There’s nothing like a breath of fresh air—unless, of course, that breath contains nasty pollutants. For over half of all Americans, getting fresh air into their lungs isn’t as easy as opening a window. That’s because, according to a new report by the American Lung Association, they live in places with unhealthful levels of pollutants like particles and ozone.

In its annual State of the Air report, the American Lung Association analyzes data collected from official air quality monitors. It found continued reduction in ozone and particle pollution—six of the 25 most ozone-polluted cities (Los Angeles, Visalia-Porterville-Hanford, Sacramento, Dallas-Fort Worth, El Centro, and Houston) had their fewest unsafe ozone days since the report began in 2000, and 16 of the cities with the highest levels of particle pollution had their lowest year-round levels in the history of the report. Factors like stronger regulation of common pollutants, cleaner power plants and more alternative-fuel vehicles may be behind the improvement.

But the picture is still dire for many U.S. cities. The ongoing drought in the West seems at least partially to blame—20 of the 25 most-polluted cities in the nation are in the West and Southwest. And some cities, like Bakersfield, CA experienced pollution spikes due to dust, grass, and wildfires.

People with health conditions like asthma are at the greatest risk from airborne pollution, but older and younger people are at particular risk. So are people living in poverty: In 2012, researchers from Yale University found that people who live in less affluent communities breathe air with more potentially dangerous compounds like vanadium and zinc. The American Lung Association report found that over 24.8 million people living at or below the federal poverty line live in places that score an F grade for at least one pollutant, and nearly 3.8 million live on counties that fail tests for ozone, short- and long-term particle pollution.

Four cities were the cleanest, experiencing no days in the unhealthy range of either ozone or particle pollution: Burlington-South Burlington, VT, Elmira-Corning, NY, Honolulu, HI, and Salinas, CA. The top four most polluted cities were all in California: Los Angeles, Bakersfield, Visalia-Porterville-Hanford, and Fresno-Madera. (Check out the complete list to see where your city stands.)

So what can be done? The American Lung Association recommends that Congress enforce and protect the Clean Air Act, which regulates the kinds of pollutants groups can emit. But individuals can play a role, too—reducing air pollution is as easy as turning off the lights when you leave the room or eating locally-grown food. You could also try grabbing a bike instead of reaching for the car keys.