Air Quality by State 2025

Pollution is the introduction of harmful contaminants or substances into the environment, causing adverse effects. Pollution in the United States is a serious issue that hurts the planet and is also harmful to humans.

Air pollution can cause various health problems such as accelerated aging of the lungs and decreased function of the lungs, development of asthma, bronchitis, and possible cancer, and cardiovascular and respiratory illness. Additionally, people can experience reduced resistance to infections, increased fatigue, and uncomfortable symptoms such as chest pain, dry throat, wheezing, headaches, or nausea. The most polluted cities in the U.S. are Los Angeles, Visalia, Bakersfield, and Fresno, all located in California.

The Clean Air Act (CAA) is the federal law that regulates air emissions and authorizes the Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) to regulate hazardous air pollutants and to protect public health and welfare. The Air Quality Index is a key tool in the CAA that allows the EPA to provide important information to the public about air quality.

Air Quality Index

The Air Quality Index (AQI) is an index calculated for four major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide. The AQI runs from zero to 500, with zero representing clean air and 500 being the most hazardous. Values of 100 or below are generally thought of as satisfactory; however, when values are above 100, air quality is considered unhealthy for certain at-risk groups of people and becomes more unsafe as the value increases.

The Air Quality Index has six categories that respond to different levels of health concerns. These are:

1. AQI is 0-50. Air quality conditions are good, and this is symbolized by the color green. Air quality poses little or no health risk.
2. AQI is 51-100. Air quality conditions are moderate, and this is symbolized by the color yellow. Those who are sensitive may experience some respiratory symptoms at this level.
3. AQI is 101-150. Air quality conditions are unhealthy for sensitive groups, and this is symbolized by the color orange. Sensitive groups include older adults, children, and those with lung conditions or diseases.
4. AQI is 151-200. Air quality conditions are unhealthy, and this is symbolized by the color red. At this level, everyone may begin to feel the effects of the air quality, and sensitive groups will feel more intense effects.
5. AQI is 210-300. Air quality conditions are very unhealthy, and this is symbolized by the color purple.
6. AQI is 301-500. Air quality conditions are hazardous, and this is symbolized by the color maroon.

States with the Best Air Quality

Hawaii

Hawaii has an air quality index of 21.2, the cleanest average air in the U.S. This is well in the good air quality index range. Honolulu is one of six cities in the U.S. ranked on all three of the American Lung Association’s cleanest cities lists for ozone, year-round particle pollution, and short-term particle pollution. The largest threats to Hawaii’s air quality are volcanic activity and climate change.

Alaska

Alaska’s air quality index is 29.1, the second-best index in the U.S. While the state has good air quality, Alaskans living in Anchorage or Mat-Su Borough will experience particle pollution problems due to wood-burning home heating. Both of these areas, however, have seen improvements in air quality in recent years. Most areas can be found in the “good” AQI range most of the year.

Washington

Washington’s air quality index is 33.5, placing it in third for the best air quality. Washington has both some of the cleanest and some of the most polluted air. Most areas in Washington remain in the “good” AQI range and sometimes reach into the “moderate” range. According to the Department of Ecology, 100% of the state’s population lives in communities that meet the state and federal air quality standard. Washington’s main sources of pollution are motor vehicles, wood smoke, and wildfire burning.

Oregon

The state with the fourth-best quality of air in Oregon. Oregon’s air quality index is 36.1. Currently, all Oregon residents live in areas that meet federal air quality standards. Two ballot measures are being pushed in Oregon for 2020: 100% Clean Economy and 100% Clean Electricity. These measures will push Oregon to be a 100% clean economy by 2050 and Oregon’s electricity to come from 100% clean, renewable, and affordable sources by 2050.

Maine

Maine’s air quality index is 36.5. Maine saw a large improvement in air quality from 2018 to 2019. However, some residents live in areas where the air is unhealthy due to emissions from power plants. Bangor is one of six cities in the U.S. ranked on all three of the American Lung Association’s cleanest cities lists for ozone, year-round particle pollution, and short-term particle pollution.

States with the Worst Air Quality

Utah

Utah’s air quality index is 51.2, which puts it in the “moderate” AQI range and makes Utah the state with the worst air quality on average. One of the reasons for Utah’s poor air quality is its mountainous topography, which can cause pollution to build up near the surface. Utah has been actively taking steps to improve air quality, such as increasing solar energy use and enacting 30 new rules that reduce emissions from sources.

Ohio

Ohio’s air quality index is 48.2, just barely keeping its average air quality within the “good” range and giving Ohio the overall second-worst air quality in the U.S. According to the American Lung Association’s 2019 “State of the Air” report found that Cleveland was the ninth-most polluted city in the U.S. for year-round particle pollution, and Cincinnati was found to be the thirteenth-worst state. Fortunately, the report also found that year-round particle pollution levels in cities such as Columbus, Akron, and Cleveland were at their lowest ever.

Georgia

Georgia’s air quality index of 48.2 ties it with Ohio as having the second-worst air quality in the United States. Atlanta is the largest metro area in Georgia that continues to struggle with air quality. Atlanta, known for its long periods of high traffic, gets most of its nitrogen oxides (the chemical that causes pollution) from tailpipe emissions from cars and trucks. According to the Georgia Department of Natural Resources, Atlanta ranks second for the most air pollution from vehicles. Additionally, the American Lung Association of Georgia found that approximately 11% of children in Georgia have asthma, almost double the national average.

West Virginia

West Virginia’s air quality is the fourth-worst overall in the U.S., with an index of 47.6. Power plants in West Virginia might be a large source of pollution for other states in addition to their own state. West Virginia’s two other main sources of pollution are car emissions and the coal industry, which has not only polluted their air but has severely damaged the state’s water. While air quality is improving across West Virginia, especially with fine particle air pollution, smog levels have increased across the state.

Indiana

Just below West Virginia with an air quality index of 47.5 in Indiana. Indiana’s main sources of pollution include the dependence on automobiles and the lack of investment in public transportation. Additionally, Indiana’s coal-fired power plants, located in the southwest part of the state, generate air pollution carried by winds to the eastern parts of the state and beyond to other states (similar to the power plants in West Virginia). Indiana ranks poorly for drinking water quality, industrial toxins, pollution health risks, and urban air quality, according to a ranking published by U.S. News and World Report.

AQI’s in the table below are averages. AQI varies by county and region in each state and can change from day to day and multiple times per day.