Why Does Delhi's Air Become So Bad Every Winter?

The Ministry of Environment's page on Delhi-NCR notes that several sources contribute to PM2.5 and that their shares change by season. A February 2026 CAQM review of studies from 2015 to 2025 estimated winter PM2.5 at about 27% secondary particles, 23% transport, 20% biomass burning, 15% dust and 9% industry.

Vehicles. Within the city, vehicles are the biggest locally generated source in winter. Their relative share rises further as dust declines and combustion sources dominate.

Burning. The biomass share shouldn't be read as stubble burning alone. It also includes municipal waste burning. Farm fires also swing year to year. During October 15 to November 30, they averaged 4.9% of Delhi's PM2.5 in 2025, against 15.5% in 2024.

Construction, households and industry. Residential emissions, construction and waste burning have shown little improvement. Industry and power plants add their own share.

Secondary particles. These aren't emitted directly. They form in the air when gases such as nitrogen oxides and sulphur dioxide react, which is why cutting visible smoke alone doesn't clear the air.

Weather. Winter conditions make it harder for pollutants to disperse. Cold, still air near the ground traps emissions over the city.

Crossing borders. One analysis found that about 65% of Delhi's winter PM2.5 in 2025 came from outside the capital and about 35% from within.

No single source explains Delhi's winter smog. Fixing it means cutting emissions across sectors and states at the same time, not waiting for one visible culprit to be dealt with.