1996: the air of Delhi was black with smog. The city did not know what had engulfed it. It was breathing in poison. It did not know that dirty air had crept upon it. There was no information about air pollution and its hazards. All assumed it was just dark winter days. Nearly three decades ago, in November 1996, the Centre for Science and Environment (CSE) published Slow Murder—The Deadly Story of Vehicular Pollution in India. The book started with an investigation into the pollution-under-control (PUC) certificate system; it asked if Delhi or any city could clean its air by checking the tailpipe emissions of each car. It asked if the city needed more measures—from emission technology to fuel quality. This was the first such enquiry and it brought results.
Slow Murder launched CSE’s campaign to clean Delhi’s air. Remember, this was the time when air pollution was not on the discussion table. It was not on anyone’s agenda. In fact, the question CSE was asked often was: ‘Why are you so worried about some black air?’ The then lieutenant governor of Delhi had said that it was only dust, nothing to be worried about! The then health minister said air pollution was not a health concern.
Naming the book Slow Murder was a verdict on the capital city’s air emergency. Pollution does not kill instantly, but leads to the suppression of the body’s immune system, destroying lung function or adding to the cancer and cardiovascular disease burden. ‘It was slow, but murder nevertheless.’ The book delved into the nexus of the government and industry that smothered thousands into slow death. Reporting on the book, Down To Earth magazine put three faces on the cover of its November 15, 1996 edition: the Union Minister of Environment and Forests (there was no climate change in the ministry’s title then) Jai Narain Prasad Nishad; the Union Minister of Petroleum (there was no natural gas then) T R Baalu; and Rahul Bajaj, the owner of Bajaj Motors—India’s leading two- and three-wheeler manufacturer of the time). And the cover had the headline: ‘Nailed! for slow murder’. Why? Because CSE’s research showed that proposals for vehicular standards were being shuffled between agencies. This was the time when India had no Bharat Stage (BS) I or II vehicle emission standards. The proposal for cleaner fuels was being bandied about, without any resolution. This was the time when sulphur in fuel was 10,000 parts per million (ppm) and above (currently, with BS VI, sulphur is down to 10 ppm).
Rahul Bajaj was on the cover because of the extremely polluting 2-stroke technology that two-and three-wheelers used. Bajaj had a monopoly on vehicles at the time—this was before the advent of 4-stroke technology, which saw the rise of Hero Honda, and personal car mobility, which saw the rise of Maruti-Suzuki and then other companies. CSE’s fight was to bring policy for fuel-technology standards and to use this to drive out polluting vehicles. This is what is now called the first generation reform. The then Vice President of India, K R Narayanan, released the book at his official residence. It brought attention. CSE followed this with a public meeting at the capital’s FICCI Auditorium in November 1996.
But as it happens, nothing really moved. A year went by. The automobile industry pushed against any reform and the government shuffled papers. In 1997, CSE again went to the public. This time, it had data to show the number of deaths because of pollution. CSE analysis (based on a World Bank model) showed that in just four years—between 1991 and 1995—there had been a shocking 30 per cent increase in the number of premature deaths because of air pollution. Cardiologist Naresh Trehan shared two photographs of lungs. He said when he operated he knew where the patients hailed from through the colour of their lungs. The first photograph was that of a Delhite, Trehan said. The second, showing pink lungs, was of a resident from Himachal Pradesh. Even non-smokers from Delhi had black lungs. All this spurred action. In December 1997, then Union Minister of Environment and Forests (MOEF), Saifuddin Soz, issued a white paper Delhi’s pollution, which became the basis of an action plan. In January 1998, MOEF constituted the Environment Pollution (Prevention & Control) Authority (EPCA) for the National Capital Region (NCR).
In June, 1998, EPCA released its first report on priority actions in which it detailed steps that were needed to clean Delhi’s air. These included control on diesel vehicles and a move towards Compressed Natural Gas (CNG). In July, the Supreme Court’s top bench, then headed by chief justice A S Anand, issued directions based on EPCA’s report with deadlines. It set a deadline for conversion of all three-wheelers and diesel buses to CNG. Delhi was now on schedule for a clean-up. But even as the Supreme Court proposed, the government disposed. Powerful vested interests reigned. They did not allow anything to move. Why? Because diesel had big friends.
This was also the time when science had discovered the real pollution villain—small particulates. Till then, we had only one pollutant in our lexicon, Suspended Particulate Matter or SPM. Now, evidence showed that what was really toxic was the smaller particulates, then called Respirable Suspended Particulate Matter (RSPM), because they are small enough to inhale. The first survey of RSPM—later called PM 10 and then PM 2.5, as the size of the particulate became clearer and its deadly toxicity better understood—was done in 1998. Monitoring by the Central Pollution Control Board (CPCB) showed that the level of RSPM in Delhi was five times above the national 24-hour standard. This was deadly. But where did RSPM come from?
Global research was pointing at diesel— depending on the size and toxicity of the particulate; and explaining not just on how the fuel was burnt, but also the nature of the fuel itself. Studies had found that 90 per cent of the exhaust from a diesel vehicle was below 1 micron in size and that it was coated in highly carcinogenic poly-aromatic hydrocarbons (PAH) and other toxins. In 1998, after a decade of work, the California Air Resources Board formally designated diesel particulates as toxic air contaminants. This was followed by studies from Japan, which discovered the most potent carcinogen in diesel exhaust. The news was unpalatable to the automobile industry. They hit back and things got messy.
In 1998, the car industry was about to bloom—old players like Hindustan Motors, with its Ambassador car, were being edged out by the zippy Maruti Suzukis and Hyundais of the world. And waiting in the wings was Telco, or now Tata Motors, which till then, had a grand presence only in the commercial truck and bus segments. Tata Motors had worked out a scheme to break into the Japanese-Korean-dominated car industry with a product that was not only savvy, but also cheap. How? It would use diesel, which was till then reserved for public transport. It was just about to launch the diesel-powered Sumo. This was unacceptable to pollution control activists. EPCA (with CSE’s founder director, the late Anil Agarwal, as a member) had already petitioned the Supreme Court asking for diesel to be banned for use in buses. CSE wanted the fleet to move to cleaner CNG. CSE could not want cars to start using the dirty and toxic diesel.
Soon, another report was released by CSE in 1999 provocatively titled ‘Engines of the Devil—why dieselisation of the private automobile fleet should be banned’. Tata Motors was not amused. In April 1999, Tata Motors sued CSE for defamation, slapping us with a case of Rs 100 crore for writing an article in business newspaper Business Standard on the toxicity of diesel. CSE took the matter to the public. Tata withdrew the notice, but the fight moved underground. Tata Motors hired a public relations firm, Burson-Marsteller Roger Pereira Communications, to campaign for diesel. Soon pamphlets (this was before the age of the Internet) on myths and facts about diesel, which quoted anonymous experts, appeared on the tables of decision-makers. Many non-profits sprung up to defend diesel, one of which was, ironically, called ‘Citizens Against Pollution’ and had a one-point agenda to defend diesel. Disinformation became the name of the game.
In April 1999, EPCA submitted a report to the apex court on restrictions for plying diesel-driven (private) vehicles in NCR. It called for a ban based on emerging public health issues and the extent of pollution in Delhi. The Supreme Court was listening. The court-appointed amicus curiae, Harish Salve, took up the matter asking for the ‘forthwith suspension of registration of diesel vehicles until further orders’. The learned judges of the country’s apex court concurred, saying that the ‘very right to life of the citizens is at stake’. The government was asked to respond. The fight became dirty; personal attacks on CSE’s director Anil Agarwal, who was then battling a serious form of cancer, became common. The automobile industry, led by Tata Motors, used their full might to defeat this move. In court, Tata Motors lawyers, F S Nariman and Arun Jaitley, P Chidambaram for Fiat and Kapil Sibal for Maruti argued that particulate matter was not dangerous. Tata Motors’ affidavit in court said, ‘I deny that smaller the particulate, the more harmful it is’ and ‘I deny that RSPM is more deadly because they are breathed deep into the lungs and lodged there’. This was the refrain.
The Supreme Court, faced with this barrage of opposition from the industry, decided that instead of banning diesel for private cars, it would push for a drastic improvement in fuel and emission standards. Meanwhile, the government believed it was not its war at all. So in April 1999, the apex court gave its sentence. It said that only those vehicles that were Euro I compliant could be registered in NCR as of June 1999; but it gave only one year for the transition to Euro II. It directed that from April 1, 2000, only Euro II vehicles could be registered. In the interim period—May 1, 1999 to April 1, 2000—it set a quota for 250 diesel cars per month and 1,250 petrol cars per month to be registered in NCR on a first-come-first-served basis. In one stroke, the apex court introduced emission standards for vehicle technology and fuels, and gave the automobile industry six months to make the transition.
Industry lawyers argued for ‘more time’, saying the industry would be hit. The court answered: ‘People cannot breathe and you are asking for more time.’ The industry’s argument that people were registering cars not conforming to emission standards outside NCR, made the judges say, ‘There are no limits to ingenuity.’ They stood firm, saying to automobile lawyers, ‘Whether you speak one or two sentences, we will only pass a sentence.’ The rest is history. The automobile industry met the deadline. The oil industry provided cleaner fuel. The air of Delhi benefitted. But we had lost the critical fight—private diesel cars zoomed ahead. Clean air benefits were squandered away. But then came another turn, a rather pivotal one.
On February 16, 2001, the court of the Chief Justice of India was the centre of attention. The final deadline of April 2001 for converting all buses, autorickshaw and taxis to CNG to reduce toxic diesel particulate pollution was very close. The Delhi government wanted the deadline to be extended by another three years. The court rejected the plea. This was just the beginning of the shadow of things to come. Delhi was turning into a battlefield to block CNG implementation. Anyone impatient with the inaction today must know what it takes to push for action. Every excuse to delay implementation was put forth in the court. The disinformation campaign made claims of unreliability of the ‘untested’ CNG technology and unavailability of gas and cylinders. But the judges were firm on the deadline. They said: ‘We are conscious of the fact that from April 1, 2001, inconvenience is likely to be caused to commuting public, but this ‘urban chaos’ (to use the expression of the administration) which may arise... is creation of the administration and private operators.’ The judges said giving a ‘blanket extension would amount to putting a premium on the lapses and inaction of the administration and private transport operators.’ Only a limited extension was granted till September 30, 2001, but with rigid conditions. The then principal secretary, transport, was made personally responsible for compliance; diesel buses would be allowed to ply only with his authorisation.
As diesel buses were decommissioned, violence erupted on April 3, 2001. Angry commuters resorted to arson and set buses ablaze. Politicians too started the blame game for the rioting. ‘Anti-CNG lobby does a war dance’ was the headline in The Indian Express on the morning of April 7, 2001. Media quoted lawyers saying that the matter was leading to a constitutional crisis. On April 4, the apex court said it was ‘distressed at... defiant attitude on the part of the Delhi administration to comply with our orders. The attitude is wholly objectionable and not acceptable.’ Fearing contempt proceedings, the Delhi government filed an affidavit on April 18, 2001, saying it was committed to implement the court’s directions. The contempt proceedings were dropped on April 27, 2001.
The matter was reduced to a puerile high school debate on CNG v diesel. Saboteurs started a game of zeroes to block CNG. Detractors advocated the use of ultra low sulphur diesel, with less than 0.005 per cent sulphur, as an alternative to CNG. But when asked to make a recommendation to the court, through EPCA, it quietly mentioned low sulphur diesel (0.05 per cent), which is the BS II fuel standard. Deleting a zero was clever; to confuse the court with an alternative that did not exist. Then came the bogey that there was no gas, and that CNG was unworkable. CNG was portrayed as anti-poor. The plight of bus and autorickshaw drivers waiting in long queues made hearts bleed. This was used to whip up anger against CNG. The government claimed the city’s transport could not run on one fuel, because any hitch in the pipeline could bring the city to a halt, and demanded that EPCA recommend 0.05 per cent sulphur diesel as a world standard clean fuel. Detractors spawned myths claiming that CNG is unsafe and emits more ultrafine particulates; that CNG causes cancer, and not diesel. A study conducted for a bus and truck engine manufacturer in the US got baptised as a Harvard study, and found its way to every policymaker to debunk CNG. CSE had to counter this disinformation to clear the smokescreen of lies.
Everyone seemed to have missed the point that the Supreme Court was not promoting CNG or any specific fuel, but only trying to clean up the air. The original order of July 28, 1998, had asked for CNG and other clean fuels. Following directions, the EPCA defined clean fuels in 2001, and specified CNG, LPG, unleaded petrol with low benzene used with catalytic converters, and clean diesel with 10 ppm sulphur used with advanced particulate traps, as alternatives. The Union government then submitted a hurried interim report of the newly-constituted Auto Fuel Policy Committee to the court, asking it not to prescribe fuels and technology, but only set emissions standards. Such an approach would not work because Euro II diesel buses emitted 46 times higher particulate matter than CNG. It is diesel emissions that the World Health Organization (WHO) had branded as a carcinogen for its clear links with lung cancer. The only hope for public health in the face of a sustained campaign to sabotage the move to CNG was the firm stand taken by the Supreme Court, which refused to give in to pressure tactics. The court said: ‘No diesel. No diesel. No diesel.’
After several missed deadlines, Salve reacted in the court in March 2002: ‘This is the fifth time that the Union and state governments were coming to court with the prayer that diesel buses be allowed. If this was a private party, it would amount to abuse of process and the party would be made to pay the costs.’ In April 2002, the court brought down the curtains on the capital’s diesel buses. It imposed a penalty of Rs 1,000 per diesel bus per day, and slapped a penalty on the Union government for repeatedly seeking a modification of the CNG order. This helped change the focus from resistance to implementation. Bus operators threatened to protest, but this time the state government did not back down. Sensing that the government had changed its mood, bus operators were back on the roads, paying fines and buying new CNG buses. On December 1, 2002, the media reported: ‘Finally its bye bye diesel buses.’ The CNG crisis had affected everyone in Delhi. Once implemented, there was palpable public recognition that CNG had improved the capital’s air quality. Before the Delhi assembly elections in 2003, political parties vied with each other to take credit for CNG. Delhi’s democracy had matured.
While the clean fuel and technology agenda remained unfinished, the exploding number of cars and two-wheelers threatened to undo the incremental gains. The game of pollution control cannot be won by simply catching up, but only by leaping ahead. The court directives had helped to stabilise air pollution levels. But the breathing space that Delhi had gained could be lost if vehicle numbers and congestion increased overwhelmingly. In 2004, the apex court heard for a firm action plan for an integrated public transport strategy and vehicle restraint measures. Salve converted the statement of concern into an interim application. The chief justice bench served a notice to the Delhi government for a strategy to control vehicle numbers and congestion. For the first time, the Delhi government submitted a plan for an integrated network of metro and bus rapid transit networks, and light rail, specifying agencies and timelines for implementation. CSE then pitched for a parking strategy and vehicle taxation to restrain car numbers and usage.
October 2015 was yet another turning point. The Supreme Court stepped in once again to combat Delhi’s deadly winter pollution. Through successive winters, the capital was waking up to more wheeze, cough and suffocation. CSE was continuously tracking evidence on pollution trends, health and sources to raise fuss and push for action. This time the science was stronger. A new source inventory and apportionment study by the Indian Institute of Technology (IIT), Kanpur, ranked key polluters in the city, and also found diesel cars responsible for 70-80 per cent of PM2.5 from vehicles. This allowed better source-wise decisions. The Supreme Court cracked the whip on toxic diesel emissions from trucks, cars and taxis; waste burning; construction and road dust; and, called for enhancing public transport, walking and cycling infrastructure. The focus now shifted from the local to the regional air-shed of Delhi. The court’s directives now included not only Delhi, but the entire NCR. This helped nail the episodic problem of farm fires in Punjab and Haryana.
The court rulings also finally established that diesel cars are more polluting. Misuse of low-taxed diesel by cars came under fire. The apex court, in its ruling of December 16, 2015, said: ‘It is noteworthy that diesel vehicles of 2,000 cc [cubic centimetres] and above and SUVs [sports utility vehicles] are generally used by more affluent sections of our society and …more prone to cause higher levels of pollution.’ It imposed a temporary ban on big diesel cars and SUVs for the winter. History repeated itself when the car industry opposed the court’s move to nail diesel cars with the ‘polluter pays’ principle. The judges said this will make people aware that they are buying more polluting vehicles. While the pending proposal from EPCA shows that diesel cars should pay at least 20 per cent of the car value to pay for the extra fuel tax that petrol cars pay over lifetime, the diesel car industry got away by volunteering a mere 1 per cent environment compensation charge to escape the ban.
The car industry once again led the charge in the courtroom to plead that diesel cars are clean. On January 5, 2016, the then chief justice T S Thakur said: ‘You are saying diesel vehicles pollute less. So are your vehicles emitting oxygen? People’s lives are at stake and you are interested in selling cars.’ The heat on diesel catalysed the momentous step taken by the Union government on September 16, 2016, to skip Euro V and directly leapfrog to Euro VI emissions standards by 2020. Only at this level do petrol and diesel emissions nearly equalise. This was a game-changing decision.
The fact that Delhi could succeed once again in slowing the pollution peaks during winter kindled hope. But the momentum lost steam post winter. The post-Diwali smog in 2016—the worst in 17 years—showed that the city was still not prepared. Standing in the Supreme Court on November 8, 2016, CSE heard the observation of the chief justice, ‘The limit has gone beyond human tolerance… It has become dangerous for human life. It is a disaster and you must have a disaster management plan for this. The time has come when you have to have a policy; a concrete plan.’
Today, Delhi has an ambitious plan for e-buses but it is just not taking the shape or progressing at the speed required so that the growth of private vehicles can be stemmed. In 2023, the number of private vehicles registered in the city doubled over the previous year; this in spite of the fact that petrol and diesel prices are up and a substantial share of the household budget is spent on transportation. The explosion of personal cars not only adds to the congestion but also negates all the expenditure on increasing roads, building flyovers and highways, and improving technology and fuel. There are old vehicles that are still polluting; Delhi’s scrappage programme has not been effective. And even if new vehicles are cleaner, add too many of them and the benefits are lost. It is simple maths!
The key source in this battle for clean skies and clear lungs is the fuel that we burn—from household chulhas to factories to thermal power plants. In most cases, it is biomass or coal. The Supreme Court banned the use of pet coke—the dirtiest of such fuels and the Delhi government banned the use of coal, which was later extended to the entire NCR. It was also agreed that thermal power plants would clean up or shut down. Action on this has been patchy to say the least. The lesson from the transition to CNG is that people need alternatives for a ban to be effective. When diesel buses were stopped, CNG supply had to be assured. It also had to be feasible in terms of cost. The Supreme Court agreed that this needed fiscal measures to keep clean fuel cheaper than dirty fuel. Now, even as coal is banned, the price of natural gas makes industry uncompetitive. It will not work.
However, Delhi seems to be at risk of losing its long-term air quality gains. As assessment of the annual trend in PM2.5 during 2024 by CSE showed persistent and consistent rise for the second consecutive year since 2022. This cannot be seen as an annual aberration due to meteorological factors. Consistent rise indicates the impact of growing pollution in the region. The annual PM2.5 concentration had increased to 104.7 μg/m³ in 2024—a 3.4 per cent rise from 2023—and this was more than twice the national ambient air quality standard of 40 μg/m³. The PM2.5 levels had increased after consistent decline and stabilisation between 2018 and 2022. Local and regional sources of pollution including vehicles, industries, open burning of waste, use of solid fuels, construction and dust sources had offset the gains—undermining the longer term progress over the past decade.
Delhi cannot hide behind the smoke screen of farm fires any more. Despite a 71.2 per cent drop in stubble fire incidents during the October–December 2024, the winter pollution had remained elevated, upsetting the annual trend. Real-time data from Delhi’s monitoring stations for the winter period (October 1-December 31, 2024) revealed a troubling 26 per cent surge in peak pollution levels compared to the previous year during the early phase of winter. The city experienced 17 days of severe or worse air quality, alongside two extended smog episodes with average smog intensity of 371 μg/m³ and 324 μg/m³, respectively which kept the average levels elevated.
This 2024 round up assessment of the annual PM2.5 trends came as a resounding warning about the heightened risk from the rising air pollution levels and loss of air quality gains from the past actions. Delhi could stabilise annual PM2.5 levels since 2018 (if the existing air quality monitors are considered) and even recorded a declining trend since 2013 (if the oldest five stations are considered) following the implementation of the directives from the Supreme Court on energy transition in transport and industry sectors.
A steady rise in total number of vehicle registrations in Delhi was observed up until 2016, increasing by about 33 per cent compared to 2011-12. Yearly registration started to drop between 2016 and 2021, by about 35 per cent, due to a combination of slumping automotive sector and the COVID-19 pandemic. Motorisation made a strong recovery in the year 2022-23, when numbers rose by a staggering 47 per cent. Two-wheelers dominated the numbers, after having fallen by about 42 per cent during the pandemic in 2020-21, while private cars faced a slight decrease of 13 per cent between 2019 and 2021. Passenger segments, cabs and autorickshaws faced severe decline, going down 90 per cent and 65 per cent, but bounced back to increase by 84 per cent, and 99 per cent respectively in 2021-22.
During financial year (FY) 2011 and FY 2023, the mix of vehicle segments remained the same—two-wheelers continued to dominate, with a share greater than 60 per cent a year, followed by cars, claiming a substantial 30 per cent share, losing merely 1.9 percentage points after 10 years. Cabs formed less than 1 per cent share. Goods carriers increased by only 1.8 per cent, and formed a minor share of less than 4 per cent. Passenger three-wheelers had a 3.2 per cent share, 0.5 per cent lower than the last decade. Other categories (such as commercial two-wheelers, buses and off-road) collectively comprised less than 1 per cent of the total mix. Two-wheeler share had never reduced below the 60 per cent mark, peaking at 74.8 per cent in 2018-19. However, the two-wheeler market was heavily hit by the pandemic, reducing to 66.5 per cent in 2021-22. Cars, however, took a very different trajectory, seeing a relative increase in their share during the pandemic period, peaking for the first time in 11 years in 2021-22 at 33.46 per cent. Coincidentally the same year the two-wheeler sales slumped, indicating a clear shift towards cars among commuters.
As a soft measure to check rampant increase of on-road vehicles, especially personal vehicles in the state, Delhi’s transport department (GNCTD) deregistered 48, 77,646 old petrol and diesel vehicles in January 2022. This was done in accordance with the National Green Tribunal’s (NGT) direction to ban 10-year-old or older diesel vehicles and 15-year-old or older petrol vehicles to ply on NCT of Delhi’s roads. According to the ‘Economic Survey of Delhi 2022-23’, as a result of the deregistration drive, the number of registered vehicles in the state dropped by 54.76 per cent, from 79,17,898 to 1,22,53,350. This included 1,300,000 cars and 2,900,000 two-wheelers. The transport department noted that although the vehicles have been deregistered, not all of them have stopped operating on the roads. About 700,000 vehicles in 2024 had taken a no-objection certificate for re-registrations in other states, and about 100,000 had been scrapped. In addition, if the Traffic Police in Delhi spots these deregistered vehicles plying on the roads, they will be impounded and scrapped.
Delhi state’s electric vehicle (EV) share stood at 11.78 per cent out of all newly registered vehicles in FY 2023-24, from a non-existent market in 2011-12 with total registrations of just 723. While the state was unable to meet its state EV policy target of 25 per cent electrification by FY 2023-24, Delhi was one of India’s first states to cross the 10 per cent mark in FY 2022-23 and had the highest electric vehicle share. The state implemented its comprehensive EV policy in 2020 featuring an ambitious target, supported by purchase subsidies and tax rebates for both private and commercial vehicles. It included scrappage incentives for replacing old vehicles with EVs and supported retrofitting companies to convert conventional vehicles. Notably, Delhi was the first to include battery-swapping station operators in its policy, offering direct financial aid for setting up charging stations and managing battery sales. Its widespread charging network covered over 97 per cent of developed areas in 3 km x 3 km grids.
What emerged as a worrying trend is the increasing dependence on private vehicles. Since 1994, the modal shares have steered more towards private transport. In 1994, private mode (two-wheeler and car) share was 17 per cent, and bus share was 42 per cent. In 2007, the private vehicle share rose to 23 per cent while public transport (bus and metro) share dropped to 30 per cent. In 2018, 29 per cent trips were made on private modes, and 24 per cent using a bus or a metro. The number of trips in the capital had gone up by 94 per cent since 2008, and more than five times in the last 40 years. Average per capita trip rate (PCTR) had increased from 1.38 in 2007 to 1.55 in 2018, a 12 per cent increase. For motorised trips (excluding walking and cycling), PCTR was 0.905 in 2018, up from 0.87 in 2001 and 0.72 in 2018. Additionally, the average trip length in Delhi has increased from 6 km in 2007 to 10.9 km in 2018, as a result of the growth of regional centres, and increase in accessibility of these centres through roads and public transport. This marks an 81 per cent rise in the average trip length in 10 years.
In conclusion, commuters in Delhi are travelling more and for longer distances, and the rise has been exponential in the last few years. According to the National Institute of Urban Affairs, Delhi, 52 per cent of the trips carried out in Delhi are work-related, 15.4 per cent are recreational trips, and 14 per cent are education trips. Metro has the highest trip length of 16.7 km, defining its role in long-distance commute. Two-wheelers, cars, and buses are used for medium-distance commute between 8 km and 14 km. All trips longer than 5 km are made using motorised modes.
As more people choose personal vehicles, the road space becomes increasingly congested, further straining public transport systems like buses. Buses, already suffering economic losses and inadequate service quality, face the brunt of congestion, making them slower, less reliable, and more unattractive to potential users. This vicious cycle of private vehicle preference exacerbates congestion, which in turn worsens public transport performance, making it even less competitive against personal vehicles. Congestion, thus, becomes more than just a symptom of personal vehicle dependency—it actively tips the scales in favour of private transport. As public transport vehicles are stuck in traffic, journey times lengthen, reliability decreases, and the overall cost of public transport, both in terms of time and money, rises. The result is that public transport becomes less viable for commuters who can afford to switch to personal vehicles, perpetuating a cycle of private vehicle growth. And this results in a pollution spike.
Traffic congestion and prolonged vehicle idling are significant contributors to urban air pollution, emitting various harmful pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), and volatile organic compounds (VOCs). The mechanics of internal combustion engines (ICE) under idle or stop-and-go conditions are less efficient, leading to incomplete fuel combustion, which directly results in higher levels of these emissions compared to free-flowing traffic. Additionally, pollutants from idling are released close to the ground, exacerbating ground-level ozone formation, which can severely impact respiratory health, especially in densely populated urban centres. Emissions from vehicles idling in traffic also contribute significantly to secondary organic aerosols (SOAS), which have been associated with cardiovascular and respiratory conditions. The composition of these SOAS often includes ultrafine particles (UFPs) that are small enough to penetrate lung tissue and enter the bloodstream, compounding the health risks associated with prolonged exposure to urban traffic emissions.
In high-congestion scenarios, the cumulative effect of multitude of vehicles idling or accelerating intermittently generates hot spots of pollution, particularly in enclosed spaces such as the ‘tunnel like effect’ caused by tall construction on both sides of road segments or near intersections. Scientific evidence around emission rates of pollutants during idling suggests many fold increases compared to when the vehicle operates at optimal speeds (which is said to be 40-60 kmph).
Literature review cited in CSE’s report ‘Anatomy of Delhi’s Congestion’ published in February 2025, showed emissions to be three to seven times higher in congested traffic than in free-flowing conditions, with specific pollutants like NO2, CO and CO2 experiencing dramatic increases. The CSE report cited a 2024 study that shows during congestion emissions are three times higher than in free-flow conditions. Another 2023 study showed that due to congestion, PM2.5 increases by 3.5 μg/m³ and ozone (O3) increases by 1.1 parts per billion. Yet another 2016 study cited showed that the percentage increase in pollutants in three sets of runs between delay and non-delay: NO2 (166 per cent), HC (100 per cent), CO (180 per cent) and CO2 (71 per cent).
Economic costs due to congestion are also substantial. Projections estimate congestion would cost Delhi around US $14.7 billion by 2030, including pollution and fuel wastage. Daily fuel losses due to idling alone are estimated at millions of dollars. The CSE report quoted studies that projected traffic congestion cost was (including pollution+fuel) US $14,658 million for the year 2030. Vehicles caught in congestion and idling can spew pollutants several times higher than their normal emissions on roads. Since vehicles are the dominant sources of NOx levels, there is a strong correlation between vehicles and hourly changes in NOx levels. The data for seven days in Delhi (September 10-16, 2024) presented the correlation between NO2 and speed reduction compared to free-flow speed. The data indicated that during peak hours on working days (October 27-30, 2024), when travel speeds dropped, NO2 levels were notably high. The correlation coefficient for speed reduction and NO2 levels for peak hours was -0.534 which indicated a moderate negative relationship between the two with a variance of -0.28. This means that when speed decreases, NO2 levels increase, and for the data recorded during the week, about 28 per cent of the variance in NO2 could be explained by the changes in the speed.
This was first published as part of Slow Murder Continues: India’s suffocating journey of knowing and forgetting the deadly air pollution (2025), by the Centre for Science and Environment. It documents 40 years of reportage on air pollution and the fight against it in India. Download it here