A new report on air quality across the U.S. offers a cloudy prognosis on the long-term health of about 156 million residents who live in areas that received an "F" in smog and soot pollution.

Since 2000, the American Lung Association has released an annual State of the Air report analyzing air quality data across the United States. This year’s report, released on Wednesday, found the highest number of people exposed to unhealthy levels of air pollution in a decade.

Despite improvements in air quality in past decades, 156 million Americans still breathe in too much soot or ozone, says the annual State of the Air report from the American Lung Association.