After making great strides in air pollution over the past decade, the United States took a small step backwards. Forty-seven percent of the U.S. population lived in areas with unhealthy levels of air pollution, slightly higher than last year. According to the American Lung Association’s 2014 “State of the Air” report, the Fresno, Calif., metro area was the most polluted city, moving ahead of Bakersfield, Calif.

The ALA’s report measures cities based on low-lying ozone pollution, as well as both short- and long-term particle pollution. These particles, just 1/30th the diameter of a human hair, are capable of getting past our bodily defenses and causing physical harm, particularly to those who already suffer from pulmonary diseases, the very young and the elderly. The report measures both the total accumulated particle pollution over the course of a year, as well as the number of days that the air pollution hit unhealthily high levels. Based on the average levels of long-term pollution measured by the report between 2010 and 2012, these are the most polluted cities in the country.

One region in the country that is not improving is California’s Central Valley, where half of the top 10 most polluted metro areas were located. A number of factors contribute to pollution there. For one, the mountain ridge that forms the valley traps in smog from Los Angeles and San Francisco and blocks rain storms that help filter the air from coming into the region.

Click here to see the 10 most polluted cities

Janice Nolen, assistant vice president of National Policy at the ALA, said that the pollution levels in those cities have improved “but just not enough — they still have a long way to go.”

Warmer temperatures contributed to high pollution levels in some metro areas. In particular, ozone pollution levels increased in 2012 because the United States had one of the hottest summers on record. “Ozone is heavily dependent on sunlight, and when it’s especially hot, it tends to cook in the atmosphere. Higher ozone levels is a trend that we saw across the nation, especially in St. Louis and Phoenix,” Nolen said.

Traffic levels in some areas also contributed to higher air pollution levels. In the El Paso area, in particular, a growing volume of commercial vehicles carrying cargo to and from Mexico added to pollution.

Nolen explained that while corporations are engaging in more environmentally friendly practices, certain industries such as power generation and steel making are still contributing to high levels of pollution. This was the case for St. Louis and Pittsburgh, both among the most polluted cities.

High particle pollution puts residents at risk. More than 46 million Americans live in counties with unhealthy, year-round levels of particle pollution. In the Los Angeles metro area alone, more than 2 million residents live with asthma, while more than 625,000 suffer from chronic obstructive lung disease (COPD). Both make residents more vulnerable to the adverse health effects of air pollution.

While we still do not know the full extent of the physical harm pollution causes, it is well established that pollution is detrimental to one’s health, Nolen said. “We know that there is a link between particle pollution and lung cancer,” she said. “But mainly, healthy adults who have chronic diseases will have a harder time dealing with pollution.”

Based on average long-term particle pollution data collected by the U.S. Environmental Protection Agency and aggregated by the ALA between 2010 and 2012, 24/7 Wall St. reviewed the 10 most polluted metropolitan statistical areas in the country. In some instances, ALA published data by combined metropolitan statistical area. In addition to year-round particle pollution, we also considered ozone pollution and short-term particle pollution by county, which the ALA measured as the weighted number of days between 2010 and 2012 where pollution levels were deemed unhealthy for some residents, or worse. Days when air pollution was most-severe received higher weights. For each of these metropolitan areas, the ALA also noted the population and the number of people in the area with asthma and COPD.

These are the 10 most polluted cities in America.

10. St. Louis-St. Charles-Farmington, Mo.-Ill.
> Year-round particle pollution rank: 10th worst
> Ozone pollution rank: 13th worst
> Adults with asthma: 221,038
> Population: 2.9 million

More than 280,000 residents in the St. Louis metro area suffered from either pediatric or adult asthma, making them especially sensitive to particle pollution. As of 2011, nearly 26 million Americans suffered from the disease, according to the ALA. The St. Louis area’s high level of pollution is partly attributable to coal-fired generation plants. Local politicians have called for stricter environmental regulations of Ameren Corporation’s Meramec plant to reduce sulfur dioxide emissions. Recently, environmental group Sierra Club sued Ameren, claiming it violated pollution levels and demanding it lower pollution at its plants. According to ALA’s Nolen, the area also had higher ozone levels due to the summer’s temperature, which was especially high last year.

9. Phoenix-Mesa-Scottsdale, Ariz.
> Year-round particle pollution rank: 9th worst
> Ozone pollution rank: 11th worst
> Adults with asthma: 278,199
> Population: 4.3 million

The average number of high particle pollution days in the Phoenix area — days in which the air is actually unhealthy for at least some residents to breathe — increased since last year. The area also ranks among the worst metro areas in the United States for ozone pollution. The cause of much of the pollution can be traced to a steadily growing population, which increases car pollution. And because of the relatively windless dry desert air, the ground-level ozone pollution becomes smog that hovers over the area rather than dissipating. Much of the area’s population is especially vulnerable to air pollution. According to ALA data, more than 370,000 Phoenix area residents have adult or pediatric asthma. Additionally, more than 187,000 suffered from COPD, a lung disease that affects 12.7 million adults, according to the ALA.

ALSO READ: America’s Most (and Least) Healthy Cities

8. El Paso-Las Cruces, Texas-N.M.
> Year-round particle pollution rank: 8th worst
> Ozone pollution rank: n/a
> Adults with asthma: 54,409
> Population: 1.0 million

The quality of air in the El Paso metro area has suffered from a growing population in recent decades, as well as a heavy volume of commercial and passenger vehicles passing through the area. El Paso, which sits on the Texas-New Mexico border and is located roughly 25 miles from one of the largest cities in Mexico, Ciudad Juarez, had one of the fastest growing populations in the nation between 2000 and 2006 — and with the growing population came more cars. According to the U.S. Federal Highway Administration, the border crossing between El Paso and Juarez is a major source of international trade. El Paso was only one of five cities to see its year-round particle pollution levels increase in the past year.

7. El Centro, Calif.
> Year-round particle pollution rank: 7th worst
> Ozone pollution rank: 17th worst
> Adults with asthma: 11,084
> Population: 176,948

The El Centro metro area had an average of 16 high ozone days, and received failing grades for both ozone pollution as well as year-round particle pollution levels. More than 15,500 of the metro area’s 177,000 residents suffered from asthma, while roughly 5,700 suffered from COPD, making them especially susceptible to air pollution. The area borders Mexico — which has less stringent environmental regulations — and the far-larger city of Mexicali. The region’s dry desert air also allows smog to linger over the city.

6. Pittsburgh-New Castle-Weirton, Penn.-Ohio-W.Va.
> Year-round particle pollution rank: 6th worst
> Ozone pollution rank: 21st worst
> Adults with asthma: 214,860
> Population: 2.7 million

Once again, the Pittsburgh metro area was among the top 10 most polluted cities in America. However, the area recorded its lowest levels of both year-round and short-term particle pollution since the ALA began its annual study. Still for many residents, pollution may still be a problem. More than 268,000 residents, out of a population of 2.7 million suffered from asthma, while close to 150,000 suffered from COPD. Facilities operated by U.S. Steel, as well as the coal-powered Homer City power plant, have both been criticized for their environmental track record in the past. More recently, natural gas drilling has come under fire, and the state has toughened regulations to protect air quality.

5. Modesto-Merced, Calif.
> Year-round particle pollution rank: 5th worst
> Ozone pollution rank: 7th worst
> Adults with asthma: 49,029
> Population: 784,031

The Modesto metro area’s pollution problem is not limited to year-round particle pollution. Stanislaus County, which is part of the metro area, had an average of 21.2 high ozone days and 29.2 high particle pollution days, both receiving a failing grade from the ALA. The number of high particle pollution days increased in the Modesto area from the previous period as well. Like many of America’s most polluted cities, Modesto is located in the Central Valley region of California. In addition to causing significant farming problems, the drought has worsened the metro area’s pollution problem by keeping the pollution in the air. Part of the area’s drought may be attributable to the “Ridiculously Resilient Ridge” — a massive high-pressure ridge west of the region currently blocking storms coming in from the Pacific and naturally filtering the air.

4. Los Angeles-Long Beach, Calif.
> Year-round particle pollution rank: 4th worst
> Ozone pollution rank: the worst
> Adults with asthma: 1.2 million
> Population: 18.2 million

Although Los Angeles recorded its lowest levels of particle pollution, the area is still among the most polluted in the country. The metro area, which has more than 18 million residents, had an average of 77.5 days of high ozone levels, the highest in the country, and 25.2 days of high particle pollution days. The area received F grades in both measures from the ALA. Recently, the California Department of Environmental Protection released its first ever list of census areas most burdened by pollution in hopes that it will pressure local politicians to clean up the worst affected regions, including Los Angeles. The high number of residents with respiratory diseases is also likely of concern. More than 1.6 million residents suffered from asthma and more than 625,000 residents suffered from COPD.

3. Bakersfield, Calif.
> Year-round particle pollution rank: 3rd worst
> Ozone pollution rank: 3rd worst
> Adults with asthma: 52,552
> Population: 856,158

Although still one of the worst polluted metro areas in the country, Bakersfield moved up from the most polluted area last year to the third most polluted in the most recent report. Kern County, which makes up the Bakersfield metro area, had an average of 78.5 days of high ozone levels and 33.3 days of high particle pollution, both of which received a failing grade from the ALA. In late 2012, the San Joaquin Valley Air Pollution Control District passed an air pollution control plan with the goal of meeting federal EPA standards by 2019. EPA projections from that year show that only seven counties — all in California — were expected to fail emissions tests in 2020, including three in the Central Valley: Kern, Tulare and Merced.

ALSO READ: America’s Most Content (and Miserable) Cities

2. Visalia-Porterville-Hanford, Calif.
> Year-round particle pollution rank: 9th worst
> Ozone pollution rank: 2nd worst
> Adults with asthma: 36,397
> Population: 603,341

The Visalia metro area improved to its lowest levels of year-round particle pollution since last year, but air pollution in general remained a major problem. The Visalia metro area, which consists of Tulare County, had an average of 88.7 high ozone days per year, worse than all but two other U.S. counties. The county also had an average of seven high particle pollution days per year, enough to earn a failing grade from the ALA. Out of a population of roughly 600,000, more than 50,000 adults and nearly 20,000 children suffered from asthma.

1. Fresno-Madera, Calif.
> Year-round particle pollution rank: the worst
> Ozone pollution rank: 4th worst
> Adults with asthma: 68,342
> Population: 1.1 million

The Fresno metro area overtook Bakersfield as the most polluted metro area in the country in the ALA’s report. The two counties in the metro area, Madera and Fresno, were the nation’s worst and second-worst for year-round particle pollution. The air pollution got so severe this past winter that school and city officials would actually raise red flags throughout the city to warn students and parents when the air outside was dangerous to breathe, according to the Los Angeles Times. Out of its 1.1 million residents, nearly 100,000 suffered from asthma. Exhaust from agricultural operations, the area’s topography and smog drifting in from San Francisco and Sacramento all contributed to the air pollution.

Thank you for reading! Have some feedback for us?
Contact the 24/7 Wall St. editorial team.