New Delhi was the most polluted capital city last year, just like it was in 2018. That finding comes IQ AirVisual, a Swiss-based group that studies air-quality measurements from around the world.
India was home to 21 of the world’s 30 most polluted cities, the group reported.
IQ AirVisual said the study measured levels of a kind of particulate matter (PM), called PM2.5, in major population centers.
Researchers chose to examine PM2.5 because it “is the pollutant widely regarded as most harmful to human health,” the report said.
These small particles, less than 2.5 microns in length, can go deep into the lungs. High PM2.5 levels can cause health problems, including cancer and heart disease, the report notes.
Researchers examined data about PM2.5 concentrations in the air. The information was collected hourly, from ground-based monitoring systems.