Seoul's pollution levels on some days rival those of Shanghai and Beijing, major Chinese cities whose pollution problems are well-documented. In 2016, Seoul's air quality index was considered unhealthy for sensitive populations (such as children, the elderly and those with existing respiratory conditions) on 78 days. A Year in Air Quality, 2016
The number of days in which the average air quality level reached certain health thresholds defined in the Environmental Protection Agency’s Air Quality Index. The readings reflect concentrations of the pollutant PM2.5 that have been scaled to health categories in the index, or AQI.