The most polluted cities in America
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The most polluted cities in America

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While air pollution in the U.S. has improved remarkably since the passage of the Clean Air Act, the Environmental Protection Agency (EPA) estimates that 137 million people nationwide still live in counties where air pollution levels exceeded air quality standards. In 2018, the U.S. emitted 76 million tons of pollution into the atmosphere, which according to the American Lung Association, contributes to increased rates of lung cancer and reduced lifespans among American families.


Air quality standards for common pollutants that pose a threat to public health are managed by the EPA. These pollutants include ozone, fine and coarse particulate matter (PM2.5 and PM10), sulfur dioxide, lead, carbon monoxide, and nitrogen dioxide. From 1980 to 2018, the greatest air quality improvements occurred in lead, sulfur dioxide, and carbon monoxide levels. According to the EPA, efforts to remove lead from automobile gasoline alone helped reduce levels of lead in the air by 89 percent over three decades. Similarly, a 1990 update to the Clean Air Act known as the Acid Rain Program set a permanent cap on emissions of sulfur dioxide and nitrogen oxides from power plants. The cap helped reduce sulfur dioxide to 9.0 percent of the 1980 value and the level of nitrogen dioxide to 39 percent over the same time period.


While levels of nitrogen dioxide, carbon monoxide, sulfur dioxide, and lead are all below 40 percent of their respective 1980 values, ozone and particle pollution continue to present a major public health risk. With these trends in mind, researchers at BuyAutoInsurance.com analyzed data from the EPA to find the U.S. metropolitan areas with the highest levels of air pollution.

The EPA uses a measure known as the Air Quality Index (AQI) to report daily air quality, with higher values corresponding to greater levels of air pollution. Values between 0-50 are considered good; values between 51-100 are moderate; values between 101-150 are unhealthy for sensitive groups; and values above 151 are unhealthy for the general public. Researchers used AQI data to determine air quality rankings for small, midsize, and large metropolitan areas.

The findings among the small and midsize metros are in line with the findings among the large metros. Specifically, locations in California make repeated appearances on these lists. When comparing air pollution levels of large metropolitan areas, five of the top 15 metros were found to exist in the coastal state. The incidence is higher among the top 15 small and midsize metros.


Here are the metropolitan areas with the worst air quality.


The 15 Large Metros With the Worst Air Quality

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15. New York-Newark-Jersey City, NY-NJ-PA

  • Median AQI: 53
  • Max AQI: 210
  • Good air quality: 156 days per year
  • Moderate or unhealthy for sensitive groups: 206 days per year
  • Unhealthy, very unhealthy, or hazardous: 3 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

14. San Francisco-Oakland-Hayward, CA

  • Median AQI: 53
  • Max AQI: 245
  • Good air quality: 147 days per year
  • Moderate or unhealthy for sensitive groups: 206 days per year
  • Unhealthy, very unhealthy, or hazardous: 12 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

13. Detroit-Warren-Dearborn, MI

  • Median AQI: 54
  • Max AQI: 151
  • Good air quality: 137 days per year
  • Moderate or unhealthy for sensitive groups: 226 days per year
  • Unhealthy, very unhealthy, or hazardous: 2 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

12. St. Louis, MO-IL

  • Median AQI: 54
  • Max AQI: 182
  • Good air quality: 157 days per year
  • Moderate or unhealthy for sensitive groups: 206 days per year
  • Unhealthy, very unhealthy, or hazardous: 2 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

11. Pittsburgh, PA

  • Median AQI: 55
  • Max AQI: 154
  • Good air quality: 136 days per year
  • Moderate or unhealthy for sensitive groups: 228 days per year
  • Unhealthy, very unhealthy, or hazardous: 1 day per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

10. Philadelphia-Camden-Wilmington, PA-NJ-DE-MD

  • Median AQI: 55
  • Max AQI: 164
  • Good air quality: 132 days per year
  • Moderate or unhealthy for sensitive groups: 230 days per year
  • Unhealthy, very unhealthy, or hazardous: 3 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

9. Chicago-Naperville-Elgin, IL-IN-WI

  • Median AQI: 57
  • Max AQI: 177
  • Good air quality: 117 days per year
  • Moderate or unhealthy for sensitive groups: 242 days per year
  • Unhealthy, very unhealthy, or hazardous: 6 days per year
  • Most common pollutant: Fine particulate matter (PM2.5)

Photo Credit: Alamy Stock Photo

8. Las Vegas-Henderson-Paradise, NV

  • Median AQI: 61
  • Max AQI: 154
  • Good air quality: 122 days per year
  • Moderate or unhealthy for sensitive groups: 242 days per year
  • Unhealthy, very unhealthy, or hazardous: 1 day per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

7. Salt Lake City, UT

  • Median AQI: 61
  • Max AQI: 169
  • Good air quality: 138 days per year
  • Moderate or unhealthy for sensitive groups: 225 days per year
  • Unhealthy, very unhealthy, or hazardous: 2 days per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

6. Sacramento–Roseville–Arden-Arcade, CA

  • Median AQI: 61
  • Max AQI: 314
  • Good air quality: 124 days per year
  • Moderate or unhealthy for sensitive groups: 217 days per year
  • Unhealthy, very unhealthy, or hazardous: 24 days per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

5. San Diego-Carlsbad, CA

  • Median AQI: 64
  • Max AQI: 143
  • Good air quality: 73 days per year
  • Moderate or unhealthy for sensitive groups: 292 days per year
  • Unhealthy, very unhealthy, or hazardous: 0 days per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

4. Denver-Aurora-Lakewood, CO

  • Median AQI: 64
  • Max AQI: 174
  • Good air quality: 83 days per year
  • Moderate or unhealthy for sensitive groups: 274 days per year
  • Unhealthy, very unhealthy, or hazardous: 8 days per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

3. Los Angeles-Long Beach-Anaheim, CA

  • Median AQI: 77
  • Max AQI: 201
  • Good air quality: 35 days per year
  • Moderate or unhealthy for sensitive groups: 310 days per year
  • Unhealthy, very unhealthy, or hazardous: 20 days per year
  • Most common pollutant: Ozone

Photo Credit: Alamy Stock Photo

2. Phoenix-Mesa-Scottsdale, AZ

  • Median AQI: 77
  • Max AQI: 996
  • Good air quality: 42 days per year
  • Moderate or unhealthy for sensitive groups: 304 days per year
  • Unhealthy, very unhealthy, or hazardous: 19 days per year
  • Most common pollutant: Coarse particulate matter (PM10)

Photo Credit: Alamy Stock Photo

1. Riverside-San Bernardino-Ontario, CA

  • Median AQI: 97
  • Max AQI: 296
  • Good air quality: 25 days per year
  • Moderate or unhealthy for sensitive groups: 275 days per year
  • Unhealthy, very unhealthy, or hazardous: 65 days per year
  • Most common pollutant: Ozone

Methodology & Detailed Findings

Statistics on annual levels of air pollutants by metropolitan area for 2018 were obtained from the U.S. Environmental Protection Agency. For each metro, the EPA calculates a daily index value for each pollutant. The highest of those index values is reported as the daily Air Quality Index (AQI) value, and the pollutant responsible for the highest index value is considered the main pollutant for the day. For each metro, the median of the daily AQI values is reported as the overall measure of air quality for the year—the median AQI. Additionally, for each metro, the pollutant listed as the main pollutant for the greatest number of days in the year is listed as the most common pollutant in that metro. PM10 and PM2.5 represent coarse and fine particulate matter, respectively.

To improve relevance, metropolitan areas were grouped into the following cohorts based on population size:

  • Large: 1,000,000 or more
  • Midsize: 350,000-999,999
  • Small: 100,000-349,999

Metros were ordered by their median AQI. In the event of a tie, both the maximum annual AQI and the number of unhealthy, very unhealthy, or hazardous days were considered.


Many of America’s most polluted metros are in California. The rankings can be attributed to a few factors. First, California is the most populous U.S. state, which means more people are commuting, consuming, and ultimately polluting. Second, California is generally warm and sunny. Ground-level ozone pollution is more likely to form in this type of climate as the sun and heat act as accelerants. California’s hot weather has also been linked to severe wildfires, which push more dangerous particles into the air. Finally, many California cities exist on plains or in valleys that are encased by mountains. This topography helps trap pollutants in, keeping ambient air concentrations high.

Across the U.S., major sources of pollution vary by pollutant. For example, vehicles produce the largest share of carbon monoxide gas, while industrial processes and stationary fuel consumption (like power plants) are responsible for the majority of particulate matter and sulfur dioxide in the air.


Commonly, ground-level ozone pollution is caused by automobiles, power plants, chemical plants, refineries, and industrial boilers. However, it can also be created through natural sources including trees. Prolonged exposure to this type of pollution can lead to numerous health problems, particularly those related to the lungs and respiratory system.

Air pollution in the U.S. has greatly improved since the passage of the Clean Air Act. Yet, millions of Americans continue to live in areas where air pollution remains a threat. To tackle this ongoing problem, the EPA and other federal, state, and local environmental agencies are implementing new policies and programs aimed at individuals and companies. For example, the EPA SmartWay program helps companies identify more fuel-efficient ways to move goods. Similarly, cities and states across the nation are heavily investing in and promoting public transportation systems as a way to reduce air pollution and protect the environment.

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