In China, people breathe air thick with the lung-damaging pollutant ozone two to six times more often than people in the United States, Europe, Japan, or South Korea, according to a new assessment. By one metric—total number of days with daily maximum average ozone values (8-hour average) greater than 70 ppb—China had twice as many high ozone days as Japan and South Korea, three times more than the United States, and six times more than Europe.