NEW DELHI, Nov. 13 -- Delhi overtook Lahore on Friday to become the world's most polluted major city. Hazardous smog covered India's capital as air quality deteriorated and visibility dropped. The city's population of around 20 million faced increasingly polluted conditions.

Swiss air quality monitor IQAir recorded Delhi's Air Quality Index (AQI) at 334. It classified the reading as hazardous, placing Delhi first among major cities worldwide. The index measures pollution levels through the concentration of harmful substances in the air.

Meanwhile, India's Central Pollution Control Board reported an even higher AQI reading. Its monitoring system recorded 403 and classified the air quality as severe. The two readings differ because both o...