A thick smog settled over New Delhi as winter began in India last year, forcing medical professionals to declare a public health emergency. Residents swarmed local hospitals complaining of respiratory problems. Cricket players were forced to put on anti-pollution masks during a national match between India and Sri Lanka. And United Airlines canceled flights into the city, citing the air-quality concerns.

Air pollution isnt among the causes of death that medical examiners list on death certificates, but the health conditions linked to air pollution exposure, such as lung cancer and emphysema, are often fatal. Air pollution was responsible for 6.1 million deaths and accounted for nearly 12 percent of the global death toll in 2016 , the last year for which data was available, according the University of Washingtons Institute for Health Metrics and Evaluation.