New Delhi was the world’s most polluted capital for the third straight year in 2020, according to IQAir, a Swiss group that measures air quality levels based on the concentration of lung-damaging airborne particles known as PM2.5. 

• India was home to 35 of the world’s 50 most polluted cities, according to IQAir’s 2020 World Air Quality Report, which gathered data for 106 countries.  
• The findings were based on the country’s annual average of particulate matter PM2.5, airborne particles with less than 2.5 microns in diameter.  
• Prolonged exposure to PM2.5 can lead to deadly diseases, including cancer and cardiac problems. 

Source: https://indianexpress.com/article/india/new-delhi-is-worlds-most-polluted-capital-for-third-straight-year-iqair-study-7230892/