A new scientific study has found that Delhi’s suffocating air is the result of two very different kinds of pollution — one you can see and one you cannot. The study found that open fires, such as waste burning, stubble burning, and even household cooking, account for over half of the city’s particulate matter, creating the heavy smog that fills the skyline each winter. However, the invisible toxic gases that make the air chemically dangerous largely come from vehicles and industries.
Researchers from IISER Mohali, IITM Pune, and the India Meteorological Department conducted this year-long project under the Real-time Ambient Source Apportionment of Gases and Aerosols for Mitigation (RASAGAM) initiative. Using advanced pollution fingerprinting technology, they traced pollutants back to their exact sources, identifying the specific types of emissions contributing to Delhi’s air crisis.
The findings show that during the post-monsoon months, when smog levels are at their worst, biomass burning is the main contributor to particulate matter pollution. Stubble burning across northern India contributes nearly one-fourth of Delhi’s PM10 and PM2.5, while waste and household burning add another similar share. Together, open fires are responsible for more than half of Delhi’s overall particulate load. Low wind speeds and cooler temperatures during this season trap pollutants close to the surface, creating the thick, visible haze that chokes the city.
However, the study found a very different picture when it came to toxic gases known as volatile organic compounds or VOCs. These invisible pollutants are primarily released by vehicles and industries. Petrol-driven cars alone account for about one-fifth of VOC mass and nearly a third of the potential to form new fine particles. Overall, transport sources contribute more than 40 percent of the total VOC emissions in Delhi, while stubble burning makes up only a small fraction.
These VOCs include highly toxic and cancer-causing chemicals such as benzene, toluene, and xylene. Their concentrations in Delhi’s air were found to be several times higher than those in major cities in Europe or North America. The study also detected rare and previously unseen compounds, including methanethiol and certain C6 amides — some of which were identified in the atmosphere for the first time anywhere in the world.
What makes these findings especially concerning is that VOCs do not simply disappear. They react with other airborne pollutants, producing even more harmful compounds that remain suspended long after the visible haze fades. Scientists warn that these chemical reactions make the air more reactive and hazardous to breathe, increasing long-term health risks such as cancer and respiratory illnesses.
According to lead researcher Professor Vinayak Sinha from IISER Mohali, India does not currently monitor VOCs anywhere, despite their significant impact on air quality. He explained that better control over open fires and improved waste management could drastically reduce particulate matter, while addressing vehicle and industrial emissions is crucial to tackling the invisible chemical smog.
The researchers also pointed out flaws in current pollution databases that often underestimate emissions from transport and industrial sources while overestimating those from household fuels. This results in misplaced policies that fail to target the true causes of pollution.
Even though newer vehicles under BS-VI standards emit fewer pollutants, non-exhaust emissions from tyres, brakes, and road dust continue to add to the problem. Meanwhile, a large number of commercial CNG and heavy-duty vehicles operating in and around the capital contribute significantly to VOC and particulate levels.
The study concludes that Delhi’s pollution crisis has two faces. The visible haze is mostly caused by open fires, while the invisible toxicity stems from vehicular and industrial emissions. Combating both requires distinct strategies — regional coordination and waste management to stop open burning, and stricter monitoring, regulation, and enforcement to control invisible chemical pollutants.
Only by addressing both sides of the problem can Delhi hope to breathe cleaner, safer air in the years ahead.
