighbouring states. Nevada and Oregon, for example, were among the 10 worst-ranked states for pollution. California's success is likely due to its ability to pass budget decisions for its parks. The state parks foundation has advocated for spending tens of millions of dollars to address issues such as wildfires and sea-level rise.
The state's success in keeping pollution levels low is also reflected in the improvement of several measures of air quality over time. The number of parks with significant concern levels in at least one of the air quality conditions dropped from 96% to 70%, significant concern levels for unhealthy air dropped by 52%hazy skies plummeted by 94%. These improvements have been attributed to the implementation of several clean air regulations.
However, it is important to note that California has not seen the same improvements as the rest of the country. While the state has taken steps to reduce pollution, it still has a long way to go, especially when it comes to its national parks.
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Massachusetts has poor air quality
While Massachusetts meets federal clean air guidelines and air pollution has declined by 70% since the Clean Air Act was passed in the 1970s, the state still struggles with poor air quality. According to a study by Boston College's Global Observatory on Planetary Health, air pollution is responsible for an estimated 2,780 deaths a year in Massachusetts. The study, which examined the public health consequences of air pollution in the state on a town-by-town basis, also found that air pollution caused measurable cognitive loss in children exposed to fine particulate pollutants. The average level of fine particulate pollution across Massachusetts in 2019 was 6.3 micrograms per cubic meter, with levels ranging from a low of 2.77 micrograms per cubic meter in Worcester County to a high of 8.26 in Suffolk County. The U.S. Environmental Protection Agency's (EPA) standard is 12 micrograms per cubic meter, while the World Health Organization's (WHO) recommended guideline is 5.
Boston, a city in Massachusetts, generally has clean air that meets all EPA targets for criteria pollutants. However, Boston residents still breathe unhealthy air during an average of 1.3 days a year. Townsend, Amesbury, and Natick have the worst air quality in the state, with high levels of PM2.5, a type of airborne particle that includes soot, dust, dirt, metals, and chemicals. Ozone, a highly corrosive gas pollutant and key component of smog, is also a concern in Boston, contributing to the city's unhealthy pollution days.
The majority of air pollution in Massachusetts, about 95%, results from the combustion of fossil fuels. Cars, trucks, buses, planes, trains, and ships produce two-thirds of pollutant emissions, while power plants, industrial facilities, and home heating and cooking contribute the remaining third. These sources emitted a total of 938,000 tons of pollutants in 2017, the most recent year for which data is available.
While Massachusetts has made progress in reducing air pollution, the current EPA air pollution standards are not adequately protecting public health, according to the researchers from Boston College. The state should be further incentivized to transition to cleaner fuels and reduce air pollution and greenhouse gas emissions, which contribute to global climate change.
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Louisiana is the most polluted state
Soil throughout Louisiana has been contaminated by mud left behind by hurricanes, landfills, and industrial sites. Soil pollutants include oil, arsenic, lead, and wood and creosote treatment chemicals. Louisiana's low elevation, averaging only three feet above sea level in the southeastern region, makes the coast particularly vulnerable to the impacts of sea-level rise. From 1932 to 2016, Louisiana lost nearly 2,000 square miles of land, and the state is experiencing some of the highest rates of land loss globally. The Gulf of Mexico has experienced twice the global average rate of sea-level rise since 2010, and rising sea levels and sinking land have accelerated land loss in the Mississippi River Delta region.
Louisiana's wetlands are vital to the state's economy, with nearly half of the state's population living in the coastal region and contributing billions of dollars annually through fishing and tourism. However, warming water temperatures can promote harmful algal blooms, rendering seafood unsafe for human consumption. Losing wetlands also increases the damage from tropical storms and hurricanes, and the state recently cancelled a $3 billion project to restore wetlands along its steadily vanishing coastline.
In summary, Louisiana faces significant environmental challenges due to pollution and the impacts of climate change, threatening the health and well-being of its residents and the state's unique ecosystems, cultures, and industries.
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Frequently asked questions
Vermont has the lowest levels of industrial toxins and the lowest pollution-related health risks in the country, making it America's least polluted state.
The Environmental Protection Agency (EPA) uses two datasets to measure pollution. The first assesses a state's "total toxic chemical pollution per square mile of land area". The second evaluates the long-term, chronic human health effects of toxic chemical pollution, such as birth defects and cancer.
Some major sources of pollution include steel mills, auto plants, coal-powered power plants, agriculture, transportation, and wildfires.
Louisiana is the most polluted state in the US, with the second-highest pollution health risks and the third-highest levels of industrial toxins. Other highly polluted states include California, Nevada, Indiana, Delaware, and Utah.
The Clean Air Act has successfully driven pollution reduction for over 50 years. The EPA also plays a critical role in monitoring air pollution, writing rules to clean it up, and enforcing those rules.
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