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.