Toxic smog obscured India's famed monument to love, the Taj Mahal, as well as Sikhism's holiest shrine, the Golden Temple in Amritsar, and delayed flights on Thursday, becoming too thick to see through in several places.
The city of Lahore in neighbouring Pakistan ranked as the world's most polluted in winter's annual scourge across the region, worsened by dust, emissions, and smoke from fires burnt illegally in India's farming states of Punjab and Haryana.
In the city of Agra, the Taj Mahal was barely visible from the gardens in front of the 17th-century monument, while dense fog wreathed worshippers at the Golden Temple in Punjab, television images showed.
Delhi flights faced delays, with tracking website Flightradar24 showing 88% of departures and 54% of arrivals were delayed.
Officials blamed high pollution, combined with humidity, becalmed winds and a drop in temperature for the smog, which cut visibility to 300 m (980 ft) at the city's international airport, which diverted flights in zero visibility on Wednesday.
More patients flocked to hospitals, particularly children.
"There has been a sudden increase in children with allergies, cough and cold ... and a rise in acute asthma attacks," Sahab Ram, a paediatrician in Punjab's Fazilka region, told news agency ANI.
Delhi's minimum temperature fell to 16.1 degrees Celsius (61°F) on Thursday from 17 degrees C (63 degrees F) the previous day, weather officials said.
Its pollution ranked in the 'severe' category for the second consecutive day, with a score of 430 on an index of air quality maintained by the top pollution panel that rates a score of zero to 50 as 'good'.
Pollution in New Delhi is likely to stay in the 'severe' category on Friday, the earth sciences ministry said, before improving to 'very poor', or an index score of 300 to 400.
The number of farm fires to clear fields in northern India has risen steadily this week to almost 2,300 on Wednesday from 1,200 on Monday, the ministry's website showed.
Lahore, the capital of Pakistan's eastern province of Punjab, was rated the world's most polluted city on Thursday, in live rankings kept by Swiss group IQAir. Authorities there have also battled hazardous air this month.
