Disparity in the AirPeople in some of the world’s most populous countries—China, India, Pakistan—breathe the most polluted air. This map of some of the most affected parts of the globe shows readings of PM2.5, or fine-particulate matter, recorded over several months in 2020, with the highest readings selected for each place. The geographic disparity is clear: Of the world’s 50 most polluted cities, nearly all are in Asia, though the exceptional wildfires in the western U.S. briefly led to similar pollution levels in 2020.