About 134 million American were exposed to unhealthy levels of air pollution between 2014 and 2016, according to figures just released by the American Lung Association. That figure represents a substantial increase over the 125 million people who were exposed to unhealthy levels of pollution between 2013 and 2015.

The ALA 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. People who inhale high levels of these 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.

This recent rise in exposure to air pollution is primarily due to an increase in ozone gases — partially the result of the deadly wildfires that have swept through California. Meanwhile, non-gaseous particle pollution has reduced steadily in the decades since the Clean Air Act was passed.

24/7 Wall St. reviewed the 25 metropolitan areas with the highest levels of ozone pollution — also known as smog — from the American Lung Association’s annual “State of the Air 2018” report. Harmful to breathe, ozone is created when pollutants, such as gases coming out of tailpipes and smokestacks, come into contact with sunlight.

Of the over 100 million Americans living in the 25 most polluted cities, 8.5 million have asthma, including nearly 2 million children. Because housing costs tend to reflect the desirability of an area, 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 tend to be exposed to greater levels of pollution than communities in other parts of these cities. A combined 14 million people live below the poverty line in the 25 smoggiest cities.

Click here to see the 25 most polluted cities in America.
Click here to see our detailed findings and methodology.