“State of the Air” uses letter grades to measure unhealthy levels of ground-level ozone air pollution (also known as smog), and year-round and short-term spikes in particle pollution (also known as soot) over a three-year period. The 2025 report looks at the latest quality-assured air quality data, which was collected from 2021 to 2023.

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.