Delhi overtakes Lahore as world's most polluted major city

Delhi overtook Lahore on Friday as the world's most polluted major city, with the Swiss air quality group IQAir recording an index reading of 334 in the Indian capital, a level it classes as hazardous, as smog cut visibility across a city of 20 million people.

India's Central Pollution Control Board recorded 403 on its own scale and rated the air severe. Business Recorder reported that the Swiss and Indian readings differ because the two systems use different scales and methods to calculate and classify pollution. An index reading of up to 50 is considered good, with little or no health risk.

IQAir's ranking placed Delhi above Lahore on Friday.

In Lahore, air quality worsened as wind speed fell to 2 km/h, Dunya News reported, describing the rise in the city's air quality index as temporary. Dunya News did not publish a reading for Lahore.

Videos from the news agency ANI showed cars and motorcycles emerging from dense smog that also appeared to obscure street lights on main roads in Delhi.

Pollution is a seasonal problem across northern India and neighbouring Pakistan. The cold, heavy air of winter traps vehicle emissions, construction dust and smoke from agricultural burning, and the resulting smog sits over the plains until wind or rain clears it.

Both reports measured air quality on Friday morning. Delhi's reading of 334 on the IQAir scale and 403 on the Indian board's scale sit far above the 50 mark the Swiss group treats as the upper limit for clean air, and Lahore's deterioration came as the wind that normally disperses pollutants dropped almost to a standstill.