A third day of smoky air gives millions in US East Coast, Canada a new view of wildfire threat

Oct 12, 2023

How Smoke From Canada's , Wildfires Has Transformed , Skies in the U.S. 'The New York Times' reports that atmospheric scattering is why skies across the northeastern United States have turned orange amid heavy smoke from Canadian wildfires. On a normal day with a blue sky, sunlight bounces off of molecules of air in the atmosphere, scattering high energy colors with shorter wavelengths into blue light. Larger smoke particles scatter the orange and red colors in the light from the sun. The resulting surreal haze is also closely related to how the sky appears at sunset. As the sun travels closer to the horizon, only longer wavelengths can travel through the atmosphere to be perceived as the reddish hues visible at sunset. 'NYT' reports that hundreds of fires continue to burn across Eastern Canada, spreading clouds of hazardous air pollution across a large swath of the eastern U.S. Air quality warnings have been issued for areas spanning from New York to the Carolinas. Air quality warnings have been issued for areas spanning from New York to the Carolinas. On June 7, major cities experienced unhealthy levels of air pollution, including Philadelphia, Washington and New York. 'NYT' reports that while air quality is expected to improve in parts of the Northeast, smoke from the ongoing fires continues to spread across the U.S. On June 8, Canadian authorities said that approximately 250 wildfires were burning out of control in the eastern part of the country. Some of those fires have already burned for weeks.

NEW YORK — Images of smoke obscuring the New York skyline and the Washington Monument this week have given the world a new picture of the perils of wildfire, far from where blazes regularly turn skies into hazardous haze.

A third day of unhealthy air from Canadian wildfires may have been an unnerving novelty for millions of people on the U.S. East Coast, but it was a reminder of conditions routinely troubling the country's West — and a wake-up call about the future, scientists say.

"This is kind of an astounding event" but likely to become more common amid global warming, said Justin Mankin, a Dartmouth College geography professor and climate scientist. "This is something that we, as the eastern side of the country, need to take quite seriously."

A Marine Corps honor color guard rehearses Thursday near the Washington Monument cloaked in a thick layer of smoke in Washington, D.C.

Millions of residents could see that for themselves Thursday. The conditions sent asthma sufferers to hospitals, delayed flights, postponed ballgames and even pushed back a White House Pride Month celebration. The fires sent plumes of fine particulate matter as far away as North Carolina and northern Europe and parked clumps of air rated unhealthy or worse over the heavily populated Eastern Seaboard.

At points this week, air quality in places including New York, the nation's most populous city, nearly hit the top of the U.S. Environmental Protection Agency's air-pollution scale. Local officials urged people to stay indoors as much as possible and wear face masks when they venture out.

Such conditions are nothing new — indeed, increasingly frequent — on the U.S. West Coast, where residents were buying masks and air filters even before the COVID-19 pandemic and are accustomed to checking air quality daily in summer. Since 2017, California has seen eight of its 10 largest wildfires and six of the most destructive.

The hazardous air has sometimes forced children, older adults and people with asthma and other respiratory conditions to stay indoors for weeks at a time. Officials have opened smoke shelters for people who are homeless or who might not have access to clean indoor air.

"The West has always burned, as has Canada, but what's important now is that we're getting these massive amounts of smoke in a very populated region, so many, many people are getting affected," said Loretta Mickley, the co-leader of Harvard University's Atmospheric Chemistry Modeling Group.

Smoke from Canadian wildfires on Thursday obscures the view of the U.S. Capitol Building on Capitol Hill in Washington.

Fueled by an unusually dry and warm period in spring, the Canadian fire season that is just getting started could well become the worst on record. More than 400 blazes burned Thursday. Over a third are in Quebec, where Public Safety Minister François Bonnardel said no rain is expected until next week and temperatures are predicted to rise.

He said there have been no reports of injuries, deaths or home damage so far from the fires, but it remained unclear Thursday when more than 12,000 evacuees from various communities would be able to return. Manon Cyr, mayor of the evacuated town of Chibougamau, said she advised residents to be "Zen and patient. That's the most important."

She noted the real solution will be a good dose of rain.

In neighboring Ontario, a haze hung over Toronto, Canada's most populous city, where many school recess breaks, day care center activities and outdoor recreation programs were canceled or moved inside.

U.S. President Joe Biden said Thursday that hundreds of American firefighters and support personnel have been in Canada since May, and he'd offered Canadian Prime Minister Justin Trudeau "any additional help Canada needs to rapidly accelerate the effort to put out these fires." The two spoke Wednesday.

MTA employee Shanita Hancle, left, hands out masks to commuters Thursday at the entrance to a subway station in New York.

Wildfires aren't the only air-quality problems that beset major population centers around the globe.

Chemically, wildfire smoke can be more toxic than typical urban pollution, but with an asterisk: With smog, "the problem is you're in it all the time," says Jonathan Deason, an environmental and energy management professor George Washington University.

In New York City, Health Department spokesperson Pedro Frisneda said emergency rooms saw a "higher than usual" number of asthma-related visits from the blanket of smoke, estimating patients were in the "low hundreds."

The city public school system — the nation's largest — said Friday's classes would be conducted remotely. Motorists even got a break Thursday and Friday from having to move their cars for street cleaning.

In Washington, a big Pride Month celebration on the White House's South Lawn was moved from Thursday to Saturday, and a Washington Nationals-Arizona Diamondbacks game was postponed. Local officials closed public parks and suspended some road work.

Philadelphia ended trash collection ended early, for the sake of sanitation employees. Bridgeport, Connecticut's largest city, opened spaces usually used as hot-weather cooling centers so that residents could escape the unhealthy air.

A Chris Stapleton concert at a Syracuse amphitheater was pushed back, fireworks were canceled at Niagara Falls and racing was canceled at New York's Belmont Park two days before the famed Belmont Stakes. It wasn't yet clear whether the Triple Crown race itself might be affected; Gov. Kathy Hochul said that would depend on the air quality at the track Saturday.

Almost half of all Americans—45.8%, or 150 million people—live in counties with unhealthy air quality, according to the American Lung Association's 2020 State of the Air report. Poor air quality, including unhealthy ozone or particle pollution, can lead to respiratory issues that are known to shorten lifespans, contribute to the risk of lower birth weight in newborns, and cause myriad tangential health problems.

Despite this knowledge, the number of people living amid unsafe air quality continues to grow; 2020 numbers are up from 134 million people reported in 2018. To learn more about where in the country the air is the dirtiest, Stacker analyzed the 2019 Air Quality Statistics Report from the Environmental Protection Agency (EPA), released in May 2020. From there, we compiled a list of the 50 cities with the dirtiest air.

To help track air quality around the country, the Environmental Protection Agency (EPA) created National Ambient Air Quality standards. The two most relevant thresholds 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).

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 amounts of particulate matter, the city with dirtier air was determined by which city had the highest particulate matter on the second dirtiest day. Any city with an annual mean amount of particulate matter smaller than 2.5 micrometers was not included in this ranking. The second-highest days were also included 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.

Keep reading to see if your city ranks in the top 50 for dirtiest air.

You may also like: 30 incredible photos that show the true power of nature

- Average coarse particulate matter (PM10): 22 μg/m^3

- Second-highest average coarse particulate matter (PM10): 88 μg/m^3 (70.5% below EPA standard)

Joplin was incorporated in 1873, drawing people there for mineral mining including lead and zinc. The city grew in leaps and bounds around the turn of the 20th century as railroads connected it to other cities around the U.S. and Joplin established itself as a central transportation center in Missouri. Today, Joplin continues to be at the center of multiple modes of transportation including major highways, contributing to air pollution in the area.

- Average coarse particulate matter (PM10): 22 μg/m^3

- Second-highest average coarse particulate matter (PM10): 191 μg/m^3 (21.5% above EPA standard)

Payson, with a population of just over 15,000, is surrounded by the Tonto National Forest. Its proximity to Phoenix (the cities are less than 100 miles apart) and highways works against Payson's air quality. The Environment Arizona Research and Policy Center has recommended improving air quality by transitioning to clean energy and imposing stricter rules on fuel economy standards.

- Average coarse particulate matter (PM10): 22 μg/m^3

- Second-highest average coarse particulate matter (PM10): 91 μg/m^3 (64.8% below EPA standard)

- Average fine particulate matter (PM2.5): 9.3 μg/m^3 (29.0% below EPA standard)

The EPA in February 2021 settled a case with the Fairchild Semiconductor Corporation resolving allegations of hazardous waste regulations violations at the company's semiconductor manufacturing facility in South Portland, Maine. The company agreed to comply with federal regulations to lower air pollution and VOC emissions, both of which were expected to contribute to improved air quality for the city and surrounding areas.

- Average coarse particulate matter (PM10): 22 μg/m^3

- Second-highest average coarse particulate matter (PM10): 90 μg/m^3 (66.7% below EPA standard)

- Average fine particulate matter (PM2.5): 8.4 μg/m^3 (42.9% below EPA standard)

Open-air farm burning has contributed to air-quality issues throughout the Sacramento region. In February 2021, the California Air Resources Control Board agreed to phase out open-air agricultural burning by 2025.

You may also like: How climate change has affected each state

- Average coarse particulate matter (PM10): 22 μg/m^3

- Second-highest average coarse particulate matter (PM10): 87 μg/m^3 (72.4% below EPA standard)

- Average fine particulate matter (PM2.5): 6.8 μg/m^3 (76.5% below EPA standard)

A large air pollution contributor in Santa Barbara comes from ash stirred up 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.

- Average coarse particulate matter (PM10): 23 μg/m^3

- Second-highest average coarse particulate matter (PM10): 80 μg/m^3 (87.5% below EPA standard)

- Average fine particulate matter (PM2.5): 10.6 μg/m^3 (13.2% below EPA standard)

Air pollution throughout the Birmingham region of Alabama has dropped more than most other American urban areas since 2009, according to a 2020 study. Neverthess, the area in 2019 was ranked 14th-worst in the country for year-round particulate air pollution.

- Average coarse particulate matter (PM10): 23 μg/m^3

- Second-highest average coarse particulate matter (PM10): 83 μg/m^3 (80.7% below EPA standard)

- Average fine particulate matter (PM2.5): 6.4 μg/m^3 (87.5% below EPA standard)

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

- Average coarse particulate matter (PM10): 23 μg/m^3

- Second-highest average coarse particulate matter (PM10): 86 μg/m^3 (74.4% below EPA standard)

- Average fine particulate matter (PM2.5): 14.2 μg/m^3 (15.5% above EPA standard)

Pittsburgh's air quality has been so notoriously bad that three-quarters of a century ago, the metropolitan area earned the unusual descriptor of "Hell with the Lid Off." Much has changed since, with air quality improving as steel mills shut down over several decades leading into the 1980s. A 2021 report from Pittsburgh Works found that Allegheny County met federal standards at all eight local air monitors for the first time.

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 529 μg/m^3 (71.6% above EPA standard)

- Average fine particulate matter (PM2.5): 5.2 μg/m^3 (130.8% 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.

You may also like: 50 common weather terms, explained

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 52 μg/m^3 (188.5% below EPA standard)

- Average fine particulate matter (PM2.5): 7.8 μg/m^3 (53.8% below EPA standard)

Colorado's Front Range Urban Corridor includes major state cities including Boulder, Colorado Springs, and Pueblo. The region has consistently struggled to meet air-quality standards laid out by the EPA. Wildfires regularly disrupt efforts to improve the air throughout the Boulder area.

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 50 μg/m^3 (200.0% below EPA standard)

- Average fine particulate matter (PM2.5): 10.7 μg/m^3 (12.1% below EPA standard)

In Texas’ Hidalgo County, Brownsville, Corpus Christi, Laredo, and McAllen rank among the cleanest when it comes to ozone pollution. But McAllen, Brownsville, and Houston also rank among the 25 cities with the worst particle-pollution levels. Particle pollution has been shown to contribute to the risk for health issues raning from strokes to cancer.

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 70 μg/m^3 (114.3% below EPA standard)

Monroe, Michigan, falls within Monroe County—a region regularly tagged with air-quality alerts or "ozone action days." In 2020, DTE Energy Co. reached an agreement with the federal government to lower air pollution from its coal-fired power plants in Southeast Michigan.

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 163 μg/m^3 (8.0% above EPA standard)

- Average fine particulate matter (PM2.5): 7.6 μg/m^3 (57.9% below EPA standard)

Much of the pollution in this region 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.

- Average coarse particulate matter (PM10): 24 μg/m^3

- Second-highest average coarse particulate matter (PM10): 116 μg/m^3 (29.3% below EPA standard)

- Average fine particulate matter (PM2.5): 9.4 μg/m^3 (27.7% below 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.

You may also like: 30 global airports that could be flooded by 2100

- Average coarse particulate matter (PM10): 25 μg/m^3

- Second-highest average coarse particulate matter (PM10): 65 μg/m^3 (130.8% below EPA standard)

- Average fine particulate matter (PM2.5): 12.6 μg/m^3 (4.8% above 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.

- Average coarse particulate matter (PM10): 25 μg/m^3

- Second-highest average coarse particulate matter (PM10): 108 μg/m^3 (38.9% below EPA standard)

- Average fine particulate matter (PM2.5): 2.1 μg/m^3 (471.4% 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.

- Average coarse particulate matter (PM10): 25 μg/m^3

- Second-highest average coarse particulate matter (PM10): 48 μg/m^3 (212.5% below EPA standard)

- Average fine particulate matter (PM2.5): 10.7 μg/m^3 (12.1% below EPA standard)

Oil production in Texas’ Permian Basin was expected to double between 2018 and 2023, with direct consequences to air quality in the region. Laredo Petroleum, a major player in the region, has seen significant growth in the last several years.

- Average coarse particulate matter (PM10): 25 μg/m^3

- Second-highest average coarse particulate matter (PM10): 129 μg/m^3 (16.3% below EPA standard)

- Average fine particulate matter (PM2.5): 7 μg/m^3 (71.4% below EPA standard)

California State Parks have worked to improve air quality around Southern California's Oceano Dunes, which cover 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.

- Average coarse particulate matter (PM10): 26 μg/m^3

- Second-highest average coarse particulate matter (PM10): 57 μg/m^3 (163.2% below EPA standard)

- Average fine particulate matter (PM2.5): 12.5 μg/m^3 (4.0% above EPA standard)

A major factor for air pollution in the Indianapolis region 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.

You may also like: 30 of the most devastating hailstorms in U.S. history

- Average coarse particulate matter (PM10): 26 μg/m^3

- Second-highest average coarse particulate matter (PM10): 71 μg/m^3 (111.3% below EPA standard)

- Average fine particulate matter (PM2.5): 7.6 μg/m^3 (57.9% below EPA standard)

The Kansas City region has a little more than a dozen days annually that do not meet EPA air-quality standards. Much of this is due to smog, or ground-level ozone.

- Average coarse particulate matter (PM10): 26 μg/m^3

- Second-highest average coarse particulate matter (PM10): 97 μg/m^3 (54.6% below EPA standard)

- Average fine particulate matter (PM2.5): 5.1 μg/m^3 (135.3% 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.

- Average coarse particulate matter (PM10): 27 μg/m^3

- Second-highest average coarse particulate matter (PM10): 81 μg/m^3 (85.2% below EPA standard)

- Average fine particulate matter (PM2.5): 5.5 μg/m^3 (118.2% 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, a Minnesota town that boarders Wisconsin.

- Average coarse particulate matter (PM10): 28 μg/m^3

- Second-highest average coarse particulate matter (PM10): 159 μg/m^3 (5.7% above EPA standard)

- Average fine particulate matter (PM2.5): 13.4 μg/m^3 (10.4% 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.

- Average coarse particulate matter (PM10): 29 μg/m^3

- Second-highest average coarse particulate matter (PM10): 63 μg/m^3 (138.1% below EPA standard)

- Average fine particulate matter (PM2.5): 10.7 μg/m^3 (12.1% 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 more than 8 million pounds of particulates into the atmosphere.

You may also like: How weather has shaped human history

- Average coarse particulate matter (PM10): 29 μg/m^3

- Second-highest average coarse particulate matter (PM10): 104 μg/m^3 (44.2% below EPA standard)

- Average fine particulate matter (PM2.5): 8.3 μg/m^3 (44.6% 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.

- Average coarse particulate matter (PM10): 29 μg/m^3

- Second-highest average coarse particulate matter (PM10): 80 μg/m^3 (87.5% below EPA standard)

- Average fine particulate matter (PM2.5): 9.6 μg/m^3 (25.0% below EPA standard)

The Energy Policy Institute at the University of Chicago estimates that the average Merced resident could live seven 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.

- Average coarse particulate matter (PM10): 29 μg/m^3

- Second-highest average coarse particulate matter (PM10): 83 μg/m^3 (80.7% below EPA standard)

- Average fine particulate matter (PM2.5): 8 μg/m^3 (50.0% below EPA standard)

Much of the air pollution in these regions comes 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.

- Average coarse particulate matter (PM10): 29 μg/m^3

- Second-highest average coarse particulate matter (PM10): 94 μg/m^3 (59.6% below EPA standard)

- Average fine particulate matter (PM2.5): 6.3 μg/m^3 (90.5% below EPA standard)

Sahara dust in 2020 moved across Puerto Rico in a concentration that hadn't been recorded in 50 years. The territory has been under pressure in recent years by the EPA to reduce emission levels, which some say could be achieved with more effort put toward green infrastructure.

- Average coarse particulate matter (PM10): 30 μg/m^3

- Second-highest average coarse particulate matter (PM10): 104 μg/m^3 (44.2% below EPA standard)

- Average fine particulate matter (PM2.5): 10.6 μg/m^3 (13.2% below 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.

You may also like: Fastest-warming cities in the U.S.

- Average coarse particulate matter (PM10): 31 μg/m^3

- Second-highest average coarse particulate matter (PM10): 79 μg/m^3 (89.9% below EPA standard)

- Average fine particulate matter (PM2.5): 10.9 μg/m^3 (10.1% 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.

- Average coarse particulate matter (PM10): 32 μg/m^3

- Second-highest average coarse particulate matter (PM10): 108 μg/m^3 (38.9% below EPA standard)

- Average fine particulate matter (PM2.5): 8.8 μg/m^3 (36.4% below EPA standard)

Nogales’ air quality is regularly a point of concern in Arizona, as people living closer to the U.S./Mexico border often use more wood for fires, contributing to higher PM levels than in northern cities like Tucson. Long-term exposure to lower air quality such as in Nogales has been linked to health issues ranging from asthma to reduced lung function.

- Average coarse particulate matter (PM10): 32 μg/m^3

- Second-highest average coarse particulate matter (PM10): 153 μg/m^3 (2.0% above EPA standard)

- Average fine particulate matter (PM2.5): 8.6 μg/m^3 (39.5% 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 has found that part of the problem lies in mountains in the area trapping polluted air.

- Average coarse particulate matter (PM10): 33 μg/m^3

- Second-highest average coarse particulate matter (PM10): 79 μg/m^3 (89.9% below EPA standard)

A binational air-quality committee in 2021 created a fund to help monitor air quality in El Paso, Texas, and Juarez, Mexico. The fund, if goals are met, would funnel $100,000 into supporting three air-quality monitors and system improvements.

- Average coarse particulate matter (PM10): 35 μg/m^3

- Second-highest average coarse particulate matter (PM10): 129 μg/m^3 (16.3% below EPA standard)

- Average fine particulate matter (PM2.5): 9.6 μg/m^3 (25.0% 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.

You may also like: Fastest-warming states in the U.S.

- Average coarse particulate matter (PM10): 35 μg/m^3

- Second-highest average coarse particulate matter (PM10): 111 μg/m^3 (35.1% below EPA standard)

- Average fine particulate matter (PM2.5): 10 μg/m^3 (20.0% 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.

- Average coarse particulate matter (PM10): 35 μg/m^3

- Second-highest average coarse particulate matter (PM10): 99 μg/m^3 (51.5% below EPA standard)

- Average fine particulate matter (PM2.5): 10.5 μg/m^3 (14.3% 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.

- Average coarse particulate matter (PM10): 35 μg/m^3

- Second-highest average coarse particulate matter (PM10): 174 μg/m^3 (13.8% above EPA standard)

- Average fine particulate matter (PM2.5): 7.9 μg/m^3 (51.9% 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.

- Average coarse particulate matter (PM10): 36 μg/m^3

- Second-highest average coarse particulate matter (PM10): 141 μg/m^3 (6.4% below EPA standard)

- Average fine particulate matter (PM2.5): 7.7 μg/m^3 (55.8% 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.

- Average coarse particulate matter (PM10): 36 μg/m^3

- Second-highest average coarse particulate matter (PM10): 155 μg/m^3 (3.2% above EPA standard)

- Average fine particulate matter (PM2.5): 9.7 μg/m^3 (23.7% below 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.

You may also like: U.S. cities with the dirtiest air

- Average coarse particulate matter (PM10): 36 μg/m^3

- Second-highest average coarse particulate matter (PM10): 170 μg/m^3 (11.8% above EPA standard)

- Average fine particulate matter (PM2.5): 5.9 μg/m^3 (103.4% 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.

- Average coarse particulate matter (PM10): 39 μg/m^3

- Second-highest average coarse particulate matter (PM10): 234 μg/m^3 (35.9% above EPA standard)

- Average fine particulate matter (PM2.5): 11.2 μg/m^3 (7.1% below 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.

- Average coarse particulate matter (PM10): 39 μg/m^3

- Second-highest average coarse particulate matter (PM10): 139 μg/m^3 (7.9% below EPA standard)

- Average fine particulate matter (PM2.5): 5 μg/m^3 (140.0% 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.

- Average coarse particulate matter (PM10): 44 μg/m^3

- Second-highest average coarse particulate matter (PM10): 162 μg/m^3 (7.4% above EPA standard)

- Average fine particulate matter (PM2.5): 10.8 μg/m^3 (11.1% 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.

- Average coarse particulate matter (PM10): 46 μg/m^3

- Second-highest average coarse particulate matter (PM10): 284 μg/m^3 (47.2% above EPA standard)

- Average fine particulate matter (PM2.5): 12.9 μg/m^3 (7.0% 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.

- Average coarse particulate matter (PM10): 49 μg/m^3

- Second-highest average coarse particulate matter (PM10): 304 μg/m^3 (50.7% above EPA standard)

- Average fine particulate matter (PM2.5): 12.3 μg/m^3 (2.4% 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.

- Average coarse particulate matter (PM10): 53 μg/m^3

- Second-highest average coarse particulate matter (PM10): 149 μg/m^3 (0.7% below EPA standard)

- Average fine particulate matter (PM2.5): 8 μg/m^3 (50.0% 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.

- Average coarse particulate matter (PM10): 54 μg/m^3

- Second-highest average coarse particulate matter (PM10): 382 μg/m^3 (60.7% above EPA standard)

- Average fine particulate matter (PM2.5): 13 μg/m^3 (7.7% 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.

- Average coarse particulate matter (PM10): 59 μg/m^3

- Second-highest average coarse particulate matter (PM10): 222 μg/m^3 (32.4% above EPA standard)

- Average fine particulate matter (PM2.5): 11.3 μg/m^3 (6.2% below 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.

You may also like: Dramatic satellite observations that show the true scale of Arctic change

Two out of every five Americans—40%, or 137 million people—live in counties with unhealthy air quality, according to the American Lung Association's 2022 State of the Air report. Poor air quality, including unhealthy ozone or particle pollution, can lead to long-term health effects, including respiratory and cardiovascular issues that can lead to premature death.

Air quality can be impacted and degraded by industrial emissions, ambient emissions such as those from cars and residential heating systems, and natural events, including wildfires and drought. Air quality can vary significantly by season and by region, even within the same city. In most places, however, there are several sources simultaneously contributing to air pollution.

To learn more about where in the country the air is the dirtiest, Stacker analyzed the 2021 Air Quality Statistics Report from the Environmental Protection Agency, released in May 2022 to compile a list of the 50 cities with the dirtiest air.

To help track air quality around the country, the Environmental Protection Agency (EPA) created National Ambient Air Quality standards. The two most relevant thresholds 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).

Air quality was ranked according to the amount of coarse particulate matter (PM10) in the air in metropolitan areas. The higher the amount of particulate matter, the dirtier the quality of air. In the case of cities with equal amounts of particulate matter, the city with dirtier air was determined by which city had the highest particulate matter on the second dirtiest day.

Data on coarse particulate matter was available for 176 metropolitan areas. 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.

- Average coarse particulate matter (PM10): 25 μg/m^3

- PM10 2nd Max 24-hr: 177 μg/m^3 (18.0% above EPA standards)

- Average fine particulate matter (PM2.5): Not available

Idaho's air quality is most significantly and negatively impacted by wildfires, both in-state and those burning in neighboring states to the west. Over the last decade, Idaho's air quality has reached unhealthy-to-hazardous ranges due to an increase in wildfires and wind patterns blowing toxic smoke from west to east.

- Average coarse particulate matter (PM10): 25 μg/m^3

- PM10 2nd Max 24-hr: 193 μg/m^3 (28.7% above EPA standards)

- Average fine particulate matter (PM2.5): Not available

Wildfires pose the greatest air quality risk in Deming. The new infrastructure being built in the region as part of the Biden Administration's Bipartisan Infrastructure Law, while a boon for safer and more efficient transportation, may bring great levels of vehicle emissions.

- Average coarse particulate matter (PM10): 26 μg/m^3

- PM10 2nd Max 24-hr: 101 μg/m^3 (32.7% below EPA standards)

- Average fine particulate matter (PM2.5): 8.8 μg/m^3 (26.7% below EPA standards)

Winter air stagnation has been a problem for the Minneapolis-St. Paul-Bloomington region. Light winds and little to no precipitation mean that pollutants cannot be cleared from the air and stay trapped close to the ground under pressure systems.

- Average coarse particulate matter (PM10): 26 μg/m^3

- PM10 2nd Max 24-hr: 115 μg/m^3 (23.3% below EPA standards)

- Average fine particulate matter (PM2.5): 8 μg/m^3 (33.3% below EPA standards)

Wildfires and dust pollution from the Oceano Dunes are the primary contributors to poor air quality in the region.

- Average coarse particulate matter (PM10): 26 μg/m^3

- PM10 2nd Max 24-hr: 120 μg/m^3 (20.0% below EPA standards)

- Average fine particulate matter (PM2.5): Not available

Vehicle and industrial emissions and volatile organic compounds are primarily to blame for low air quality in Joplin. In 2022, ProPublica identified several regions of southwest Missouri, close to where Joplin is located, as hot spots of toxic air pollution. A town located under an hour from Joplin was estimated to have an industrial cancer risk 27 times what the EPA considers acceptable.

- Average coarse particulate matter (PM10): 26 μg/m^3

- PM10 2nd Max 24-hr: 129 μg/m^3 (14.0% below EPA standards)

- Average fine particulate matter (PM2.5): 11.1 μg/m^3 (7.5% below EPA standards)

Air pollution in Chico is caused by a mixture of vehicular emissions, agricultural dust, fossil fuel production emissions, household energy utilization, and wildfires. Chico's location at the base of the Sacramento Valley, surrounded by the Cascade Range to the north and the Sierra Nevada range to the east and south, enables emissions to accumulate in its lower atmosphere.

- Average coarse particulate matter (PM10): 27 μg/m^3

- PM10 2nd Max 24-hr: 81 μg/m^3 (46.0% below EPA standards)

- Average fine particulate matter (PM2.5): 12.9 μg/m^3 (7.5% above EPA standards)

More than half of Pittburgh's pollution can be traced to industrial sources like steel plants. Vehicular emissions are also a contributing factor, disproportionately impacting low-income neighborhoods in Allegheny County.

- Average coarse particulate matter (PM10): 27 μg/m^3

- PM10 2nd Max 24-hr: 103 μg/m^3 (31.3% below EPA standards)

- Average fine particulate matter (PM2.5): 11 μg/m^3 (8.3% below EPA standards)

Low water levels of the Great Salt Lake coupled with dry conditions allow toxic dust from the lakebed to be kicked up into the atmosphere by the wind. This toxic dust includes metals and arsenic, which can be carried and distributed over great distances.

- Average coarse particulate matter (PM10): 27 μg/m^3

- PM10 2nd Max 24-hr: 134 μg/m^3 (10.7% below EPA standards)

- Average fine particulate matter (PM2.5): Not available

Most of Pahrump's coarse particulate matter pollution comes from the use or disturbance of unpaved roads and vacant lands, according to the Nevada Division of Environmental Protection. The Pahrump Valley often experiences high winds and dust storms, so when disturbances of roads and land kick up dust, it can quickly become hazardous under the right conditions.

- Average coarse particulate matter (PM10): 27 μg/m^3

- PM10 2nd Max 24-hr: 138 μg/m^3 (8.0% below EPA standards)

- Average fine particulate matter (PM2.5): 11.4 μg/m^3 (5.0% below EPA standards)

Wind patterns and wildfire smoke from neighboring states such as Montana are the primary cause of low air quality in the region.

- Average coarse particulate matter (PM10): 27 μg/m^3

- PM10 2nd Max 24-hr: 147 μg/m^3 (2.0% below EPA standards)

- Average fine particulate matter (PM2.5): Not available

Seasonal wildfires which continue to grow more intense in the west are producing massive amounts of particulate matter air pollution. Because Pocatello is surrounded by mountains, air pollutants from fires are more easily trapped in that topographical region.

- Average coarse particulate matter (PM10): 28 μg/m^3

- PM10 2nd Max 24-hr: 89 μg/m^3 (40.7% below EPA standards)

- Average fine particulate matter (PM2.5): 12.7 μg/m^3 (5.8% above EPA standards)

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 pollution.

- Average coarse particulate matter (PM10): 28 μg/m^3

- PM10 2nd Max 24-hr: 93 μg/m^3 (38.0% below EPA standards)

- Average fine particulate matter (PM2.5): 7.8 μg/m^3 (35.0% below EPA standards)

Wildfire smoke is the main source of unhealthy air in Kalispell. The fossil fuel industry is also prevalent in Montana. In 2020, activists sued the state for violating the state Constitution that guarantees residents "the right to a clean and healthful environment."

- Average coarse particulate matter (PM10): 28 μg/m^3

- PM10 2nd Max 24-hr: 113 μg/m^3 (24.7% below EPA standards)

- Average fine particulate matter (PM2.5): 10.1 μg/m^3 (15.8% below EPA standards)

Boise is subject to the pollution caused by a rapidly growing population—such as vehicular emissions and residential wood burning, as well as wildfires in the region. Its location in a valley makes it more difficult for ozone and fine particulate matter to be cleared from the atmosphere.

- Average coarse particulate matter (PM10): 28 μg/m^3

- PM10 2nd Max 24-hr: 143 μg/m^3 (4.7% below EPA standards)

- Average fine particulate matter (PM2.5): 9.2 μg/m^3 (23.3% below EPA standards)

Watertown is vulnerable to the effects of Canadian wildfires that burn to the north. Wind can carry smoke from Alberta to North and South Dakota, and parts of Minnesota.

- Average coarse particulate matter (PM10): 28 μg/m^3

- PM10 2nd Max 24-hr: 262 μg/m^3 (74.7% above EPA standards)

- Average fine particulate matter (PM2.5): 9.7 μg/m^3 (19.2% below EPA standards)

Dust is the primary air pollutant in the Bishop region, worsened by extensive drought conditions in the west. Wildfires from surrounding areas have also blanketed Bishop with hazardous smoke and ash.

- Average coarse particulate matter (PM10): 31 μg/m^3

- PM10 2nd Max 24-hr: 89 μg/m^3 (40.7% below EPA standards)

- Average fine particulate matter (PM2.5): 11.3 μg/m^3 (5.8% below EPA standards)

Smoke from seasonal rangeland burning contributes to bouts of poor air quality in Wichita. The controlled burns, which typically occur in March and April, are necessary for ranchers to control invasive species, foster better grazing for cattle, and preserve the tallgrass prairie ecosystem. Vehicular and industrial emissions are also at play in Wichita.

- Average coarse particulate matter (PM10): 31 μg/m^3

- PM10 2nd Max 24-hr: 284 μg/m^3 (89.3% above EPA standards)

- Average fine particulate matter (PM2.5): 12.4 μg/m^3 (3.3% above EPA standards)

Air pollution in Reno can largely be attributed to regional wildfires. Its position in the Truckee Meadows Valley makes it a prime location for pollution to settle and difficult for the environment to naturally clear.

- Average coarse particulate matter (PM10): 32 μg/m^3

- PM10 2nd Max 24-hr: 85 μg/m^3 (43.3% below EPA standards)

- Average fine particulate matter (PM2.5): 12.4 μg/m^3 (3.3% above EPA standards)

Industrial sources are the main contributors to air pollution in the region. Southwest Detroit alone is home to an oil refinery, steel mill, wastewater treatment plant, a coal-fired power plant and a gas-fired power plant, heavily traveled highways, and a six-lane bridge.

- Average coarse particulate matter (PM10): 32 μg/m^3

- PM10 2nd Max 24-hr: 114 μg/m^3 (24.0% below EPA standards)

- Average fine particulate matter (PM2.5): 8.4 μg/m^3 (30.0% below EPA standards)

As is characteristic of many other locations throughout Missouri, St. Joseph's air quality is negatively impacted mainly by industrial and ambient emissions.

- Average coarse particulate matter (PM10): 32 μg/m^3

- PM10 2nd Max 24-hr: 132 μg/m^3 (12.0% below EPA standards)

- Average fine particulate matter (PM2.5): Not available

The Sierra Vista-Douglas region is prone to air pollution from wildfires and dust from prolonged drought conditions.

- Average coarse particulate matter (PM10): 32 μg/m^3

- PM10 2nd Max 24-hr: 161 μg/m^3 (7.3% above EPA standards)

- Average fine particulate matter (PM2.5): 10.6 μg/m^3 (11.7% below EPA standards)

Industrial sources are a primary cause of air pollution in St. Louis. In March 2023, the EPA released an order called the Good Neighbor Rule, instructing Missouri and 22 other states to reduce air pollution levels, particularly industrial emissions. Nitrous oxides emitted from industrial facilities contribute to dangerous ground-level ozone.

- Average coarse particulate matter (PM10): 33 μg/m^3

- PM10 2nd Max 24-hr: 103 μg/m^3 (31.3% below EPA standards)

- Average fine particulate matter (PM2.5): 10.5 μg/m^3 (12.5% below EPA standards)

Houston is home to oil refineries and petrochemical facilities that generate harmful air pollutants. The facilities are often located in close proximity to Black and Latino residential neighborhoods, causing them to bear a disproportionate burden of negative health impacts.

- Average coarse particulate matter (PM10): 33 μg/m^3

- PM10 2nd Max 24-hr: 113 μg/m^3 (24.7% below EPA standards)

- Average fine particulate matter (PM2.5): 13.8 μg/m^3 (15.0% above EPA standards)

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. voted to approve the Clean Air Action Plan in 2017 to help transition diesel trucks to run on electric power.

- Average coarse particulate matter (PM10): 33 μg/m^3

- PM10 2nd Max 24-hr: 162 μg/m^3 (8.0% above EPA standards)

- Average fine particulate matter (PM2.5): 11.7 μg/m^3 (2.5% below EPA standards)

Wildfire smoke is a primary polluter of the region's air, followed by ambient sources including passenger vehicle and freight emissions.

- Average coarse particulate matter (PM10): 34 μg/m^3

- PM10 2nd Max 24-hr: 154 μg/m^3 (2.7% above EPA standards)

- Average fine particulate matter (PM2.5): Not available

Air pollution from oil and gas production, particularly nitrogen oxide, ozone, and fine particulates, impacts the health of many Americans, including Wyoming residents, according to a 2023 study. Wyoming is also impacted by wildfire smoke from regional fires.

- Average coarse particulate matter (PM10): 35 μg/m^3

- PM10 2nd Max 24-hr: 103 μg/m^3 (31.3% below EPA standards)

- Average fine particulate matter (PM2.5): 11.3 μg/m^3 (5.8% below EPA standards)

Kansas City is subject to the impacts of industrial air pollution, particularly soot from coal-fired power plants, diesel emissions, and wood-burning devices. The impacts of these emissions worsen in the winter as the demand for heating increases.

- Average coarse particulate matter (PM10): 35 μg/m^3

- PM10 2nd Max 24-hr: 137 μg/m^3 (8.7% below EPA standards)

- Average fine particulate matter (PM2.5): 9.8 μg/m^3 (18.3% below EPA standards)

Much of the Davenport region's air pollution is caused by industrial sources or transportation. Sulfur dioxide from coal-fired power plants and nitrous oxide from ambient vehicular emissions are the most common pollutants.

- Average coarse particulate matter (PM10): 35 μg/m^3

- PM10 2nd Max 24-hr: 178 μg/m^3 (18.7% above EPA standards)

- Average fine particulate matter (PM2.5): 11.4 μg/m^3 (5.0% below EPA standards)

Smog is a pervasive problem in Cincinnati, worsened by vehicular emissions. Pollutants from coal-fired power plants, as well as diesel emissions from freight transportation, are also prevalent.

- Average coarse particulate matter (PM10): 36 μg/m^3

- PM10 2nd Max 24-hr: 129 μg/m^3 (14.0% below EPA standards)

- Average fine particulate matter (PM2.5): 15 μg/m^3 (25.0% above EPA standards)

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.

- Average coarse particulate matter (PM10): 36 μg/m^3

- PM10 2nd Max 24-hr: 152 μg/m^3 (1.3% above EPA standards)

- Average fine particulate matter (PM2.5): 9.9 μg/m^3 (17.5% below EPA standards)

Transportation, construction, and industrial facilities are the primary sources of harmful emissions and ozone in Las Vegas. Long-term drought conditions and intensified wildfires also contribute to the region's poor air quality.

- Average coarse particulate matter (PM10): 37 μg/m^3

- PM10 2nd Max 24-hr: 230 μg/m^3 (53.3% above EPA standards)

- Average fine particulate matter (PM2.5): Not available

Wildfire smoke from fires in neighboring states like Idaho is a main driver of poor air quality in Laramie. In 2022, wildfires as far south as Arizona impacted air quality in parts of southern Wyoming.

- Average coarse particulate matter (PM10): 39 μg/m^3

- PM10 2nd Max 24-hr: 93 μg/m^3 (38.0% below EPA standards)

- Average fine particulate matter (PM2.5): 10.1 μg/m^3 (15.8% below EPA standards)

The Denver region is prone to several different types of air pollution including vehicle emissions, wildfire smoke, and industrial emissions. The topography of the region worsens the impacts of these pollutants as they settle in the lower atmosphere.

- Average coarse particulate matter (PM10): 40 μg/m^3

- PM10 2nd Max 24-hr: 199 μg/m^3 (32.7% above EPA standards)

- Average fine particulate matter (PM2.5): 9.3 μg/m^3 (22.5% below EPA standards)

Wildfire smoke has the most significant impact on Yuma's air quality. The University of Arizona also identifies vehicle emissions from Interstate 8, pollutants emitted from surrounding agricultural fields, and particulate matter stirred up at the Yuma Proving Ground also impact air quality.

- Average coarse particulate matter (PM10): 41 μg/m^3

- PM10 2nd Max 24-hr: 166 μg/m^3 (10.7% above EPA standards)

- Average fine particulate matter (PM2.5): 10.8 μg/m^3 (10.0% below EPA standards)

Vehicle emissions are a primary contributor to poor air quality in the Chicago area. In addition to transportation-related emissions, communities bisected by major highways are also home to many industries and as such, are overburdened by air pollution.

- Average coarse particulate matter (PM10): 41 μg/m^3

- PM10 2nd Max 24-hr: 168 μg/m^3 (12.0% above EPA standards)

- Average fine particulate matter (PM2.5): 8.1 μg/m^3 (32.5% below EPA standards)

Located on the high plains, Rapid City is subject to droughts and high winds, making it susceptible to a phenomenon called fugitive dust. Fugitive dust is generated by mining and construction activity, gravel roads, and wind erosion.

- Average coarse particulate matter (PM10): 42 μg/m^3

- PM10 2nd Max 24-hr: 119 μg/m^3 (20.7% below EPA standards)

- Average fine particulate matter (PM2.5): 9.7 μg/m^3 (19.2% below EPA standards)

In recent years, San Diego has seen a significant rise in dangerous ozone pollution, caused mostly by passenger vehicle emissions. As recently as 2021, San Diego allowed industries to expel cancer-causing air pollutants at a rate much higher than most of California.

- Average coarse particulate matter (PM10): 42 μg/m^3

- PM10 2nd Max 24-hr: 198 μg/m^3 (32.0% above EPA standards)

- Average fine particulate matter (PM2.5): 12.4 μg/m^3 (3.3% above EPA standards)

Like neighboring Fresno, Madera's air pollution is largely caused by farming emissions and transportation emissions.

- Average coarse particulate matter (PM10): 43 μg/m^3

- PM10 2nd Max 24-hr: 147 μg/m^3 (2.0% below EPA standards)

- Average fine particulate matter (PM2.5): 17.2 μg/m^3 (43.3% above EPA standards)

Fresno's air pollution is characterized by high concentrations of nitrogen dioxide from passenger vehicles, freight, and gas-powered farming equipment emissions.

- Average coarse particulate matter (PM10): 45 μg/m^3

- PM10 2nd Max 24-hr: 149 μg/m^3 (.7% below EPA standards)

- Average fine particulate matter (PM2.5): 10.9 μg/m^3 (9.2% below EPA standards)

Nearly all of the air pollution in Nogales, located on the Southern U.S. border, comes from Mexico, according to the Arizona Department of Environmental Quality. Nogales is situated downhill from Mexico, and as a result, this cross-border air pollution settles in the lowest topographical points.

- Average coarse particulate matter (PM10): 49 μg/m^3

- PM10 2nd Max 24-hr: 242 μg/m^3 (61.3% above EPA standards)

- Average fine particulate matter (PM2.5): 20 μg/m^3 (66.7% above EPA standards)

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

- Average coarse particulate matter (PM10): 49 μg/m^3

- PM10 2nd Max 24-hr: 439 μg/m^3 (192.7% above EPA standards)

- Average fine particulate matter (PM2.5): 10.4 μg/m^3 (13.3% below EPA standards)

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.

- Average coarse particulate matter (PM10): 50 μg/m^3

- PM10 2nd Max 24-hr: 227 μg/m^3 (51.3% above EPA standards)

- Average fine particulate matter (PM2.5): 17.8 μg/m^3 (48.3% above EPA standards)

Known as the Inland Empire, the Riverside-San Bernardino-Ontario region is home to heavy pollution emitters, including freeways, railways, and industrial complexes. Black and Latino communities are disproportionately impacted by air pollution in the Inland Empire, according to the People's Collective for Environmental Justice.

- Average coarse particulate matter (PM10): 52 μg/m^3

- PM10 2nd Max 24-hr: 153 μg/m^3 (2.0% above EPA standards)

- Average fine particulate matter (PM2.5): 9.2 μg/m^3 (23.3% below EPA standards)

A binational air-quality committee in 2021 created a fund to help monitor air quality in El Paso, Texas, and Juarez, Mexico. The fund would funnel $100,000 into supporting three air-quality monitors and system improvements.

- Average coarse particulate matter (PM10): 52 μg/m^3

- PM10 2nd Max 24-hr: 198 μg/m^3 (32.0% above EPA standards)

- Average fine particulate matter (PM2.5): 20.7 μg/m^3 (72.5% above EPA standards)

According to the American Lung Association, population growth and topography are two of the biggest contributors to poor air quality in Visalia-Porterville. With more residents come more smog-producing vehicles, and heavy pollution naturally settles in valleys and basins like the one where Visalia-Porterville is located.

- Average coarse particulate matter (PM10): 52 μg/m^3

- PM10 2nd Max 24-hr: 221 μg/m^3 (47.3% above EPA standards)

- Average fine particulate matter (PM2.5): 11.3 μg/m^3 (5.8% below EPA standards)

The American Lung Association notes that transportation emissions are a major contributor to high levels of ozone in Albuquerque. In July 2022, New Mexico enacted the Clean Car Rule, which is expected to eliminate 130,000 tons of greenhouse gas emissions and more than 1,700 tons of ozone-forming pollutants by 2050.

- Average coarse particulate matter (PM10): 52 μg/m^3

- PM10 2nd Max 24-hr: 234 μg/m^3 (56.0% above EPA standards)

- Average fine particulate matter (PM2.5): 10.3 μg/m^3 (14.2% below EPA standards)

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.

- Average coarse particulate matter (PM10): 52 μg/m^3

- PM10 2nd Max 24-hr: 249 μg/m^3 (66.0% above EPA standards)

- Average fine particulate matter (PM2.5): 6.6 μg/m^3 (45.0% below EPA standards)

As Tuscon's industry and population grew industrialized and the population increased, emissions from power plants and tailpipes rose. In recent years the city has experienced an unprecedented severity of ground-level ozone.

- Average coarse particulate matter (PM10): 55 μg/m^3

- PM10 2nd Max 24-hr: 180 μg/m^3 (20.0% above EPA standards)

- Average fine particulate matter (PM2.5): 15.6 μg/m^3 (30.0% above EPA standards)

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

- Average coarse particulate matter (PM10): 76 μg/m^3

- PM10 2nd Max 24-hr: 225 μg/m^3 (50.0% above EPA standards)

- Average fine particulate matter (PM2.5): 13.2 μg/m^3 (10.0% above EPA standards)

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 110 days of "degraded" air quality.

Data reporting by Emma Rubin. Story editing by Robert Wickwire. Copy editing by Robert Wickwire. Photo selection by Abigail Renaud.

Get local news delivered to your inbox!

As conservative politicians and activists push for limits on discussions of race, gender and sexuality, some students say the measures targeti…

The Supreme Court on Thursday issued a surprising ruling in favor of Black voters in a congressional redistricting case, ordering the creation…

The pilot of the business jet that flew over Washington and crashed in Virginia appeared to be slumped over and unresponsive before the crash,…

North Dakota Gov. Doug Burgum has announced his candidacy for the 2024 Republican presidential nomination. See the full list of candidates.

One of the men killed was an 18-year-old who was graduating on Tuesday afternoon, and the other was a 32-year-old man.