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.

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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.