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U.S. cities with the dirtiest air

  • U.S. cities with the dirtiest air
    1/ Pxhere

    U.S. cities with the dirtiest air

    Poor air quality can rapidly deteriorate human health; it can lead to respiratory issues, which can, in turn, shorten lifespans. This is why it is vital for communities to monitor air pollution and make strides toward reducing it for the safety of its residents. To help identify poor-quality air, the Environmental Protection Agency has created National Ambient Air Quality standards. The two most relevant standards are 150 micrograms of pollutants per cubic meter of air over a 24-hour period for coarse particulate matter (PM10) and 12 micrograms of pollutants for fine particulate matter (PM2.5).

    For this article, Stacker researched the 50 American cities with the worst quality of air. This ranking was based on 2018 statistics and took into account 193 United States cities that the EPA reported their PM10 values for. Twenty-one of the cities included in this list came from just the state of California alone.

    The air quality was ranked according to the amount of particulate matter (PM) between 2.5 and 10 micrometers in the air. The higher the amount of particulate matter, the dirtier the quality of air. In the case of cities with equal amount of particulate matter, the city with dirtier air was determined by which city had the highest particulate matter on the second dirtiest day. Cities that had an annual mean amount of particulate matter smaller than 2.5 micrometers were not included in this ranking. The second-highest days were also included in order to rule out unusually low outliers. 

    One important note about the data is that rankings were determined by typical or average values. Exceptionally high or low values were regarded as outliers and not used to quantify the mean average particulate matter upon which the rankings were based. We also included other explainers as to factors, such as industries and climate factors, that could also account for low air quality in these cities.

    You may also like: 50 ways the weather could change in the next 50 years

  • #50. Chico, CA
    2/ cobalt and chrome // Flickr

    #50. Chico, CA

    - Average coarse particulate matter (PM10) in 2018: 25 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 81 μg/m^3 (46% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 13.7 μg/m^3 (14% above EPA standard)

    A top cause of air pollution in the Chico area comes from the smoke from wildfires common in Northern California, in which temperature inversions trap particulate matter and make it hard for it to escape. Chico became a safe-haven for those impacted by the Camp Fire of November 2018. Its population numbers have since skyrocketed, with reports from residents of traffic jams, housing shortages, and overly crowded public places, all likely to increase overall pollution in the area. 

  • #49. Santa Maria-Santa Barbara, CA
    3/ greglaskiewicz // Wikimedia Commons

    #49. Santa Maria-Santa Barbara, CA

    - Average coarse particulate matter (PM10) in 2018: 25 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 103 μg/m^3 (31% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.6 μg/m^3 (28% below EPA standard)

    A large air pollution contributor in Santa Barbara comes from ash stirred up in the air in the aftermath of wildfires in surrounding areas. Local government agency Santa Barbara County Air Pollution Control District aims to help protect the environment and residents from the impact of air pollution, with on-staff experts in meteorology, engineering, and environmental science. 

  • #48. Laramie, WY
    4/ Kari // Flickr

    #48. Laramie, WY

    - Average coarse particulate matter (PM10) in 2018: 25 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 113 μg/m^3 (25% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 5 μg/m^3 (58% below EPA standard)

    Wildfires, such as the Badger Creek Wildfire in 2018, have contributed significantly to the smoke pollution in the air. The Department of Atmospheric Science at the University of Wyoming has put up air quality monitors on top of the UW Engineering building to provide an accurate air quality index for Laramie.

  • #47. Redding, CA
    5/ BoltSnypr // Wikimedia Commons

    #47. Redding, CA

    - Average coarse particulate matter (PM10) in 2018: 25 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 137 μg/m^3 (9% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: Not available

    A major contributor to air pollution in Redding is climate change and its effects on the ozone layer. Eight cities in California, including the Redding-Red Bluff area, saw an increase in unhealthy ozone days (throughout 2014–2016), according to the American Lung Association's 2018 State of the Air report. The study awarded Redding-Red Bluff an F grade, making it one of the smoggiest places in the country. 

  • #46. San Diego-Carlsbad, CA
    6/ rvd5star // Wikimedia Commons

    #46. San Diego-Carlsbad, CA

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 54 μg/m^3 (64% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10 μg/m^3 (17% below EPA standard)

    The heat waves and wildfires common in San Diego have played a major role in increasingly unhealthy ozone levels. The American Lung Association explains that part of the  problem is that the mountains in the area trap the polluted air. While the group found from 2014–2016 there was a 42% increase in the average number of smoggy days per year over 2012–2014, the average is still significantly down from 1998–2000, which averaged 95 annual smoggy days. 

  • #45. Indianapolis-Carmel-Anderson, IN
    7/ Public Domain

    #45. Indianapolis-Carmel-Anderson, IN

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 55 μg/m^3 (63% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.9 μg/m^3 (9% below EPA standard)

    A major factor for pollution is the lack of public transportation, which fuels high usage of cars and high emissions into the air. Another notable contributor is the coal-fired power plants that are carried by winds to other parts of Indiana as well.

  • #44. Sierra Vista-Douglas, AZ
    8/ Public Domain

    #44. Sierra Vista-Douglas, AZ

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 88 μg/m^3 (41% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 5.6 μg/m^3 (53% below EPA standard)

    The main contributor to air pollution in Sierra Vista is the high-ozone count in the atmosphere. Out of 228 metro areas, the American Lung Association ranked Sierra Vista #123 for high-ozone days, but the metro surprisingly tied for first place for the country's cleanest metro area in regards to 24-hour particle pollution. 

  • #43. Kalispell, MT
    9/ Dan Petesch // Wikimedia Commons

    #43. Kalispell, MT

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 99 μg/m^3 (34% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 7.5 μg/m^3 (38% below EPA standard)

    High traffic, especially during the wintertime, causes the amount of particulate matter in the air to increase. Temperature inversions are also another contributor to poor air quality in this area.

  • #42. Rock Springs, WY
    10/ Mia & Steve Mestdagh // Flickr

    #42. Rock Springs, WY

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 193 μg/m^3 (29% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 5.7 μg/m^3 (53% below EPA standard)

    A significant source of pollution comes from the oil and gas industries in the area and from the trona mines. Wyoming's Department of Environmental Quality has installed ambient air quality monitors in Rock Springs.

  • #41. Bishop, CA
    11/ jankgo // Wikimedia Commons

    #41. Bishop, CA

    - Average coarse particulate matter (PM10) in 2018: 26 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 353 μg/m^3 (135% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 7.4 μg/m^3 (38% below EPA standard)

    The goal of government agency Great Basin Unified Air Pollution Control District is to help protect the environment and residents of Alpine, Mono, and Inyo Counties, the last of which Bishop is located within. GBUAPCD monitors multiple air quality cameras in the region in order to make sure the region is meeting federal and state air quality standards. 

  • #40. Baton Rouge, LA
    12/ bluepoint951 // Flickr

    #40. Baton Rouge, LA

    - Average coarse particulate matter (PM10) in 2018: 27 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 84 μg/m^3 (44% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.3 μg/m^3 (14% below EPA standard)

    Most of the particulate matter in the atmosphere comes from the coal-emitting power plants, wood burning, vehicle emissions, and wildfires. Efforts to clean up coal-fired power plants and retiring diesel engines have helped to reduce the pollution in the air.

  • #39. St. Joseph, MO-KS
    13/ Rcool1234 // Wikimedia Commons

    #39. St. Joseph, MO-KS

    - Average coarse particulate matter (PM10) in 2018: 27 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 87 μg/m^3 (42% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.9 μg/m^3 (26% below EPA standard)

    A huge source of pollution is the airborne mercury released by the many coal-fired power plants in the area. In 2011, Missouri was found to be one of the states producing the most mercury pollution in the country by an Environment America report, however, as of January 2019 mercury emissions in Missouri have decreased by at least 75%. 

  • #38. Gillette, WY
    14/ Joanna Poe // Flickr

    #38. Gillette, WY

    - Average coarse particulate matter (PM10) in 2018: 27 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 113 μg/m^3 (25% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 4.7 μg/m^3 (61% below EPA standard)

    A big contributor to air pollution is the weather conditions in the area. Conditions such as cold weather, low wind, and a great deal of snow increase the formation of ozone in the atmosphere.

  • #37. Oxnard-Thousand Oaks-Ventura, CA
    15/ Ken Lund // Flickr

    #37. Oxnard-Thousand Oaks-Ventura, CA

    - Average coarse particulate matter (PM10) in 2018: 27 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 160 μg/m^3 (7% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.2 μg/m^3 (15% below EPA standard)

    Much of the pollution in this area comes from the pesticide use, power plants, and Superfund cleanup sites. The area's air quality is also impacted by smoke from wildfires in the state: In 2018, the Ventura County Health Care Agency offered its residents free face masks after brush fires led to poor air quality. 

  • #36. Salinas, CA
    16/ BrendelSignature // Wikimedia Commons

    #36. Salinas, CA

    - Average coarse particulate matter (PM10) in 2018: 28 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 76 μg/m^3 (49% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 6.9 μg/m^3 (43% below EPA standard)

    The California Air Resource Board in 2018 began inspections of heavy duty trucks in the Salinas Valley to ensure compliance with measures to decrease vehicle emissions in the area. Measures to decrease the impact of diesel vehicles include fuel-saving technology, diesel soot filters, and upgrades to engines. 

  • #35. Muskogee, OK
    17/ Valis55 // Wikimedia Commons

    #35. Muskogee, OK

    - Average coarse particulate matter (PM10) in 2018: 28 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 155 μg/m^3 (3% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: Not available

    Much of the pollution in Muskogee comes from sulfur dioxide in the air, which is a byproduct of the fossil fuels burned by power plants and industrial facilities in the area.

  • #34. Detroit-Warren-Dearborn, MI
    18/ Pixabay

    #34. Detroit-Warren-Dearborn, MI

    - Average coarse particulate matter (PM10) in 2018: 29 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 85 μg/m^3 (43% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 11.9 μg/m^3 (1% below EPA standard)

    Much of the air pollution in the area comes from the automobile industry and vehicle emissions from the multitude of trucks on local roads. More than 150 sites in southwest Detroit emit dangerous particles and chemicals into the air, such as sulfur dioxide, nitrous oxide, and PM2.5. The International Journal of Environmental Research and Public Health reported that 5.5% of annual deaths in Detroit can be linked to PM2.5 exposure. 

  • #33. Duluth, MN-WI
    19/ Tony Webster // Wikimedia Commons

    #33. Duluth, MN-WI

    - Average coarse particulate matter (PM10) in 2018: 29 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 89 μg/m^3 (41% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 5.7 μg/m^3 (53% below EPA standard)

    A significant contributor to air pollution in this northern Minnesota region comes from the smoke from the wildfires that occur in Canada. The Minnesota Pollution Control Agency reported that smoke from July 2019 wildfires in Manitoba and Ontario in Canada made its way to northern Minnesota, adding that visibility in the area decreased to under 1 or 2 miles. MPCA issued air quality alerts to surrounding towns, including Duluth. 

  • #32. San Luis Obispo-Paso Robles-Arroyo Grande, CA
    20/ Capt. Sean Smith // U.S. Air National Guard photo

    #32. San Luis Obispo-Paso Robles-Arroyo Grande, CA

    - Average coarse particulate matter (PM10) in 2018: 29 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 123 μg/m^3 (18% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.8 μg/m^3 (27% below EPA standard)

    California State Parks are aiming to improve the air quality around Southern California's Oceano Dunes, which covers 18 miles along the coast, where many enjoy off-roading and camping. Conservationists have expressed concern that these activities are endangering plant and animal species, as well as increasing overall pollution in the area. A four-year plan intends to install dust-control measures and increase vegetation. 

  • #31. Sacramento-Roseville-Arden-Arcade, CA
    21/ U.S. Coast Guard photo

    #31. Sacramento-Roseville-Arden-Arcade, CA

    - Average coarse particulate matter (PM10) in 2018: 29 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 252 μg/m^3 (68% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 14.4 μg/m^3 (20% above EPA standard)

    A top cause of pollution here is the stagnancy in the air. It's extremely difficult for pollutants to leave the atmosphere in Sacramento.

  • #30. Cleveland-Elyria, OH
    22/ Ken Lund // Flickr

    #30. Cleveland-Elyria, OH

    - Average coarse particulate matter (PM10) in 2018: 30 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 74 μg/m^3 (51% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 11.1 μg/m^3 (8% below EPA standard)

    Much of Cleveland's air pollution comes from coal-burning power plants and vehicle emissions, which come from cars, trucks, trains, and boats in the area. Byproducts of lawn care equipment used in the region also contribute to the pollution.

  • #29. Wichita, KS
    23/ Shannon McGee // Flickr

    #29. Wichita, KS

    - Average coarse particulate matter (PM10) in 2018: 31 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 86 μg/m^3 (43% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.2 μg/m^3 (32% below EPA standard)

    Two major contributors of air pollution in Wichita are byproducts from industrial plants and smog from neighboring cities in Oklahoma and Texas.

  • #28. St. Louis, MO-IL
    24/ Pxhere

    #28. St. Louis, MO-IL

    - Average coarse particulate matter (PM10) in 2018: 31 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 91 μg/m^3 (39% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 11.1 μg/m^3 (8% below EPA standard)

    Much of the air pollution from the area comes from the high temperatures during the summer, which drive ozone formation, and from the sulfur dioxide emitted from power plants and other industries in the area.

  • #27. Stockton-Lodi, CA
    25/ Clotee Pridgen Alloc… // Wikimedia Commons

    #27. Stockton-Lodi, CA

    - Average coarse particulate matter (PM10) in 2018: 31 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 212 μg/m^3 (41% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 17.6 μg/m^3 (47% above EPA standard)

    More cities impacted by smoke from the region's wildfires include Stockton and Lodi in the San Joaquin Valley (south of San Francisco). The burning of carbon-based fuels, including vehicle emissions, is also to blame. In January 2019, the California Air Resources Board announced a plan for cleaning up the valley's dangerous air and meeting federal standards for fine particle pollution. 

  • #26. Payson, AZ
    26/ Alan Levine // Wikimedia Commons

    #26. Payson, AZ

    - Average coarse particulate matter (PM10) in 2018: 31 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 239 μg/m^3 (59% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: Not available

    A 2018 report by the Environment Arizona Research & Policy Center found that Payson, which is less than 100 miles northeast of Pheonix, experienced 87 days in which the ozone levels were elevated in 2016. The report's experts recommend transitioning to clean energy and imposing stricter rules on fuel economy standards. 

  • #25. Yuba City, CA
    27/ Robert Lesiuk // Wikimedia Commons

    #25. Yuba City, CA

    - Average coarse particulate matter (PM10) in 2018: 31 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 245 μg/m^3 (63% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.3 μg/m^3 (14% below EPA standard)

    The air quality in Northern California's Yuba City is harmed by the region's wildfires and weather patterns.  Yuba County Health issued a health advisory notice for its residents after the Butte County Camp Fire in November 2018. 

  • #24. Houston-The Woodlands-Sugar Land, TX
    28/ Ken Lund // Flickr

    #24. Houston-The Woodlands-Sugar Land, TX

    - Average coarse particulate matter (PM10) in 2018: 32 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 102 μg/m^3 (32% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 11 μg/m^3 (8% below EPA standard)

    Much of the poor air quality in Houston is the result of climate change: The higher temperatures in the summer stimulate formation of ozone in the atmosphere. Particulate matter is also high in Houston from disasters such as Hurricane Harvey, which released over 8 million pounds of particulates into the atmosphere.

  • #23. Las Vegas-Henderson-Paradise, NV
    29/ ClatieK // Flickr

    #23. Las Vegas-Henderson-Paradise, NV

    - Average coarse particulate matter (PM10) in 2018: 32 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 115 μg/m^3 (23% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10 μg/m^3 (17% below EPA standard)

    The high temperatures and intense sunlight in the Las Vegas region trigger the formation of ozone in the atmosphere. Particulate matter from vehicle and tailpipe emissions also adds to the pollution here.

  • #22. Los Angeles-Long Beach-Anaheim, CA
    30/ Public Domain // Wikimedia Commons

    #22. Los Angeles-Long Beach-Anaheim, CA

    - Average coarse particulate matter (PM10) in 2018: 33 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 90 μg/m^3 (40% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 12.9 μg/m^3 (8% above EPA standard)

    The majority of air pollution in L.A. comes from vehicle emissions—especially from large trucks; the trucking industry is prominent in L.A. The Harbor Commissioners of L.A. recently voted to approve the Clean Air Action plan to help transition diesel trucks to run on electric power.

  • #21. Rapid City, SD
    31/ M. Mingda Liu // Wikimedia Commons

    #21. Rapid City, SD

    - Average coarse particulate matter (PM10) in 2018: 33 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 124 μg/m^3 (17% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 6.7 μg/m^3 (44% below EPA standard)

    Smoke from wildfires in western states affects air pollution in South Dakota. Dust is a major issue in Rapid City, which harms air quality, and is worsened by the location of quarries in the northwest corner of the city.

  • #20. Merced, CA
    32/ Paul Sullivan // Flickr

    #20. Merced, CA

    - Average coarse particulate matter (PM10) in 2018: 34 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 80 μg/m^3 (47% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 15.1 μg/m^3 (26% above EPA standard)

    The Energy Policy Institute at the University of Chicago estimates that the average Merced resident could live 7 months longer if the region could meet the World Health Organization's standards of soot exposure levels. The area would need to impose strict emissions controls to meet WHO's standards. 

  • #18. Nogales, AZ (tie)
    33/ Ken Lund // Flickr

    #18. Nogales, AZ (tie)

    - Average coarse particulate matter (PM10) in 2018: 34 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 114 μg/m^3 (24% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 9.2 μg/m^3 (23% below EPA standard)

    The unpaved roadways in Nogales are the main source of the high particle emissions in Nogales. The Unified Cargo Processing program is meant to reduce wait times for commercial trucks at the U.S.-Mexico border, which should lessen emissions leaking into the air.

  • #18. El Paso, TX (tie)
    34/ Pixabay

    #18. El Paso, TX (tie)

    - Average coarse particulate matter (PM10) in 2018: 34 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 114 μg/m^3 (24% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.5 μg/m^3 (29% below EPA standard)

    A big cause of pollution is the shared border with Mexico, which has much less stringent clean air regulations. The Drive Clean Texas campaign urges El Paso residents to be more mindful of how they drive and how that affects the air pollution.

  • #17. Minneapolis-St. Paul-Bloomington, MN-WI
    35/ Public Domain // Wikimedia Commons

    #17. Minneapolis-St. Paul-Bloomington, MN-WI

    - Average coarse particulate matter (PM10) in 2018: 35 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 144 μg/m^3 (4% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.9 μg/m^3 (26% below EPA standard)

    Much of the air pollution in these regions come from inversions, which is when warm air gets sandwiched between cold air layers. This formation prevents pollutants from being able to disperse out into the atmosphere.

  • #16. Denver-Aurora-Lakewood, CO
    36/ Anne Hornyak // Wikimedia Commons

    #16. Denver-Aurora-Lakewood, CO

    - Average coarse particulate matter (PM10) in 2018: 36 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 146 μg/m^3 (3% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.2 μg/m^3 (15% below EPA standard)

    The major contributors to air pollution are increased vehicle traffic and increased building construction. Another issue is the smoke from wildfires in western states drifting eastward toward Denver.

  • #15. Modesto, CA
    37/ Fabrice Florin // Flickr

    #15. Modesto, CA

    - Average coarse particulate matter (PM10) in 2018: 36 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 184 μg/m^3 (23% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 17.2 μg/m^3 (43% above EPA standard)

    The large agricultural sector in Modesto contributes to emissions released that affect the atmosphere. Modesto is located in the San Joaquin Valley, which as a whole experiences poor air quality. 

  • #14. Cincinnati, OH-KY-IN
    38/ WebXplorer // Flickr

    #14. Cincinnati, OH-KY-IN

    - Average coarse particulate matter (PM10) in 2018: 37 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 83 μg/m^3 (45% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.6 μg/m^3 (12% below EPA standard)

    Coal-fired power plants are a major contributor of air pollution in the Cincinnati area. Another cause of pollutants is diesel emissions from the heavy duty transportation vehicles, such as large trucks, that are heavily used in the region.

  • #13. Las Cruces, NM
    39/ David Herrera // Flickr

    #13. Las Cruces, NM

    - Average coarse particulate matter (PM10) in 2018: 37 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 325 μg/m^3 (117% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 9.4 μg/m^3 (22% below EPA standard)

    Much of the air pollution in this region is not from vehicle emissions or manufacturing, but rather from the many dust storms that pass through the region.

  • #12. Davenport-Moline-Rock Island, IA-IL
    40/ Ken Lund // Flickr

    #12. Davenport-Moline-Rock Island, IA-IL

    - Average coarse particulate matter (PM10) in 2018: 39 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 140 μg/m^3 (7% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.8 μg/m^3 (27% below EPA standard)

    The pollution in the air in the Davenport area is from the particles emitted by vehicular traffic. The stagnant air also prevents pollutant particles from being able to freely disperse.

  • #11. Albuquerque, NM
    41/ Ron Cogswell // Flickr

    #11. Albuquerque, NM

    - Average coarse particulate matter (PM10) in 2018: 40 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 133 μg/m^3 (11% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.4 μg/m^3 (30% below EPA standard)

    Much of the air pollution in Albuquerque comes from the burning of fuels such as gas, oil, and coal: Airports and vehicle traffic are massive sources of emissions. A 2017 report by the Environment New Mexico Research & Policy Center found that Albuquerque had 113 days of high smog pollution in 2015. 

  • #10. Madera, CA
    42/ Jcarrello // Wikimedia Commons

    #10. Madera, CA

    - Average coarse particulate matter (PM10) in 2018: 40 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 140 μg/m^3 (7% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 14 μg/m^3 (17% above EPA standard)

    The American Lung Association's 2019 State of the Air report ranked the Madera-Fresno-Hanford area the worst in the country for 24-hour soot and the fourth-worst in smog. Several events were canceled by the Madera Parks and Community Services during the time period ALA looked at (2015–2017) due to poor air quality exacerbated by extremely high temperatures. 

  • #9. Yuma, AZ
    43/ Lance Cpl. George Melendez // USMC

    #9. Yuma, AZ

    - Average coarse particulate matter (PM10) in 2018: 42 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 227 μg/m^3 (51% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 8.7 μg/m^3 (28% below EPA standard)

    The Yuma County Farm Bureau has made active educational efforts to teach Yuma's agricultural sector about best management practices to protect their atmosphere. Growers in the area must follow practices such as conservation irrigation, reduced speeds on roads, and planting based on soil moisture, according to a Jan. 2, 2018 issue of the Yuma Sun

  • #8. Tucson, AZ
    44/ Laura Bolyard // NPS Photo

    #8. Tucson, AZ

    - Average coarse particulate matter (PM10) in 2018: 43 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 206 μg/m^3 (37% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 5.9 μg/m^3 (51% below EPA standard)

    The high-pressure weather systems coupled with intense heat and solar radiation contribute to high ozone layers in this region. Pima County, where Tucson is located, regularly issues ozone warnings to residents.

  • #7. Fresno, CA
    45/ Creative Commons // Wikimedia Commons

    #7. Fresno, CA

    - Average coarse particulate matter (PM10) in 2018: 45 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 125 μg/m^3 (17% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 17.1 μg/m^3 (43% above EPA standard)

    A major source of air pollution in Fresno is ozone from vehicle emissions. Emissions from the agricultural and construction industries in the area are also part of the problem.

  • #6. Riverside-San Bernardino-Ontario, CA
    46/ Ken Lund // Wikimedia Commons

    #6. Riverside-San Bernardino-Ontario, CA

    - Average coarse particulate matter (PM10) in 2018: 51 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 143 μg/m^3 (5% below EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 17.4 μg/m^3 (45% above EPA standard)

    The National Science Foundation gave Riverside a $1.2 million dollar grant to provide curriculum about air pollution to high school students in the area and help promote awareness of the problem.

  • #5. Visalia-Porterville, CA
    47/ kallahar // Wikimedia Commons

    #5. Visalia-Porterville, CA

    - Average coarse particulate matter (PM10) in 2018: 51 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 152 μg/m^3 (1% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 17.3 μg/m^3 (44% above EPA standard)

    Since the burning of solids (e.g. wood and pellets) causes air pollution, residents can only burn these materials between November and February to reduce air pollution.

  • #4. Bakersfield, CA
    48/ Public Domain // Wikimedia Commons

    #4. Bakersfield, CA

    - Average coarse particulate matter (PM10) in 2018: 54 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 171 μg/m^3 (14% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 18.1 μg/m^3 (51% above EPA standard)

    The geography and topography of the San Joaquin Valley makes Bakersfield especially susceptible to smog formation and particulate buildups during certain weather systems.

  • #3. Hanford-Corcoran, CA
    49/ Clotee Pridgen Alloc… // Wikimedia Commons

    #3. Hanford-Corcoran, CA

    - Average coarse particulate matter (PM10) in 2018: 59 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 219 μg/m^3 (46% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 18.6 μg/m^3 (55% above EPA standard)

    The geography of the Valley which Hanford resides in contributes largely to its air pollution. The hot summers coupled with stagnant air and mountainous terrain creates makes this part of California susceptible to high-smog levels.

  • #2. El Centro, CA
    50/ Jill // Flickr

    #2. El Centro, CA

    - Average coarse particulate matter (PM10) in 2018: 60 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 351 μg/m^3 (134% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 10.4 μg/m^3 (13% below EPA standard)

    The hot, arid climate in El Centro contributes greatly to its air pollution. El Centro shares a border with Mexico, which has much looser clean air regulations than its neighbor to the north. El Centro has put in efforts with the Mexican government to prevent vehicles that do not meet U.S. emission standards from crossing the border.

  • #1. Phoenix-Mesa-Scottsdale, AZ
    51/ Pixabay

    #1. Phoenix-Mesa-Scottsdale, AZ

    - Average coarse particulate matter (PM10) in 2018: 71 μg/m^3
    - Second-highest average coarse particulate matter (PM10) over a 24-hour period: 223 μg/m^3 (49% above EPA standard)
    - Average fine particulate matter (PM2.5) in 2018: 12.6 μg/m^3 (5% above EPA standard)

    Huge contributors to the poor air in Phoenix include emissions from cars and chemical solvent waste from small industrial businesses. A 2018 report by the Environment Arizona Research & Policy Center found that in 2016, the Phoenix-Mesa-Scottsdale area had 100 days of "degraded" air quality. 

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