Meghan Hodges
Air pollution is a serious health threat that can trigger asthma attacks, harm lung development in children, and even be deadly. It is caused by various factors, including climate change, wildfires, and traffic pollution. While good air quality days are common throughout the United States, unhealthy air quality is concentrated in the western and southwestern regions. In 2021, 11 cities had unhealthy air quality more than 10% of the year, with eight of these in California and the rest in Arizona, Colorado, and Utah. However, three US cities: Urban Honolulu and Kahului, both in Hawaii, and Lake Havasu City/Kingman in Arizona, had healthy air quality on more than 99% of days in 2021. These cities can be considered among those with the least air pollution in the country.
| Characteristics | Values |
|---|---|
| Cities with the least air pollution in the US | Urban Honolulu, Kahului, and Lake Havasu City/Kingman |
| Air quality rating | Healthy air quality on more than 99% of days in 2021 |
| Air quality index (AQI) | Higher AQI means more pollutants and is less healthy |
| AQI in the US | Good air quality days are relatively common nationwide |
| AQI in metro areas | 75% of metro areas in the US had good air quality for at least two-thirds of 2021 |
| AQI in Phoenix | Less than 10% of days have an AQI in the “good” range |
| Worst air quality in the US | Southwestern US |
| Worst air quality in the Southwestern US | Phoenix-Mesa-Scottsdale metro area |
| Average AQI in the US | 123 |
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What You'll Learn
Urban Honolulu, Hawaii
Honolulu's air quality is measured by the United States Air Quality Index (US AQI), which takes into account the presence of major pollutants such as nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), carbon monoxide (CO), and fine particles like PM2.5 and PM10. In June 2021, for instance, the US AQI readings for Honolulu ranged from 16 to 21, firmly placing them in the 'good' rating bracket.
Additionally, the city's PM2.5 reading, which measures the concentration of fine particulate matter in the air, was an impressive 3.8 μg/m³ in 2020. This is well below the World Health Organization's (WHO) target of 10 μg/m³ or less, indicating exceptionally clean air. This reading placed Honolulu in 4702nd place out of all ranked cities worldwide, showcasing its remarkably low pollution levels.
However, it is important to note that Honolulu still faces some sources of pollution. Emissions from power plants, for instance, contribute to the presence of pollutants like black carbon, volatile organic compounds (VOCs), mercury, lead, and cadmium. Construction sites and road repairs can also temporarily increase pollution levels through the use of heavy machinery and the disturbance of large amounts of earth. Despite these sources of pollution, Honolulu has maintained its reputation for exceptionally clean air.
Climate change and increasing heat levels pose a potential threat to Honolulu's pristine air quality. The city's geographical location makes it susceptible to the impacts of climate change, which can exacerbate the formation of ozone pollution and make it more challenging to maintain clean air. Despite these challenges, Urban Honolulu remains a leader in clean air, thanks in part to the Clean Air Act, which has driven pollution reduction for over 50 years.
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Kahului, Hawaii
Air pollution is a serious health threat, affecting the lives of millions of people worldwide. Particle pollution, for instance, can be smaller than 1/30th of a human hair's diameter, and when inhaled, can cause severe harm to the lungs and even trigger asthma attacks. In this context, let's explore Kahului, a city in Hawaii, and its efforts to maintain clean air.
The state of Hawaii benefits from its remoteness, being surrounded by vast expanses of the Pacific Ocean. This natural barrier helps dissipate pollutants and prevents the accumulation of harmful emissions from industrial activities prevalent on continental landmasses. The ocean breeze contributes to the dispersion of air pollutants, ensuring a constant supply of relatively cleaner air.
Additionally, Hawaii has demonstrated a strong commitment to environmental protection. The state has implemented policies and initiatives to reduce pollution and improve air quality. For example, the Hawaii State Energy Office promotes the use of clean energy and energy efficiency, which helps reduce the state's carbon footprint and mitigate air pollution caused by fossil fuel combustion.
Moreover, the state has embraced renewable energy sources such as solar and wind power. The Hawaiian Islands are ideal for harnessing these natural resources, and the state has actively encouraged their integration into the energy grid. This shift towards renewable energy not only reduces pollution but also contributes to the sustainability and resilience of Hawaii's energy infrastructure.
While Kahului, Hawaii, boasts cleaner air compared to many metropolitan areas, it is not immune to air quality challenges. Natural factors, such as volcanic activity and wildfires, can introduce particulate matter and hazardous gases into the atmosphere. Additionally, the state's heavy reliance on imported oil for transportation and other sectors contributes to air pollution, particularly in the form of ozone-forming compounds and particulate matter from vehicle emissions.
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Lake Havasu City/Kingman, Arizona
The air quality in Lake Havasu City, Arizona, is generally good, with most individuals advised to go about their normal outdoor activities. However, sensitive groups, such as those with asthma, older adults, and people with lung diseases, may experience minor to moderate symptoms from long-term exposure. On certain days, when the air quality reaches a high level of pollution, sensitive groups are advised to reduce their time spent outside to prevent symptoms such as difficulty breathing or throat irritation.
Lake Havasu City's air quality index (AQI) is subject to change and is measured by Plume Labs, which converts raw measurements into AQI values using standards and guidelines developed by the Environmental Protection Agency (EPA) and the World Health Organization (WHO). The main pollutant in Lake Havasu City is PM2.5, with a concentration of 3.9 µg/m³, which meets the WHO annual PM2.5 guideline.
Kingman, Arizona, also has good air quality, with most individuals able to enjoy outdoor activities. Like Lake Havasu City, sensitive groups may experience symptoms with long-term exposure, and they should limit their outdoor time when pollution levels are high.
Kingman's AQI is provided by Plume Labs, which uses raw data from satellite measurements to calculate the AQI, based on EPA and WHO guidelines. The main pollutant in Kingman is PM2.5, with a concentration of 1.9 µg/m³, which also meets the WHO annual PM2.5 guideline.
Both Lake Havasu City and Kingman, Arizona, offer good air quality for their residents, with only minor symptoms expected for sensitive groups during periods of high pollution. The cities provide a healthy environment for outdoor activities, with air quality that meets WHO guidelines.
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Phoenix-Mesa-Scottsdale, Arizona
Phoenix, the capital of Arizona, is the fifth most populous city in the US, with an estimated population of 1.7 million in 2019. Despite its hot, dry climate, Phoenix has developed an excellent canal and irrigation system, enabling agricultural industries to flourish.
In terms of air quality, Phoenix has seen a recent decline in PM2.5 and ozone pollution. However, 2019 showed improvements compared to the previous two years. According to the federal Environmental Protection Agency (EPA) and the Arizona Department of Environmental Quality (ADEQ), vehicle emissions are the primary source of air pollution in Phoenix, while non-vehicle emissions have decreased significantly since the 1990s.
Ozone levels in Phoenix tend to be higher in the summer than in the winter due to the abundant sunlight and heat required for ozone formation. In 2019, unhealthy ozone levels were recorded between May 3rd and September 13th. Maricopa County, of which Phoenix is the county seat, received an "F" rating from the American Lung Association for high ozone days, ranking 7th out of 228 metropolitan areas.
To improve air quality, Phoenix has implemented measures to reduce vehicle emissions and promote sustainable transportation. These include investing in transit, light rail, cycle paths, and pedestrian-friendly areas. Additionally, Phoenix aims to achieve the best air quality by 2050, with excellent air quality on at least 90% of days. At present, Phoenix achieves good or excellent visibility 70-80% of the time. The city is committed to reducing ozone and dust particulate pollution, with one of its energy suppliers reporting a 14% reduction in carbon dioxide emissions due to diversification policies.
While Phoenix has made strides towards improving air quality, it continues to face challenges, particularly regarding ozone levels during the summer months. By focusing on reducing vehicle emissions and promoting sustainable practices, Phoenix aims to create a healthier and more environmentally friendly urban environment for its residents.
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Air Quality Index (AQI)
The Air Quality Index (AQI) is a tool used by the Environmental Protection Agency (EPA) to communicate about outdoor air quality and health. The AQI is divided into six colour-coded categories, each corresponding to a different level of health concern. The higher the AQI value, the greater the level of air pollution and the greater the health concern.
An AQI value of 50 or below represents good air quality, with little to no risk to health. An AQI value over 300 represents hazardous air quality. Values at or below 100 are generally considered satisfactory, while values above 100 indicate unhealthy air quality, first for certain sensitive groups of people, and then for everyone as values increase.
The AQI is calculated for five major air pollutants regulated by the Clean Air Act. Each pollutant's AQI value of 100 corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health.
In the United States, nearly half of the population lives in areas that received a failing grade for air quality in 2025. More than 156 million people live in counties that received a failing grade for either ozone or particle pollution, with over 42 million people living in counties that failed all three air pollution measures.
Despite being the most populous city in the US, New York City has relatively clean air, with an AQI of less than 50, or "good", for the last three years of recorded data (2017-2019). During the COVID-19 lockdown period, New York observed a 25% reduction in fine particle pollution, with 100% of hours in the best US AQI "good" category.
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Frequently asked questions
Urban Honolulu and Kahului, both in Hawaii, and Lake Havasu City/Kingman, Arizona, had healthy air quality on more than 99% of days in 2021.
Air quality is measured by an area's air quality index (AQI). The Environmental Protection Agency (EPA) uses AQI ranges to grade air quality from good to hazardous.
Air pollution is a serious health threat. It can trigger asthma attacks, harm lung development in children, and even be deadly. It is also linked to an increased risk of premature birth and lower birth weight in newborns.
Air pollution comes from a variety of sources, including traffic pollution, wildfires, and industrial emissions.
Policymakers at every level of government must take steps to clean the air. The Clean Air Act has successfully driven pollution reduction for over 50 years, and the U.S. Environmental Protection Agency plays a critical role in monitoring and enforcing air quality standards.
