Delhi overtook Pakistan's Lahore as the world's most polluted city in Swiss group IQAir's live rankings, with an air quality index (AQI) score of more than 1,000. The group classifies readings above 301 as "hazardous".
The Indian capital battles severe pollution every winter as cold, heavy air traps dust and emissions, as well as smoke from illegal farm fires in the adjoining states of Punjab and Haryana.
IQAir said the concentration of PM2.5 - particulate matter measuring 2.5 microns or less in diameter that can be carried into lungs, causing deadly diseases and cardiac issues - was more than 120 times the World Health Organisation's recommended levels on Wednesday morning.
India's pollution authority, however, said the AQI was around 350, below its threshold of 400 for "severe" pollution.
The variation reflects different scales to convert pollutant concentrations into AQI, said climate expert Gufran Beig, Chair Professor at the National Institute of Advanced Studies and founder of SAFAR, the federal government's air quality monitoring agency.
The India Meteorological Department added that pollution and fog had reduced visibility to 100 metres (328 feet) in some places by around 8 a.m. (0230 GMT).
Low visibility procedures were initiated at the city's Indira Gandhi International Airport, operator Delhi International Airport Limited said in a post on social media platform X, adding that some flights might be affected.
The meteorological agency said the city's average temperature dropped to 17 degrees Celsius (63 degrees Fahrenheit) on Wednesday morning from 17.9 C the day before, and may fall again as sunlight remains cut off by the smog.
The city's environment minister said last week that the government was keen to use artificial rain to cut the smog. Authorities have on occasion closed schools, placed restrictions on private vehicles, and stopped some building work to curb the problem.
Pakistan's Punjab province, which shares a border with India, has also banned outdoor activities, closed schools and ordered some shops, markets and malls to close early.
