More than half of all Americans live in areas that have dangerously high levels of either ozone or particle pollution. The United States has made great strides controlling air pollution since the Clean Air Act was passed in 1973. However, more people are breathing potentially hazardous air than in 2015, due largely to stricter standards adopted by the EPA at the end of last year.

24/7 Wall St. reviewed the metropolitan areas with the highest levels of ozone pollution — also known as smog — from the American Lung Association’s (ALA) annual “State of the Air 2016” report. Ozone is created through a photochemical reaction when pollutants come into contact with sunlight. The Los Angeles-Long Beach, California area leads the nation with 152.5 days of unhealthful ozone levels in a single year.

Click here to see the 20 most polluted cities in America.

The report considers two types of pollution: ozone pollution and particle pollution, measured in short-term spikes in a given year and in long-term annual averages. Individuals inhaling high levels of these dangerous particles are at considerably greater risk of adverse health outcomes such as difficulty breathing and cardiovascular-related illnesses. People suffering from asthma, chronic obstructive pulmonary disease (COPD), and diabetes, as well as the elderly and the very young are at an even greater risk.

Of the 91.2 million Americans living in these 20 cities, 7.7 million have Asthma, including 1.9 million children. Largely because of high housing costs, low-income neighborhoods are often clustered around truck routes, power plants, industrial sites and other high pollution zones. For this reason, residents of poor communities within these cities also tend to be exposed to even greater levels of pollution. A combined 13.8 million people live below the poverty line in the 20 smoggiest cities.

In an interview with 24/7 Wall St., Paul Billings, senior vice president for advocacy at the ALA, explained that manmade emissions, especially from power plants, motor vehicles, and diesel exhaust are the biggest sources of pollutants in the atmosphere. Everything from paint fumes to wildfires plays a major role in short-term pollution spikes and drive up long-term pollution levels, he said.

Of the 20 most ozone polluted cities, nine are in California, by far the most drought-afflicted state, which also suffered from some of U.S.’s most devastating wildfires. Since 2012, the Rush, Rim, Rough, and Happy Camp Complex wildfires (fires burning over 100,000 acres are given names) burned a combined 858,570 acres in California. Large wildfires burned a similar number of acres in Nevada and New Mexico.

Pollution levels also depend on time of year and geographic aspects such as topography, Billings noted. Due partially to greater wood burning activity, short-term particle spikes often occur during the winter, for example, while high ozone pollution levels are more frequently observed during the summer. Smog can be trapped in valleys or along mountain ranges. Usually, however, air pollution does not stay put as it is often transported to neighboring regions by wind.

“[Air pollution] may not respect the political boundaries of states but it certainly is homegrown, man-made emissions that are translating to these pollution levels,” Billings said.

To identify the 20 most polluted cities, 24/7 Wall St. reviewed the metropolitan statistical areas with the highest levels of ozone pollution, measured in days in a year when the concentration of ozone, or smog, exceeded the EPA standard from the ALA’s 2016 “State of the Air 2016” report. Short-term particle pollution, which is measured in days with excessive particle pollution levels and long-term particle pollution, expressed as the annual average concentration of particulate matter, as well as 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 2012 through 2014.

These are America’s most polluted cities.

20. St. Louis-St. Charles-Farmington, MO-IL
> High ozone days per year: 19.2 days
> Number of days with unhealthy particle pollution: N/A
> People with asthma: 286,090
> Population: 2,910,387

There are 19.2 days on average each year of dangerously high ozone air pollution (smog) in the St. Louis-St. Charles-Farmington metro area, tied with three other urban regions for the 18th most polluted city in the country. During the high ozone level days, residents can inhale harmful pollutants that can increase the risk of aggravated asthma, difficulty breathing, cardiovascular-related health problems and ultimately an untimely death.

19. Grand Rapids-Wyoming-Muskegon, MI
> High ozone days per year: 19.2 days
> Number of days with unhealthy particle pollution: N/A
> People with asthma: 152,906
> Population: 1,421,374

The level of smog in the Grand Rapids area would not be nearly as high as it is if the metro area were not located downwind from Chicago, where ozone pollution is also among the highest in the country. Particulates are blown from Chicago to Grand Rapids, where they combine in a photochemical reaction that creates ozone, Billings explained. For this reason, the ozone pollution in Grand Rapids is actually worse than it is in Chicago, which the ALA ranked 21st worst for ozone pollution, barely missing the cut for this list.

18. Tulsa-Muskogee-Bartlesville, OK
> High ozone days per year: 19.2 days
> Number of days with unhealthy particle pollution: N/A
> People with asthma: 116,872
> Population: 1,140,635

The Tulsa-Muskogee-Bartlesville, Oklahoma area ranks among the worst polluted cities in the country. Like a number of other metro areas, however, there are fewer high ozone pollution days compared with last year’s “State of the Air” report. Of the 8.6 million area residents, 116,872 people, including 83,664 children, have asthma. These individuals are at greater risk to the negative health effects of pollution.

17. San Jose-San Francisco-Oakland, CA
> High ozone days per year: 19.5 days
> Number of days with unhealthy particle pollution: 19 days
> People with asthma: 690,097
> Population: 8,607,423

The San Francisco metro area is more densely populated than any U.S. area except for New York City. Motor vehicle exhaust is one of the major drivers of pollution. The sheer number of cars, higher likelihood of traffic congestion, and greater industrial activity in the San Jose-San Francisco area largely explains its ranking as one of the smoggiest cities. The level of particle pollution in the air around the city reaches potentially hazardous levels during 19 days in a single year, the eighth highest of cities reviewed.

16. El Paso-Las Cruces, TX-NM
> High ozone days per year: 19.5 days
> Number of days with unhealthy particle pollution: 5.5 days
> People with asthma: 75,755
> Population: 1,050,120

The high level of smog in the El Paso-Las Cruces area, which reaches unhealthful levels 19.5 days out of the year on average, is exacerbated by the hot, dry climate. Like other areas in the Mid- and Western United States, El Paso-Las Cruces is also vulnerable to wildfires, which contribute to short-term spikes in PM pollution in particular. The Whitewater-Baldy Complex wildfire, one of the area’s largest in recent memory, burned nearly 300,000 acres in the Gila National Forest north of Las-Cruces in 2012.

15. Houston-The Woodlands, TX
> High ozone days per year: 22.2 days
> Number of days with unhealthy particle pollution: 0.3 days
> People with asthma: 448,924
> Population: 6,686,594

Houston is home to a number of oil and gas, manufacturing and other heavy industrial companies, notably Shell Oil Company, Kinder Morgan, Marathon Oil Corp., and Apache Corp. Billings noted that emissions from gasoline and byproducts of oil and gas extraction operations contribute to air pollution. Of the 6.7 million Greater Houston area residents, more than 1 million live in poverty. Poorer communities often face higher exposure to pollutants such as ozone.

14. New York-Newark, NY-NJ-CT-PA
> High ozone days per year: 24.3 days
> Number of days with unhealthy particle pollution: 4.3 days
> People with asthma: 2,285,782
> Population: 23,632,722

New York City is the nation’s most populated — and most crowded — city. Hundreds of thousands of people drive into New York every day, and the concentrated dose of motor vehicle exhaust is the major driver of smog in the area. Billings noted that nearby areas such as Fairfield County, Connecticut frequently have among the highest ozone levels simply because they are downwind from NYC.

13. San Diego-Carlsbad, CA
> High ozone days per year: 27.5 days
> Number of days with unhealthy particle pollution: 3.5 days
> People with asthma: 259,926
> Population: 3,263,431

The level of smog in the San Diego area worsened from the last period measured by the ALA. As in other parts of the Western United States, especially California, ozone levels increase in the summer. Southern California has endured extreme drought conditions over the past several years, which have likely contributed substantially to ozone pollution.

12. El Centro, CA
> High ozone days per year: 28.2 days
> Number of days with unhealthy particle pollution: 7.2 days
> People with asthma: 14,390
> Population: 179,091

El Centro is one of seven historically polluted cities where the number of days with unhealthy particle pollution levels — the short-term measurement of PM pollution — reached the worst level ever recorded in the city in 2012-2014. Each, particle pollution reaches unhealthy levels during 7.2 days on average, trailing just 12 other cities. El Centro also has among the most dangerously high ozone level days a year.

11. Dallas-Fort Worth, TX-OK
> High ozone days per year: 29.7 days
> Number of days with unhealthy particle pollution: 0.7 days
> People with asthma: 496,180
> Population: 7,352,286

The Dallas-Fort Worth metro area is one of six cities with historically high ozone pollution, but where the current level of ozone pollution is the lowest on record for the city. Still, with nearly 30 days of unhealthy levels of smog in a single year, the Dallas metro area is one of the smoggiest cities in the country.

10. Fort Collins, CO
> High ozone days per year: 32.2 days
> Number of days with unhealthy particle pollution: 0.3 days
> People with asthma: 27,677
> Population: 324,122

Fort Collins residents are not as exposed to harmful particle pollution as most other highly polluted cities. Ozone levels, however, reach dangerously high levels during 32.2 days in a single year on average due to the hot summer months and the region’s wildfire season. In 2013 and 2014, some 2,000 wildfires burned over 200,000 acres of wildland. These large and heavily polluting fires are almost always caused by humans.

9. Las Vegas-Henderson, NV-AZ
> High ozone days per year: 34.5 days
> Number of days with unhealthy particle pollution: 2.7 days
> People with asthma: 183,612
> Population: 2,315,324

According to the ALA, much of the ozone pollution in Nevada, Arizona, and the Southwest in general is largely due to climate change and severe drought conditions. In Las Vegas, like in other urban areas, a great deal of the pollution is likely from vehicle emissions. An estimated 183,612 Las Vegas area residents, including 38,006 children, have asthma. Individuals with lung conditions such as asthma are more vulnerable to the effects of pollution.

8. Denver-Aurora, CO
> High ozone days per year: 35.7 days
> Number of days with unhealthy particle pollution: 2.2 days
> People with asthma: 287,005
> Population: 3,345,261

Like in most U.S. cities, ozone levels in the Denver-Aurora area have improved in recent years. Despite the improvement, there is still the equivalent of more than one month of unhealthy ozone days in a single year in the area. Three of the top ozone polluted cities are in Colorado. Largely due to climate change, the state has documented rising summer temperatures, which in addition to motor vehicle emissions and other pollutants, have contributed to the city’s high level of ozone pollution.

7. Modesto-Merced, CA
> High ozone days per year: 40.0 days
> Number of days with unhealthy particle pollution: 32.5 days
> People with asthma: 64,166
> Population: 798,350

With 40 high ozone days on average each year, the Modesto-Merced area is one of the smoggiest U.S. cities. And unlike most cities, the Modesto area is one of only four historically polluted cities where the number of high ozone days increased from the measurement documented in last year’s report.

6. Sacramento-Roseville, CA
> High ozone days per year: 41.3 days
> Number of days with unhealthy particle pollution: 5.8 days
> People with asthma: 202,417
> Population: 2,513,103

Like most cities on this list, Sacramento-Roseville has had some of the nation’s worst ozone pollution levels for many years. While the 41.3 days a year of unhealthy ozone days in the Sacramento area is once again nearly the worst, it is also one of only six polluted cities that reported this year the lowest level of ozone pollution in the history of the ALA’s measurement of the area. This is at least due in part to California’s efforts in recent years to control air pollution, according to Billings.

5. Phoenix-Mesa-Scottsdale, AZ
> High ozone days per year: 43.2 days
> Number of days with unhealthy particle pollution: 5.3 days
> People with asthma: 447,405
> Population: 4,489,109

As is true for much of the Western United States, the Phoenix area is particularly vulnerable to large wildfires, which are enormous sources of air pollution. According to the National Interagency Fire Center, 1,543 fires in Arizona burned 205,199 acres of wildland in 2014.

4. Fresno-Madera, CA
> High ozone days per year: 103.8 days
> Number of days with unhealthy particle pollution: 44.7 days
> People with asthma: 89,916
> Population: 1,120,522

The Fresno-Madera metro area is one of only four cities suffering from more than 100 days of dangerously high ozone levels in a single year. Also, like only four other historically polluted cities reviewed by the ALA, this is up from the last period reviewed. In addition, particle pollution in the area spikes to unhealthy levels during 44.7 days out of the year, trailing only Bakersfield for the worst level of short-term PM pollution.

3. Visalia-Porterville-Hanford, CA
> High ozone days per year: 107.8 days
> Number of days with unhealthy particle pollution: 40.7 days
> People with asthma: 48,792
> Population: 608,467

While the Visalia area is nearly the most polluted city in the country by several measures of air pollution with 107.8 high ozone days a year on average, it is one of only 12 cities where the level of short-term spikes in particulate matter pollution improved since the last period reviewed by the ALA. Of the 608,467 area residents, 160,479 live in poverty. People living in low income communities are often exposed to greater levels of pollution.

2. Bakersfield, CA
> High ozone days per year: 113.3 days
> Number of days with unhealthy particle pollution: 48.7 days
> People with asthma: 70,085
> Population: 874,589

Bakersfield trails only LA in its level of ozone pollution. In terms of particle pollution, the area is by far the worst, with 48.7 days of dangerously high particle pollution. Also, both the number of days with unhealthful pollution levels and the city’s year-round average pollution level worsened from the last period reviewed by the ALA. Hot summers and extreme drought conditions in Bakersfield have contributed to the high levels of both ozone and particulate air pollution.

1. Los Angeles-Long Beach, CA
> High ozone days per year: 152.5 days
> Number of days with unhealthy particle pollution: 11.2 days
> People with asthma: 1,484,573
> Population: 18,550,288

The Los Angeles area has the worst ozone pollution of any U.S. city, as it did in nearly all of the 16 years the American Lung Association has measured it. The level of smog in LA reaches unhealthy levels during 152.5 days on average each year. During 2012 through 2014, there were 11.2 days of unhealthy particle pollution spikes. While this was the ninth highest level of short-term PM pollution, it was also the fewest number of such days ever recorded in LA.

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