Delhi woke on Saturday to an unwelcome distinction, topping the list of the world's most polluted major cities as a dense blanket of smog settled over the capital on the eve of Diwali. The reading came from the live global ranking compiled by Swiss air quality monitoring group IQAir, which placed the Indian capital ahead of every other large city on the planet at a time when millions of residents were preparing for the festival of lights.
According to IQAir, Delhi recorded a reading of 581 on the United States air quality index scale at 10.35 in the morning. That figure was more than double the reading of the second-ranked city, Lahore in neighbouring Pakistan, which stood at 285. Kolkata ranked third among the world's major cities with a reading of 198, while Mumbai occupied tenth place at 142.
The ranking, which is updated through the day, underlined the scale of the pollution crisis gripping northern India as winter approaches and the festive season reaches its peak.
India's own official measurement told a somewhat different story in numerical terms, though it pointed in the same direction. The Central Pollution Control Board put Delhi's air quality index at 363 at 10 in the morning, placing the city firmly in the "very poor" category.
The two figures cannot be compared directly, because IQAir's reading is based on concentrations of air pollutants measured on the American scale, while India's official index uses a different scale and methodology. What both systems agreed on was that the air over the capital was hazardous, and that conditions were expected to get worse before they improved.
The picture at street level was stark. Of the 59 monitoring stations across Delhi that recorded readings on Saturday, 16 reported air quality in the "severe" category and 29 remained in the "very poor" band. The worst reading of the day was recorded near the Indian Institute of Technology in Delhi, where the index touched 452.
Visibility fell sharply across several parts of the city as the smog thickened, and the grey haze that has become a familiar feature of Delhi's winters returned in full force.
The deterioration followed what local reports described as the worst pre-Diwali air quality recorded in the capital in eight years on Friday. The Air Quality Early Warning System for Delhi warned that the situation would not ease quickly.
It forecast that air quality was likely to remain in the "severe" category on November 8 and 9, covering Diwali itself and the day after, when the bursting of firecrackers, combined with vehicular emissions and the peak of the stubble-burning season in neighbouring states, was expected to add substantially to the pollution burden.
The worsening conditions prompted the authorities to act. The Commission for Air Quality Management invoked strict countermeasures under Stage III of the Graded Response Action Plan across Delhi and the National Capital Region with immediate effect on Friday. The measures included curbs on physical schooling and office commutes, with primary classes shifted to hybrid mode and employers urged to allow staff to work from home wherever possible.
The intention was to reduce both exposure, particularly among children, and the volume of traffic on the roads at a moment when the air was already carrying a heavy load of pollutants.
Delhi's Environment Minister, Manjinder Singh Sirsa, warned that the weather itself was about to turn against the city. He said conditions were expected to worsen further from the following day, with winds coming to a near-complete halt, a development that would trap pollutants close to the ground and push the index higher still.
His warning captured the central frustration of Delhi's annual air quality crisis: even as the sources of pollution are well understood, the weather patterns of early winter leave the city with little natural ventilation to clear them away.
The causes of the crisis are familiar and, in combination, formidable. Each year, as temperatures fall and wind speeds drop, the capital's air fills with a mixture of smoke from the burning of crop stubble in Punjab and Haryana, exhaust fumes from millions of vehicles, dust from construction sites and emissions from industry.
Diwali adds a further sharp spike, as firecrackers release a concentrated burst of particulate matter and toxic gases into air that is already struggling to disperse what it holds. This year the festival arrived at the peak of the stubble-burning season, when the practice is at its most intense in the fields of the neighbouring states.
Health experts have long warned that such episodes carry real consequences. Fine particulate matter penetrates deep into the lungs and enters the bloodstream, aggravating asthma and other respiratory conditions, and placing particular strain on children, the elderly and people with existing heart and lung disease. During severe episodes, doctors advise residents to limit outdoor activity, especially strenuous exercise, and to wear masks when they must go out.
Hospitals in the capital typically see a rise in patients complaining of breathlessness, coughing and irritation of the eyes and throat in the days around Diwali.
The crisis also raises questions that go beyond a single festival. Delhi's air quality has been a matter of public concern for years, and the courts, the central government and the governments of the neighbouring states have all been drawn into efforts to control it.
Measures have included restrictions on older vehicles, curbs on construction during the worst periods, efforts to provide farmers with alternatives to burning stubble, and the graded response plan itself, which escalates restrictions as the air quality deteriorates. Yet the return of severe air each winter suggests that the structural drivers of the problem remain unresolved, and that the city continues to depend heavily on favourable winds that do not always arrive.
For residents, the immediate concern is the weekend ahead. With Diwali falling on Sunday and the forecast pointing to severe air on both November 8 and 9, the capital faces two days in which celebrations and hazardous air will coincide. The authorities have appealed for restraint in the use of firecrackers and have asked people to follow health advisories.
Whether the festival passes without a further sharp deterioration will depend on the weather, on the scale of the celebrations and on the contribution from stubble burning in the surrounding region.
The ranking on Saturday served as a reminder that Delhi's pollution is not only a local problem but one that is watched around the world. A city of more than twenty million people spending the eve of its brightest festival under a grey sky is an image that travels far. The challenge for the days ahead is to ensure that the festival of lights does not leave the capital's residents breathing its darkest air of the year.
(Reporting based on Reuters and The Hindu BusinessLine.)