Explained: The Emissions Burden of Delhi’s Vehicles

Nearly four months after the Supreme Court passed an interim order effectively pausing the blanket ban on diesel vehicles older than 10 years and petrol vehicles older than 15 years plying on Delhi roads, it has issued a clarification.Coercive action can and will be taken against vehicles that meet emission standards below Bharat Stage (BS)-IV, the apex court said on 17 December, Wednesday.The earlier order was issued following public backlash against the Commission for Air Quality Management (CAQM)'s order to deny fuel to deregistered diesel and petrol vehicles starting 1 July. At the time, it was peak monsoon in Delhi, and the skies were largely free of smog. With the onset of winter, , however, air quality has steadily deteriorated, with AQI levels fluctuating between the ‘very poor’ and ‘severe’ categories since November.After the Delhi government sought action against older cars, the Supreme Court has effectively once again curbed them.So, why do vehicular emissions, especially from overage vehicles, matter? The Quint explains.Explained: Can Fitness Tests of Cars Replace Age as Delhi’s New Standard?As per the data from the Ministry of Road Transport and Highways (MoRTH), 2020, Delhi is home to approximately 13.3 million registered vehicles, a number which has grown twofold in the last one decade. After a dip during the pandemic years, two-wheelers and cars are growing at the annual pace of 15 percent. In addition, 1.1 million vehicles entered and exited the city daily—most of which are “outside the purview of transport-related regulations to limit Delhi’s air pollution,” Zerin Osho, Director, India Programme at the Institute for Governance and Sustainable Development, told The Quint. In November 2024, an analysis by the Delhi-based environmental think tank Centre for Science and Environment (CSE) revealed that transport accounted for 51.1 percent of all local emissions—significantly more than other sources like construction, road dust, waste burning, and industries.“With transport as the major polluter, for clean air and public health, do we need to address emissions from old vehicles or not? The answer is yes. The simple reason is that the older vehicles that are over 10-15 years old were built in compliance with emission standards which are much weaker than the latest BS-VI emission standards introduced in 2020,” Anumita Roychowdhury, Executive Director, Research and Advocacy at the CSE, told The Quint. For example, a 10-year-old diesel car would likely meet BS-III norms, which were the standard at the time. But compared to a BS-VI compliant vehicle, it emits up to 11 times more pollutants. The CSE study attributed the cause of a toxic pollution build-up to a “mobility crisis” in the capital, wherein the public transport is “losing out as the share of private vehicles is increasing”. “The ban should have been seen as a push to shift from private ownership to public transit,” said Sunil Dahiya, Founder and Lead Analyst, Envirocatalysts.He added,“However, this shift requires more than a ban. It demands a systematic effort to drastically improve last-mile connectivity and public transportation to make them a viable alternative for everyone." Not Just Delhi: Data Reveals New Pollution Hotspots Emerging Across IndiaIn another analysis of six Indian megacities, the CSE found that Delhi remained the most polluted, registering eight days of ‘severe+’ AQI, 12 days of ‘severe’ AQI, 68 days of ‘very poor’ AQI, and no ‘good’ AQI days in the winter of 2024-25. The think tank calculated the AQI by averaging the real-time data available on PM2.5 levels from 40 functional air quality monitoring stations in Delhi. “Delhi's toxic air is a health crisis,” stated Dahiya from Envirocatalysts. Justifying the ban on overage vehicles, the National Green Tribunal (NGT) in 2015 had said it was for the residents of Delhi so that they “do not travel closer to ill-health with each breath they take.” Three years earlier, the International Agency for Research on Cancer, which is part of the World Health Organisation (WHO), had classified diesel exhaust fumes as group one carcinogens. In other words, it had noted that there was sufficient evidence to link diesel emissions as a cause of lung cancer in humans, along with an increased risk of bladder cancer. As per the 2016 estimates of International Council on Clean Transportation, diesel vehicles will put on average over 2.8 lakh additional individuals at risk of cancer in any given year. Unlike petrol engines, diesel engines have a much lower carbon footprint because they compress the air to a much higher degree. “High compression leads to high pressure, translating into better combustion, thus reducing carbon emissions. But that also means high temperatures—and since air contains approximately 70 percent nitrogen, it leads to the formation of different nitrogen oxides (NOx),” explained Karna Bhati, an engineer in Engine Design at a tractor manufacturing company. NOx is a known cause of respiratory and cardiovascular diseases. Delhi Air Pollution: Are Harsher Penalties for Stubble Burning Missing the Mark?NOx was at the centre of the 2018 Volkswagen emissions scandal, in which the German automaker employed deceptive software to manipulate real-world pollution levels. In Netflix’s Dirty Money episode titled “Hard NOx,” the narrator underscores how the fraud contributed to respiratory illnesses and premature deaths in the US. Burning diesel also produces large amounts of fine particulate matter emissions such as PM2.5 and PM10. PM2.5 can make its way into a human body to its lungs and the bloodstream, and are linked to higher risk of strokes, heart attacks, and respiratory harms such as asthma. According to the findings of the CSE study on ‘mobility crisis’, Delhi needs to cut 60 percent of its current air pollution to meet the target of the National Ambient Air Quality Standards for PM2.5. Petrol engines, on the other hand, produce unburnt fuel during combustion, releasing carbon dioxide (CO₂), carbon monoxide (CO), and unburnt hydrocarbons into the atmosphere. “Petrol, being a highly volatile fuel, does not allow air to be compressed as much as it gets compressed in a diesel engine’s case, leading to incomplete combustion of the fuel, and the corresponding emissions through the exhaust system,” Bhati added. The greenhouse gas emissions from petrol vehicles accelerate global warming. “These compounds, along with particulate matter, are harmful pollutants. If not properly managed, their release into the atmosphere could contribute to a catastrophic climate scenario."Karna BhatiVideo | Schools Shut Amid Flood Alert: How Climate Change Is Reshaping ChildhoodBoth petrol and diesel engines come with built-in aftertreatment systems to help clean up that mess. Like, the catalytic converter, which either traps the exhaust gases or transforms them into less toxic substances. “But, of course, these components don’t last forever. Over time, catalytic converters and other aftertreatment systems wear out, depending on how you treat your car,” the senior engineer added. In August, Delhi’s Environment Minister Manjinder Singh Sirsa had reviewed a pilot project by the Delhi Pollution Control Committee (DPCC) to test catalytic converter-based retrofitting devices on BS-III and BS-IV heavy commercial vehicles. Officials had reportedly said the devices will help reduce particulate matter, carbon monoxide, hydrocarbons, and nitrogen oxides by “over 70 percent”. “Delhi still has tens of thousands of pre-BS-VI commercial vehicles such as trucks and buses that will realistically remain in operation for several more years. These older vehicles are among the dirtiest on the road, contributing disproportionately to black carbon and PM2.5 emissions. For this stock of vehicles, retrofitting could deliver quick, tangible reductions in particulate emissions, especially if paired with stricter compliance checks and targeted financial support,” Osho told The Quint. The Quint has reached out to Dr Nandita Moitra, senior scientist at the DPCC, with a detailed questionnaire on the implementation of these devices, as well as the regulatory body’s plan of action to target high emitters. The article will be updated as and when she responds. The Quint has also reached out to the CAQM on its plan of action to target vehicles with high levels of emissions. Their response is awaited."Retrofits can work, but uncertainty over their quality makes them a risky bet without robust real-world emissions testing,” cautioned Dahiya. Waterlogged Wards, Carbon Footprint: Public Healthcare's Quiet Climate CrisisA 2023 study co-authored by Gufran Beig, Founder-Director of SAFAR (System of Air Quality and Weather Forecasting and Research) and Chair Professor, National Institute of Advanced Studies, Bengaluru, analysed the change in emission loads in Delhi over a decade—from 2010 to 2020. The study, titled ‘Decadal growth in emission load of major air pollutants in Delhi’, focused on eight major air pollutants to explain the deteriorating air quality in Delhi and the alarming rise in emissions from the transport sector. “Most of the emission inventory is based on secondary data or data available from government agencies. What we did instead was run a mega campaign where hundreds of volunteers collected data from different junctions within the city.”Gufran Beig“When you generate primary data like that, what you get is what’s really on the road—the flow of traffic, the kind of vehicles, at what time, etc. This kind of gridded emission inventory can help identify the high-emission hotspots—and the moment you identify the hotspots, your mitigation strategy will not be uniform across areas with low, medium, and high emission levels,” he told The Quint. Osho conceded, saying, “This study reframes the city’s pollution challenge as a multi-pollutant problem and equips policymakers with the data needed for targeted, systemic emissions reduction strategies. It demonstrates not just how much pollution exists, but what pollutants are rising, from which sectors, and where exactly in the city they originate.” Video | "Air Pollution Is Raising Lung Cancer Risk in Non-Smokers": ExpertOut of the eight pollutants, the study found a "marginal growth" in PM2.5 (31 percent) and PM10 (3 percent) emissions over the decade, but a significant increase in black carbon (57 percent) and NOx (91 percent). “Several policies between 2010 and 2020 were key in preventing particulate emissions from spiralling—like stricter vehicle standards, fuel transitions in transport and power, and the Graded Response Action Plan (GRAP, 2017) curbs on diesel gensets, truck entry, and construction during smog episodes. These interventions explain the contrasting jumps without which Delhi’s PM growth could easily have mirrored those of black carbon and NOx."Zerin Osho, Director (India Programme), Institute for Governance and Sustainable Development"Despite that, it is not certain that the increase in PM2.5 can be construed as marginal,” added Osho. Dr Ajay Mathur, Professor of Practice in the School of Public Policy at Indian Institute of Technology, Delhi, added the study is in continuation with a lot of others which have brought up the issue of multiple pollutants. “What it does not do, however, is look at construction dust which contributes largely to PM2.5.” Air Pollution Interactive Photos: See How Delhi’s Air Has Changed in a WeekA narrow focus on vehicular emissions risks isn’t the silver bullet to fixing Delhi’s air pollution woes. In the long term, what Delhi needs is a paradigm shift with focus on reducing emission load from every sector, note experts.“This requires a governance model with clear accountability for regulators at city, state, and national levels to ensure measurable, sector-specific reductions. Enhanced accountability is non-negotiable.”Sunil Dahiya, Founder and Lead Analyst, Envirocatalysts“Beyond transport, Delhi has emissions from industry, coal-based power plants, emissions from neighbouring states of Haryana, Punjab, Uttar Pradesh, and Rajasthan, and construction and demolition activities. Dust, waste burning, and even seasonal crop residue burning in and outside Delhi, all worsen the problem,” said Osho. Beig added, “We need to adopt an airshed approach where areas that share the same sources of pollution and same meteorology are at the centre of the mitigation policies. Without that, the air quality won’t show a significant improvement.” Dr Mathur, who is the former Director General of The Energy and Resources Institute (TERI), added, “Howsoever you make the policies within these geographical regions, the point is that coordination is needed.” He added, “And the way implementation must go is that you continuously monitor the data. And that data tracking should allow for flexibility in the policy itself. If you’re learning, say, old vehicles are more polluting, it makes a case to remove them. If old vehicles are clear, then they don’t have to be removed, but there needs to be data.”(This article has been updated from The Quint's archives. It was first published in September 2025.)Air Pollution: Where Did the Supreme Court Order on Firecrackers Fall Short?

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SC Curbs Overage Cars in Delhi Again: Why Vehicular Emissions Can’t Be Ignored

Action can be taken against vehicles that meet emission standards below BS-IV, the apex court said on 17 December.

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