More than 138 million people, nearly 44% of the U.S. population, live in areas where air pollution reaches dangerously high levels during parts of the year. While exposure to low quality air has declined in recent years, some cities still experience alarming levels of pollution.

Based on “State of the Air,” a report released annually by the American Lung Association (ALA), 24/7 Wall St. reviewed the 10 leading metro areas with the highest levels of year-round particle pollution. The Fresno-Madera, California metro area led the nation with the worst long-term pollution level.

Click here to see the 10 most polluted cities

The report considers two types of pollution: ozone pollution and particle pollution, which can be measured both in short and long-term. Speaking to 24/7 Wall St. Janice Nolen, assistant vice president of national policy at the ALA, explained the three measures. Ozone is a gas formed in the atmosphere through heat, sunlight, and certain gases that can attack lung tissue. Particle pollution is tiny pieces of matter so small that they can bypass the body’s defenses and be inhaled into the lungs. The long-term particulate measure is the average, daily pollution generated by factories, fires, and transportation. Long term particle pollution is measured in micro-grams per cubic meter (ug/M3).The short-term particulate measure represents how many days of extreme air pollution occur, and usually reflects how extreme conditions such as fire or drought are creating abnormally high levels of unhealthy particles.

In the 2015 report, six cities set local records for the most days with dangerously high levels of particle pollution, including the San Francisco and Visalia, California metropolitan areas. Nolen said these were surprising results. “We’ve been doing this report for 16 years, and generally speaking, you’ll see a variation in pollution levels from one year to the next, but almost never do you set new records because we’ve made huge improvements in reducing pollution.” Nolen went on to explain that these cities, all in the Western United States, had just begun to experience drought conditions — that are still ongoing — when these data were collected from 2011 through 2013. The drought has only worsened since 2013. “When we get into next year’s report and the year after, I think we’ll see even higher levels in these areas.”

Several of the most polluted cities, notably Pittsburgh and Cleveland, are former manufacturing and production strongholds. While these areas are no longer the industrial strongholds of their heydays, enough industrial and manufacturing production remains for the cities to maintain their standings among the most polluted places in the country. While Pittsburgh’s skies are no longer blackened with the smoke from steel factories, it still generates a great deal of particulate pollution. According to Nolen, the biggest source of pollution in the area is the U.S. Steel plant.

Several of the most polluted cities are located in or near the Central Valley in California. In addition to the wildfires and drought that have affected the region, the area’s natural topography exacerbates the pollution problem. The Central Valley is surrounded by mountains on three sides, which traps the pollution from the San Francisco Bay Area in the valley. Five of the 10 most polluted cities, including parts of the San Francisco metro area, are in the California Central Valley.

Air pollution, both ozone and particulate, contributes to lower life expectancy. In particular, polluted air poses significant health risks to individuals who already have certain respiratory conditions. While there is no evidence that proves that air pollution is the reason that asthma has increased in prevalence in the last 40 years, Nolen said, air pollution exacerbates the effects of asthma and can be the cause of an asthma attack. In Los Angeles, one of the cities with the worst air pollution in the country, more than 1.6 million residents are living with asthma.

While conditions in some cities have worsened, the air quality in many of the cities with the worst air pollution has improved since last year’s report. Nolen explained that every five years, the Environmental Protection Agency determines the safe levels of ozone and particle pollution. Cities and communities around the country have to formulate and implement plans to reduce pollution in order to meet national air quality standards set by the EPA. People in cities such as Philadelphia, which reported its lowest levels of annual particle pollution to date in the 2015 report, have the Clean Air Act to thank. Nolen attributed more efficient vehicles, cleaner-burning fuel, cleaner power plants and an overall reduction in emissions and pollution since 1970 to the Clean Air Act.

To identify the 10 most polluted cities, 24/7 Wall St. reviewed the metropolitan statistical areas with the highest levels of year-round particle pollution from the ALA’s 2015 “State of the Air” report. The number of area residents with asthma, including the number of adults and the number of pediatric cases of the disease, also came from the ALA. The incidence of cardiovascular disease, and the number of residents who have been diagnosed with chronic obstructive pulmonary disease (COPD) at some point in their lives also came from the ALA. All estimates of pollution levels are based on three-year annual averages from 2011 through 2013.

These are America’s 10 most polluted cities.

10. Cleveland-Akron-Canton, OH
> Average year-round particle pollution: 12.5 ug/M3
> People with asthma: 339,464
> Population: 3,501,538
> High ozone days per year: 10.8

Home to more than 3.5 million people, the Cleveland metro area has some of the most polluted air in the country. Cleveland was actually one of five cities that yielded its lowest yearly average particle pollution in the ALA’s 2015 report. Despite the improvement, the metropolitan area still had a much higher than acceptable level of particle pollution. Particle pollution is often the byproduct of the burning of fossil fuels in such places as factories, power plants, and personal vehicles. Manufacturing activity — a long time major economic engine in the Cleveland area — has certainly contributed to the city’s poor air quality. Nearly 340,000 Cleveland residents suffer from asthma, and 232,000 area adults have been diagnosed with COPD. Especially vulnerable to pollution related health risks are the area’s more than 1.3 million residents younger than 18 or older than 65.

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9. Pittsburgh-New Castle-Weirton, PA-OH-WV
> Average year-round particle pollution: 13.4 ug/M3
> People with asthma: 256,406
> Population: 2,659,937
> High ozone days per year: 12.5

During the golden age of steel production in Pittsburgh, the area was one of the most polluted cities in the United States. By many accounts, between the factories and the smoke from the coal that was used to heat residents’ homes, the sky was black with smoke before 9 a.m. every morning. While Pittsburgh is a much less industrial city today and the air quality has improved somewhat, the city remains one of the most polluted cities in the country. According to Nolen, the biggest source of pollution in the area is the U.S. Steel plant. The city received failing grades in ozone pollution as well as in long-term and short-term particle pollution. More than a quarter of a million Pittsburgh residents have asthma, a condition which can be exacerbated by air pollution.

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8. Cincinnati-Wilmington-Maysville, OH-KY-IN
> Average year-round particle pollution: 13.6 ug/M3
> People with asthma: 213,812
> Population: 2,196,629
> High ozone days per year: 12.2

Like most cities with high particle pollution, the Cincinnati metro area also has a problem with severe ozone pollution. The greater Cincinnati area has been under several air quality advisories this summer due to dangerously high ozone levels. Similar to California’s Central Valley, high traffic volumes in the Cincinnati area contribute to high pollution levels. In addition, the city is located in a valley, which, like the mountains surrounding Central Valley, helps trap emissions.

7. San Jose-San Francisco-Oakland, CA
> Average year-round particle pollution: 13.8 ug/M3
> People with asthma: 744,481
> Population: 8,469,854
> High ozone days per year: 8.8

The San Francisco Bay Area, which includes the cities of Oakland and San Jose, has the third-largest combined population at risk of illness due to air pollution, at close to 8.5 million. The metro area reported the most days with high particle pollution on record because of the addition of San Joaquin county to the region. The county was the only one in the metro area to fail the ALA’s test for ozone levels and short- and long-term particle pollution. San Joaquin county had an average of 54 days a year in either orange-level conditions, which are unhealthy for people with sensitive respiratory tracts, or in red conditions, which are unhealthy for everyone.

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6. El Centro, CA
> Average year-round particle pollution: 14.3 ug/M3
> People with asthma: 15,439
> Population: 176,584
> High ozone days per year: 16.2

Like many of the most polluted metro areas in California, El Centro residents suffer from high levels of both ozone and particle pollution. Located in Imperial County, El Centro residents have experienced an average of 16 high-ozone days a year, which is sixth-worst in the country. Known more commonly as smog, high ozone levels occur when fumes from burning fossil fuels are exposed to sunlight. Prolonged exposure to high ozone levels can exacerbate existing respiratory conditions and contribute to early death. More than 15,000 El Centro residents live with asthma, and 5,700 adult residents have been diagnosed with COPD.

5. Los Angeles-Long Beach, CA
> Average year-round particle pollution: 15.1 ug/M3
> People with asthma: 1,607,111
> Population: 18,351,929
> High ozone days per year: 117.7

Los Angeles port’s cargo volume this past July was the highest ever in its over 100-year history. According to the EPA, high traffic through the port creates higher emissions that contribute to poor air quality throughout the region. High concentrations of power plants, including numerous oil and gas, petroleum, and electricity plants, release tens of millions of metric tons of carbon emissions each year. While the area is among the nation’s most polluted, Los Angeles reported its lowest pollution levels since the ALA started producing this report. Still, in Los Angeles, residents have experienced an average of 117.7 days of high ozone levels each year, the most of any U.S. metropolitan area.

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4. Modesto-Merced, CA
> Average year-round particle pollution: 15.7 ug/M3
> People with asthma: 69,027
> Population: 788,719
> High ozone days per year: 22.3

Modesto residents have experienced an average of 22 high ozone days per year. Only seven metro areas in the country had a greater number of high ozone days. Located almost directly in the center of California’s Central Valley, an area surrounded on three sides by mountains that limit wind and annual rainfall, polluted air stagnates in Modesto. The city was one of six in the country that failed to meet federal air quality standards and where particle pollution worsened since last year’s report. Nearly 70,000 Modesto residents suffer from asthma, and over 25,000 area adults have been diagnosed with COPD.

3. Visalia-Porterville-Hanford, CA
> Average year-round particle pollution: 17.0 ug/M3
> People with asthma: 52,749
> Population: 605,103
> High ozone days per year: 82.7

Visalia is located almost directly in between San Francisco and Los Angeles. The city had had an average of nearly 83 high-ozone days a year, the second highest rate in the country. The Visalia metro area was one of six California metros to break the top 10 for ozone pollution, ranking second in the nation despite this year reporting the fewest days of unhealthy ozone levels in its reporting history. On the other hand, the city, like many other Valley cities, experienced its worst year for particulate pollution.

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2. Bakersfield, CA
> Average year-round particle pollution: 17.3 ug/M3
> People with asthma: 75,406
> Population: 864,124
> High ozone days per year: 69.7

Bakersfield residents have endured nearly 70 high-ozone level days a year, the third highest rate compared with other U.S. cities. Still, this was a considerable improvement for the area when compared to years past since the ALA began reporting on air quality. As in other California metro areas, however, particularly those in the state’s Central Valley, annual particle pollution worsened last year, partly due to the severe drought conditions in the region. Also, the topography and industrial composition of the Valley is conducive to air pollution problems. Adding to the region’s especially poor air quality is pollution generated by cars and trucks travelling through the region’s major throughways and the toxic gases from manure on the millions of acres of the area’s farmland.

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1. Fresno-Madera, CA
> Average year-round particle pollution: 18.1 ug/M3
> People with asthma: 96,760
> Population: 1,107,661
> High ozone days per year: 68

Air pollution improved in Fresno-Madera, but the metro area is still the most polluted in the country, as it was in the ALA’s 2014 report. As in other parts of California, the statewide, multi-year drought has likely worsened air quality in the area over the past several years. Hot and dry weather can promote dust storms and wildfires, which trap particulates generated by power plants and other carbon emitters. Fresno residents have experienced an average of 68 high-ozone days a year, the sixth highest rate in the country.

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