Delhi air turns lethal as winter smog tightens grip

Delhi’s winter smog has returned with brute force, pushing air quality into the ‘Hazardous’ zone across large parts of the capital and triggering emergency restrictions, even as official data shows the city had recorded its cleanest January–November period in eight years.

By mid-December, pollution levels surged so sharply that Delhi briefly became the most polluted major city in the world. On 14 December, air quality index readings crossed 600 at several monitoring stations, with Anand Vihar touching 644. Real-time citywide AQI peaked at 491 on the CPCB scale, firmly in the ‘Hazardous’ category.

PM2.5 at choking levels

The crisis is being driven primarily by fine particulate matter, PM2.5, the most dangerous pollutant for human health. During winter months, PM2.5 concentrations in Delhi routinely reach 250 to 300 µg/m³. In December, readings of around 259 µg/m³ were recorded, more than 17 times higher than the World Health Organization guideline of 15 µg/m³.

Coarser particles, PM10, have also spiked, with example readings of 352 µg/m³, while carbon monoxide levels have touched 396 ppb in some areas. On the AQI scale, such conditions are associated with emergency-level health risks, especially for children, the elderly and those with pre-existing respiratory or cardiac conditions.

Public health experts often describe Delhi’s peak winter air as equivalent to smoking 30 to 50 cigarettes a day. Hospitals report a surge in respiratory distress cases each year during these episodes, prompting school closures, work-from-home advisories and restrictions on outdoor activity.

Seasonal pattern repeats

Data shows Delhi’s pollution consistently worsens between October and February, with November and December emerging as the most severe months. In previous years, November PM2.5 averages have crossed 220 µg/m³, and December has frequently seen prolonged smog episodes with AQI levels breaching 500.

The reasons are well known. Low wind speeds and temperature inversions during winter trap pollutants close to the ground. Vehicular emissions, construction and road dust, industrial activity and the burning of firecrackers and effigies add to the toxic mix. Regional factors also play a role, particularly crop stubble burning in neighbouring Punjab and Haryana.

A major source-apportionment study has shown road dust alone contributes 38 per cent of PM2.5 and 56 per cent of PM10 in the city, followed by vehicles and industrial emissions.

A year of contrasts

What makes the current crisis striking is the contrast with the rest of the year. For the first eleven months, Delhi recorded an average AQI of 187, an improvement over 201 in the previous year and 190 the year before that. Only three days crossed the ‘Severe’ threshold of 400 during this period, down sharply from double-digit counts in recent years.

March delivered its best average air quality in years, with an AQI of 170, while July was cited as the cleanest July in a decade, averaging 78, helped by sustained monsoon rainfall. Yet despite these improvements, the city failed to record a single ‘Good’ air quality day, defined as AQI below 50, through August.

October proved to be the turning point. With an average AQI of 232, it was the worst October in three years, setting the stage for the December collapse.

Emergency measures back in force

As pollution levels remained stubbornly high, authorities invoked GRAP Stage 4 on 13 December, the strictest set of curbs under the Graded Response Action Plan. These include halting construction activity, restricting truck entry and tightening controls on polluting industries.

Earlier, from 1 November, a ban on BS4 and BS5 diesel vehicles was enforced to curb vehicular emissions during winter. In December, the government ordered 50 per cent work-from-home attendance for offices and shifted schools to hybrid or online modes to reduce exposure and traffic volumes.

Crisis without closure

Despite years of emergency measures, Delhi’s winter pollution continues to follow a grim script: months of incremental improvement wiped out by a few weeks of extreme smog. The December spike has once again exposed the limits of short-term restrictions in the absence of deeper structural changes in transport, urban dust management, industrial regulation and regional coordination.

As temperatures drop further and wind speeds remain low, residents face the prospect of weeks of hazardous air, underscoring a familiar reality: for Delhi, cleaner air remains seasonal, fragile and easily reversed.