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.

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.

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.

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.

“ 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.

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