New Delhi: In its horizontal expanse, the history of Delhi engulfs the rise and fall of many empires: from the Indraprastha of the epics to the Rajput kings, the Sultans, the Mughals and the British, to being the nerve centre of the world’s largest democracy.
And like its layers of history, Delhi’s seasons were once famed, particularly its harsh winter. Once upon a time,Dilli ki Sardi had its distinct charm—this was the time to soak the sun outdoors, crack warm moongphali (peanut in shell) and savour the streetsidegajar ka halwa.
Delhi’s winters are now quantified in more morbid terms: air quality index and particulate matter. The sources of toxic smog which hangs like a shroud over the city every winter are diligently enumerated, like the many layers of history. While some pollutant sources are round-the-year phenomena, like emissions from vehicles and industries in the neighbourhood, others are transient, like stubble burning and biomass burning for winter heating.
The 2023 World Air Quality report by IQAir, a global air quality monitoring platform, found Delhi to be the most polluted city on the planet with an annual PM 2.5 reading of 92.7 micrograms/cubic meter. The World Health Organization (WHO) considers a PM 2.5 reading of less than or equal to 5 to be permissible, while Indian air quality standards place it at a much higher 40 micrograms/cubic meter.
On Sunday (27 October) morning, despite favourable wind speeds leading to a slight improvement in air quality, Delhi’s PM 2.5 reading was still at a distressing 205.4, over five times the Indian standard.
PM 2.5 is fine particulate matter, 2.5 microns or less in diameter, about 20 times smaller than the thickness of human hair. It is the most common pollutant tracked globally because of its power to inflict damage. The tiny particles can easily travel and permeate human organs like the lungs, cause inflammation and tissue damage. Exposure to PM2.5 is directly linked to cardiovascular and neurological diseases, and risk of premature death. Prolonged exposure can lead to asthma, stroke and cancer. It also impairs cognitive development in children.
The WHO estimates seven million premature deaths worldwide every year due to ambient air pollution. And as per a study published in Lancet Planetary Health in July, an estimated 12,000 deaths every year in Delhi can be attributed to air pollution. The mortality level is the highest in the country, and Delhi accounts for more than a third of the total air pollution related deaths estimated for 10 Indian cities.
Over the years, and despite multiple efforts, air pollution levels have deteriorated in the national capital region (NCR)—which covers Delhi and parts of neighbouring Haryana, Rajasthan and Uttar Pradesh, home to over 21 million people. In 2020, the union government set up a Commission for Air Quality Management for NCR. Way back in 2001, the city mandated the use of CNG as a fuel for public transport, replacing diesel. Later, it set emission norms for coal-based power plants in the vicinity and introduced a policy to phase out old and polluting vehicles. Instances of farm fires, which cause a temporary spike in winter pollution, have been on a steady decline over the years. So, why hasn’t air quality improved? Are we barking up the wrong tree? Is a pollution source hiding in plain sight?
Earlier this month, on 14 October, Delhi’s air quality index slipped into the ‘poor’ category and worsened to ‘very poor’ within a week. By month end, air quality is likely to move to the ‘severe’ category when the bursting of crackers around Diwali coincides with peak farm stubble burning in adjoining states.
The toxic air season of Delhi spans from October to February, or 5 out of 12 months in a year. Last year, a resident spent 56% of their annual hours at PM 2.5 levels exceeding 50, five times the WHO safe limit, shows data from IQAir. Less than 2% of the annual hours were spent with a more breathable PM 2.5 reading of less than 15.
A host of factors lead to poor air around the year—vehicles running on petrol and diesel, emissions from nearby industries and power plants, construction dust and biomass burning for cooking. During winter, biomass burning for heating and the burning of paddy stubble adds to the pollution load.
Meteorological conditions prevailing during winter lead to a sharp plunge in air quality. For instance, last year, Delhi’s PM 2.5 load in November was seven times that of August when monsoon rains cleaned the air by washing away pollutants. In winter, the air near the soil surface is dense and cold. The warm air above blocks the cooler air, trapping it by forming an atmospheric lid. This is known as winter inversion, which coupled with low wind speed prevents the dispersion of pollutants in the wider atmosphere, turning Delhi into a gas chamber.
A 2023 review on sources of pollution by the Delhi government’s environment department shows considerable seasonal variation. During summer, the share of dust sources range between 31-34%, which reduces to 6-15% during winter. The share of combustion sources—vehicles, industry and biomass burning—increases to 85-94% in winters.
In addition to these sources and meteorological conditions, Delhi’s air quality is also impacted by the so-called ‘valley effect.’ Delhi lies in the Indo-Gangetic plains surrounded by hills and mountains such as the Himalayas and the Aravalli, a topography which traps polluted air in the valley.
Data from the SAFAR (System of Air Quality and Weather Forecasting and Research) portal shows that on 23 October, the transport sector contributed 13% to the PM 2.5 load while stubble burning contributed 16% (at its peak, stubble burning contributed to 30% of the pollution load). About 35% of the pollutant load was from neighbouring cities and towns spread across Haryana, Uttar Pradesh and Rajasthan. Delhi’s internal sources contributed a little over 20%; 28% of the pollution was attributed to unidentified ‘other’ sources.
A large chunk of Delhi’s air pollution load is from sources which lie outside the city’s boundary, said Abhishek Kar, senior programme lead at the Council on Energy, Environment and Water (CEEW), a think tank. “Management and control are not easy because pollution sources are dispersed—individual farmers burning stubble, households burning solid fuels for cooking and heating, and individual car users burning fossil fuels. The nodal authority (Commission for Air Quality Management) can order and instruct but it has no legal authority to enforce.”
A 2021 CEEW study showed that between 62-65% of Delhi’s PM 2.5 load comes from outside of the city. During the peak winter phase (15 December to 15 January), household cooking and heating contributed most to PM 2.5 pollution (32%). This is a reason why measures limited to city limits—like taking cars off the road, a ban on diesel generators, a stop to construction activities, vacuum cleaning and water sprinkling of roads—often fail to clean the air.
Since the use of biomass as a heat source in and around Delhi goes up as winter progresses, the residential sector becomes the single-largest contributor by 15 December, the CEEW study stated. It suggested encouraging households to shift to cleaner fuels for cooking and space heating.
Farmers are often disproportionately blamed for polluting Delhi’s air. Due to a narrow window of two to three weeks, between the harvest of rice in October to planting the wheat in November, farmers burn the paddy stubble left on the field by mechanical harvesters. But over the past few years, instances of stubble burning have declined sharply, with more and more farmers incorporating crop stubble into the soil (as a nutrient source) or supplying it to biofuel manufacturers and for use as boiler fuel.
Satellite data tracked by the Indian Agricultural Research Institute showed that between 15 September and 24 October this year, Punjab and Haryana recorded 2,364 instances of stubble burning, a sharp decline compared to 16,422 instances of stubble burning seen in 2020 (till 24 October).
For the entire rice harvest season (15 September to 30 November) total farm fires in Punjab and Haryana were 55% lower in 2023 compared to 2020. Last year, unusually low wind speed trapped the polluted air locally and made the winter exceptionally bad despite lesser smoke from farm stubble fires, said an analysis by the Centre for Science and Environment.
Investments in mechanization (like balers for collecting stubble) and warehouses (to store stubble) can further reduce instances of stubble burning, said Suhas Baxi, co-founder and chief executive officer of BiofuelCircle, a supply chain startup connecting farmers with end users of biomass.“In areas where we are operating for the second or third year, we are seeing increasing interest from farmers and local entrepreneurs to invest and be a participant in this supply chain,” Baxi added.
The National Capital Region is the most polluted part of India due to the high density of polluting sources and reduced ventilation trapping pollutants. Besides, Delhi occupies less than 3% of the NCR’s area, making it susceptible to outside sources of pollution.
Interestingly, there are contributing factors which have escaped the attention it deserves. A PM 2.5 inventory analysis of Delhi-NCR by the International Forum for Environment, Sustainability and Technology (iForest), shows that residential cooking and heating in the region using solid biomass contributes more than half (51.5%) of the region’s annual PM 2.5 load. Of this, just residential cooking using biomass contributes 41%. The contribution of industrial and vehicular emissions comes at a significantly lower share, 25% and 6%, respectively.
A shift from polluting biomass-based cooking to cleaner LPG can significantly improve air quality, argues Chandra Bhushan, president and CEO of iForest. “Currently, we are burning 600 million tonnes of biomass in the open, for cooking and heating, without any pollution control equipment. In comparison, vehicular emissions are largely controlled,” he said.
To solve its air trouble, Delhi must adopt an approach like Beijing did, Bhushan suggests.
The Chinese capital took a series of steps beginning 2013 to clean up its air. It adopted a regional approach spanning Beijing-Tianjin-Hebei, engineering a shift to cleaner cooking and heating fuels, moving polluting industries out of the region, and capping coal consumption and vehicle numbers. As a result, PM 2.5 levels in the region saw a 25% reduction between 2013 and 2017. Last year, Beijing’s annual PM 2.5 reading was only a third that of Delhi’s.
In India, higher subsidies for clean cooking fuel can clean up Delhi’s air and reduce premature deaths due to indoor pollution. A 75% subsidy on LPG cylinders for low-income families in NCR will cost less than ₹7,000 crore annually, iForest estimates. This is a small price to pay when compared to the burden of breathing dirty air.
Delhi and its neighbouring regions can move to cleaner heating fuels in winter by using electric heating equipment. Additional steps include the implementation of 2015 emission norms for power plants and phasing out plants older than 30 years. Developing a green belt around Delhi can reduce dust and prevent desertification.
Due to the committed efforts in Beijing, the average life expectancy of its residents rose by 4.6 years. In comparison, warns a 2024 report from the University of Chicago, air pollution is reducing the life span of a Delhi resident by 7.8 years. In other words, by cleaning up its air, Delhi residents can add over seven years to their lives. That is a good incentive to act and take dirty air more seriously—round the year—and not just in winters.
