Delhi moved ahead of Lahore on Friday to become the world's most polluted major city, as air quality in the Indian capital fell to hazardous levels and a thick blanket of smog cut visibility across the city of around 20 million people.

Figures from Swiss air quality group IQAir put Delhi's air quality index (AQI) at 334, a reading classed as hazardous and the highest among major cities tracked around the world. India's official Central Pollution Control Board (CPCB) recorded an even higher figure of 403, which it rated as severe.

The two readings differ because the Swiss and Indian systems use different scales and methods to calculate and classify air pollution. On the Indian scale, a reading of up to 50 is considered good, with little or no health risk, while readings from 401 to 500 fall in the severe category.

Visibility falls as smog blankets the capital

Videos from news agency ANI showed cars and motorcycles emerging from dense smog on the city's main roads, with street lights partly hidden by the haze. The India Meteorological Department issued an alert for light to moderate fog across parts of Delhi on Friday, and Indian media reported that the Delhi government had deployed water sprinklers in an effort to settle dust.

The deterioration continued a steady worsening through the week. Delhi's 24-hour average AQI stood at 299, in the poor category, on 10 November, before rising to 313, classed as very poor, on 11 November and 350 on 12 November. Friday's reading of 403 marked the first time the capital had entered the severe band since 19 January, when the AQI reached 410.

According to CPCB data, 33 of Delhi's 58 monitoring stations recorded severe air quality on Friday. Several locations reported readings above 450, with IIT Delhi among the worst affected at 487, followed by Chandni Chowk at 476.

Why Delhi's air turns toxic every winter

Pollution is a seasonal problem across much of northern India and neighbouring Pakistan. As winter sets in, cold, heavy air traps vehicle exhaust, construction dust and smoke from agricultural burning close to the ground, allowing pollution to build up instead of dispersing.

The Commission for Air Quality Management has had Stage III of its Graded Response Action Plan in force since 6 November. The stage brings curbs on non-essential construction and traffic restrictions, alongside measures such as hybrid classes for younger school pupils when ordered.

Delhi's doctors had already warned earlier in the week that the toxic air was a serious health risk, and the severe category is defined by the CPCB as air that affects healthy people and seriously affects those with existing diseases. People with asthma, heart conditions or other respiratory illnesses are among the most exposed when readings climb above 400.

Lahore drops to second place

Lahore, which had topped the global pollution ranking on Thursday, slipped to second place as Delhi's air worsened. The two cities have traded the unwanted top spot repeatedly this month as winter smog has settled over the region, with Lahore recording hazardous readings of its own in recent days.

The ranking is a snapshot rather than a daily average, and it can change through the day as wind, temperature and pollution sources shift. But the direction of travel in Delhi has been clear all week, with each day's average worse than the one before.

Attention will now turn to whether the smog lifts over the weekend or deepens further. Forecasters and pollution monitors will be watched closely, and any change in wind or temperature could bring a rapid improvement, or none at all.

For the latest figures, readers can check the CPCB dashboard and IQAir's live city ranking. The Reuters report on Delhi passing Lahore as the world's most polluted city is available here, and Indian coverage of the CPCB's severe reading is available here.