Every breath in India kills, slowly but certainly. In the 2009-19 decade, about 3.8 million deaths were attributed to air pollution above the country’s air quality guidelines of 40 micrograms per cubic metre (μg/m³), according to a study by Sweden-based medical university Karolinska Institutet. ‘Compared to the stricter guidelines recommended by the World Health Organization (WHO)—only 5 micrograms per cubic metre—the figure rises to 16.6 million deaths. That’s almost 25 per cent of all mortality during the study period,’ said the university. The entire population of India lives in areas where PM2.5 (fine particulate matter with a diameter of 2.5 μm or less) levels exceed who guidelines, the study highlighted. The Natural Resources Defense Council, an international advocacy group, offers a context to PM2.5, ‘For the sake of comparison, most bacteria are at least five microns across. The diameter of a red blood cell is six microns. A strand of hair is around 70 microns wide. You could fit several thousand PM2.5 particles on a period.’ Every 10 μg/m³ increase in PM2.5 concentration led to an 8.6 per cent increase in mortality among people of 655 Indian districts between 2009 and 2019, the Karolinska Institutet study warned.
In this most populous country in the world, almost every citizen is exposed to deadly pollution while performing the most basic existential activity: breathing. And it adds to their disease burden. Various studies point out that PM2.5 concentrations continue to increase in many areas. Moreover, PM2.5 particles can travel hundreds of kilometres, bringing the spectre of pollution to larger geographies.
Air pollution is a major concern across Indian cities, with significant variations in air quality levels between large metropolitan areas and smaller cities. A new and alarming trend has emerged—smaller cities, once considered relatively cleaner, are now experiencing dangerously high pollution levels. Byrnihat, a small industrial town on the Assam-Meghalaya border, has emerged as India’s new pollution hotspot, surpassing even Delhi in annual PM2.5 levels. According to the Central Pollution Control Board (CPCB), Byrnihat in 2024 recorded an alarming average PM2.5 concentration of 133.4 μg/ m³, exceeding the national standard by a staggering 3.3 times. This marks a significant shift in India’s air pollution crisis, as smaller towns are now registering pollution levels once thought to be a problem only for metro cities. The air quality in Byrnihat has deteriorated to such an extent that a drastic 70 per cent reduction in PM2.5 levels is required to meet the annual standard. In comparison, Delhi, which has long been in the spotlight for its hazardous air, recorded an annual average PM2.5 concentration of 104.9 μg/m³, reflecting a 4 per cent increase from the preceding 2024’s 101 μg/m³. The most polluted cities in India, as highlighted in an air quality report released by Swiss air quality technology company IQAir along with CPCB data, revealed alarming PM2.5 concentrations. The notion that air pollution is a ‘big city problem’ is rapidly crumbling. The data indicates that smaller towns and industrial hubs are now at the frontline of India’s air pollution crisis.
The consistent rise in pollution levels across cities highlights the deep-rooted air quality crisis that extends beyond the metros. Other highly polluted cities include Gurugram, Haryana (91.7 μg/m³); Sri Ganganagar, Rajasthan (87.2 μg/m³); Faridabad, Haryana (84.7 μg/m³); Greater Noida, Uttar Pradesh (83.6 μg/m³); and Muzaffarnagar, also in Uttar Pradesh (83.4 μg/m³). The data indicates that industrial belts and rapidly urbanising regions are bearing the brunt of escalating emissions. The air quality trends also suggest that smaller cities, previously overlooked in national air pollution discourse, are becoming critical zones requiring urgent intervention. A report from IQAir echoed these findings. While there are slight variations in numbers, the underlying message remains the same: India’s air pollution problem is expanding in scale and intensity, engulfing newer regions in a worsening crisis.
Irrespective of their diverse geographic and climatic contexts, the megacities of India—Delhi, Mumbai, Kolkata, Bengaluru, Hyderabad and Chennai—experienced worsening PM2.5 levels during the winter of 2024-25 (October 1, 2024-January 31, 2025), according to Delhi-based advocacy non-profit Centre for Science and Environment (CSE). This has emerged from a new analysis of real-time PM2.5 data in these cities during the winter period. While Delhi, located in the land-locked Indo-Gangetic Plains (IGP) with adverse meteorology, had recorded the highest level of pollution during winter, Kolkata—also at the tip of IGP—ranked second. The megacities outside IGP —Mumbai, Chennai, Hyderabad and Bengaluru—despite having more advantageous climatic conditions and natural ventilation, also experienced increases in average PM2.5 concentrations. Other than Delhi and Chennai, all other megacities recorded city-wide winter averages that were comparatively lower than the average of the past three winters; but the concentrations across different locations had been high, leading to high exposures. The peaking of pollution during winter in any climatic zone is a sign of the underlying problem of persistent air pollution in these rapidly urbanising and motorising cities. While Delhi’s winter air quality often dominates public discourse, rising pollution levels in other megacities remain largely overlooked. Despite some improvement in seasonal pollution trends, winter pollution continues to remain high or rise locally.
A four-year analysis (2021-24) of air quality in 11 metropolitan cities by Respirer Living Sciences, a Delhi-based climate technology startup, shows alarming levels of particulate pollution across India’s major urban centres. It found that all 11 metropolitan cities monitored consistently breached the National Ambient Air Quality Standards for PM10, with pollution levels remaining stubbornly high despite various policy interventions. The most severe conditions were observed in northern India, where cities like Delhi, Patna, Lucknow and Chandigarh recorded particularly dangerous air quality. Delhi’s Anand Vihar monitoring station measured PM10 concentrations of 313.8 μg/m³ in 2024, while Patna’s Samanpura area saw levels reach 237.7 μg/m³—all exceeding the national safety standard of 60 μg/m³.
Even cities traditionally considered to have better air quality failed to meet standards. While some southern and coastal cities like Bengaluru, Chennai and Hyderabad showed modest improvements at certain monitoring sites, none managed to achieve consistent compliance with safety norms over the four-year period. Ronak Sutaria, founder and chief executive officer (CEO) of Respirer Living Sciences, said, ‘This isn’t about occasional spikes—we’re seeing chronic, year-round pollution that exposes urban populations to dangerous particulate levels on a sustained basis.’ He added, ‘Our data shows no evidence of a meaningful, long-term downward trend in most locations.’
Rural areas are also reporting high levels of air pollution. In 2022, the annual average of PM2.5 was as poor in rural India as urban India, according to an analysis carried out by non-profit Climate Trends based on satellite-based data generated by scientists with the Indian Institute of Technology (IIT), Delhi. According to the analysis, in 2022, the average annual PM2.5 level was 46.4 μg in rural India, barely below the urban level of 46.8 μg. (The national limit is 40 μg.) The urban and rural levels of PM2.5 in India since 2017 showed almost similar pollution concentration, with hardly any differences within the two sets of figures, said the report.
‘A deep insight into state-level aerosol pollution in India: Long-term (2005-2019) characteristics, source apportionment, and future projection (2023)’, a paper by Abhijit Chatterjee of the Bose Institute, Kolkata, said, ‘The air pollution issue in the country in the recent decade would not be resolved unless we take the rural parts into account.’ In India, 47 per cent of the population lived outside the air quality monitoring network and 62 per cent did not have access to daily alerts on the local air quality index. The entire rural India stands outside the purview of the air pollution network.
Incidentally, according to another analysis by CSE, the rural population suffered more than its urban counterpart in terms of lifespan lost due to exposure to PM2.5. CSE’s analysis showed that village inhabitants, on average, lost over five years and two months of lifespan due to air pollution exposure, while city dwellers lost about four years and five months. While the analysis records a lifespan loss of over eight years for rural residents in Uttar Pradesh, in Bihar and Haryana, village-dwellers are found to lose over seven years on average.
The severity of air pollution in India can be gauged from this fact. In 2018-21—a period traversing three phases of the covid-19 pandemic (pre, during and post)—India witnessed the maximum levels of human-induced air pollution, notwithstanding the lockdown and drastic reduction in movements and economic activities. This was according to a study published in journal Nature on May 17, 2023. This period saw a surge in air pollution owing to the development of transportation, industrial power plants, green space dynamics and unplanned urbanisation in the country, notes the study. Researchers Bijay Halder, Iman Ahmadianfar, Salim Heddam, Zainab Haider Mussa and Leonardo Goliath carried out machine-learning-based country-level annual air pollution monitoring using Sentinel‑5P satellite and Google Earth Engine (GEE). Sentinel-5P monitored the atmospheric air pollutants and chemical conditions from 2018 to 2021, while the cloud computing-based GEE platform was used to analyse air pollutants and chemical components in the atmosphere. The years 2020 and 2021 saw drastic changes in Air Quality Index (AQI), whereas 2018 and 2019 saw low AQI throughout the year. Delhi, Kolkata, Mumbai, Pune and Chennai recorded huge fluctuations in terms of air pollution during the study period.
It is not just particulate matter that is polluting India’s air. A study by CSE revealed a disturbing trend: Ground-level ozone pollution is on the rise across India’s major cities. This invisible gas, unlike the more familiar fine particulate matter, poses a serious health threat, particularly to those with respiratory problems. Ground-level ozone is not directly emitted from any source. It is produced from complex interaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) that are emitted from vehicles, power plants, factories, and other combustion sources and undergo cyclic reactions in the presence of sunlight to generate ground-level ozone. VOCs can also be emitted from natural sources, such as plants. Those with respiratory conditions, asthma, chronic obstructive pulmonary disease, and particularly children with premature lungs and older adults, are at serious risk. This can inflame and damage airways, make lungs susceptible to infection, aggravate asthma, emphysema, and chronic bronchitis and increase the frequency of asthma attacks leading to increased hospitalisation.
The CSE report, ‘Air Quality Tracker: An invisible threat’, analysed metropolitan areas of Bengaluru, Chennai, Kolkata, Mumbai and Pune. It looked at data for Delhi-National Capital Region (NCR), Greater Ahmedabad, Greater Hyderabad, Greater Jaipur and Greater Lucknow. All 10 areas studied witnessed exceedances of the national ozone standard, with Delhi being the most affected. Smaller cities like Ahmedabad and Pune are experiencing a particularly rapid increase in ozone pollution, the report found.
CSE researchers tracked trends from the period between April 1 to July 18, covering the years 2020 to 2024. Between April and July 2024, Delhi-NCR recorded 176 days of ground-level ozone exceedances, the highest among the 10 metropolitan areas studied. The analysis was based on granular, real-time data (15-minute averages) from CPCB. Mumbai and Pune both had 138 days, followed by Jaipur with 126 days and Hyderabad with 86 days. Kolkata had 63 exceedance days, Bengaluru 59, Lucknow 49 and Ahmedabad 41. Chennai had the fewest exceedances, with just nine days. Comparing the number of exceedances in July 2024 with July 2023, showed an increase in 7 out of 10 metropolitan areas analysed in this study. Smaller metropolitan areas had shown the most increase with Ahmedabad, registering a 4,000 per cent jump in the number of exceedances. Pune follows with 500 per cent increase and Jaipur 152 per cent increase. Hyderabad registered 115 per cent rise in number of exceedance days. The Mumbai Metropolitan Region (MMR) and Bengaluru registered 18 per cent and 1 per cent increase respectively. Delhi-NCR and Lucknow reported a change of less than 5 per cent, with the Kolkata Metropolitan Area (KMA) and Chennai had seen a drop in number of exceedances by 19 per cent and 40 per cent respectively.
Contrary to expectations, ozone levels were elevated even at night, with Mumbai recording the most instances of night-time exceedances. The duration of ozone exposure is concerning, lasting an average of 12-15 hours across most cities, the researchers find. High-end and green neighbourhoods, with lower levels of other pollutants, are ironically more susceptible to ozone buildup. While summer is the peak season for ozone, the problem persists year-round in many areas, particularly in sunnier southern cities.
Furthermore, ozone accumulates not only in metropolitan areas but also travels long distances, creating a regional pollutant that demands both local and regional action. Currently, insufficient monitoring, limited data, and ineffective trend analysis methods have hampered understanding of this growing public health risk, the report adds. The complex chemistry of ground-level ozone makes it a difficult pollutant to track and mitigate. Due to its highly toxic nature, the national ambient air quality standard for ozone is set only for short-term exposures (one-hour and eight-hour averages), with compliance measured by the number of days that exceed these standards. This necessitates early action, the researchers underlined.
Indoor pollution is another persistent threat. India’s poor indoor air quality can impair cognitive development in children less than two years, when brain growth is at its peak, according to a study published in the journal eLife in April 2023 by the researchers from the University of East Anglia (UEA). The negative impact on children’s brain development could have long-term consequences for life, warned the study. Indoor air quality is primarily linked to cooking fuels.
This study was the first to establish an association between poor air quality and cognitive problems in infants under two. ‘Prior work has shown that poor air quality is linked to cognitive deficits in children, as well as to emotional and behavioural problems, which can have a severe impact on families,’ said John Spencer, lead researcher from UEA’s School of Psychology. The team collected in-home air quality data from rural India, focusing on PM2.5 levels. The researchers worked with families from various socio-economic backgrounds in Shivgarh, a village in Uttar Pradesh. They observed poor air quality in households that used solid cooking materials such as cow dung cake. Infants from these houses had lower visual memory scores at six and nine months of age. They also had slower visual processing speeds from six to 21 months. Very small particulate fragments in the air are a major concern as they can move from the respiratory tract into the brain, Spencer adds.
According to the ‘State of Global Air, 2019’ report, India has reduced its proportion of households cooking with solid fuels from 76 per cent in 2005 to 60 per cent in 2017 due to improved access to liquefied petroleum gas. Yet solid fuel use remains high among the lower income groups. This still accounts for about two-thirds of the PM2.5-related neonatal disease burden.
India also has a considerable gender-based air pollution exposure disparity. Due to unequal access to basic social goods, mortality is worsened when women have a lower socio-economic status. Moreover, women from the lower income class use traditional indoor stoves for cooking and heating with very poor ventilation, especially in urban areas.
These are fuelled by biomass and produce carbon monoxide, hydrocarbons and particulate matter and account for 24 per cent of ambient air pollution from PM2.5. These women, disproportionately exposed to indoor air pollution and due to their pre-existing poor nourishment, face greater threat to their respiratory, cardiovascular and reproductive health.
Climate change caused by global warming is a new disrupter in our lives. Scientists are unequivocal now that the atmospheric changes will have profound impacts on air pollution. For a Delhite, or for that matter any resident of a polluted city, it has become an unconscious habit during the winter months to keep looking at the trees for signs of leaves swaying in the wind. This is the way of knowing if there is wind in the city to blow away and lighten the heavy blanket of smog that wraps Delhi and the entire IGP during winter. Stillness of air is bad news. Every winter, the deadly smog in Delhi and IGP takes the centre-stage of public attention when changes in atmospheric conditions trap massive swathes of pollutants close to our nose level. The complex interplay of wind and temperature that contributes to this serious health crisis is often not well understood.
The killer smog experienced every winter is the result of what is commonly known as ‘winter inversion’. Cooler earth surfaces during winter reduce temperatures near the ground and prevent the air from rising up to disperse. The upper layer of warmer air caps and traps the cold air beneath. This leads to a massive trapping of pollution under the inversion layer. In such a situation, the wind is the only saviour. Pollution can disperse only if the wind blows in the city and the region. Wind speed and direction, combined with the changes in air temperature; influence the pollution concentration across the land surface. The temperature changes in the air cause air pressure differences. The warm air rises and moves and leaves behind low pressure areas. The gases move from high to low pressure areas and disperse pollution. While gases are less dense in low pressure areas, they are highly concentrated in high pressure ones.
The insidious link between wind patterns and pollution concentration came out starkly from a study carried out by CSE during the winter of 2023-24 in Delhi. The crop fire incidents in Punjab and Haryana during that winter had not changed much compared to the previous winter, and there was no other unusual increase in episodic pollution activities in the region. At the same time, there were considerable rains during that season. But the concentration of PM2.5 still increased in Delhi, impacting the overall annual levels. The contributory factor was change in wind speed. During November, the average surface wind speed in Delhi was about 9.8 metres per second, which was the slowest average speed recorded compared to the previous six years. The wind speed was about 21 per cent slower during that period. While the vertical movement of air and pollutants was already restricted due to the inversion, the horizontal movement of pollutants also got restricted due to slower wind speed. This contributed towards an increase in annual average level of PM2.5, undoing the longer term downward trend in Delhi pollution. This was a deadly combination of very high pollution levels in the region and slowing down of wind speed.
Such an overwhelming influence of atmospheric conditions also came out very sharply in the Mumbai region during the 2023 winter, when the pollution episode in the city hit the headlines. Mumbai otherwise has a natural advantage of a coastal geography, where comparatively higher surface wind speeds enable faster dispersion of pollutants. Also, wind reversal helps blow away the pollutants. Gufran Beig, a noted atmospheric scientist formerly with the Indian Institute of Tropical Meteorology, has observed on several occasions that a large-scale disturbance in weather systems has been noted in the region since 2022. This, combined with growing local pollution, has impacted the urban air quality of the city and the region. During October-November 2023, the surface wind speed had slowed down in most parts of western India that led to the accumulation of pollutants. Also, delayed retreat of the monsoon in 2023 and anticyclone circulation impacted the trend.
Scientists have now cautioned that the growing global warming that is expected to impact the weather systems worldwide will also have a significant impact on pollution concentration across different local geographies. Local evidence of such an impact has begun to emerge even in India. Scientific studies carried out in IGP show that increase in carbon dioxide (CO2) emissions and the attendant warming tend to reduce the surface wind speed in IGP. This, in turn, is expected to result in higher winter-time fine particulate. Thus, meteorological changes associated due to global warming can aggravate the pollution challenge in IGP, requiring more accelerated reduction in pollution. Another study published in the journal Advancing Earth Space Sciences in September 2022 showed a similar correlation between PM2.5 concentration and the trend in heat-trapping CO2 emissions in IGP. The study estimates the reduction of surface wind speed with increasing CO2. This was expected to result in higher average winter-time PM2.5 concentrations (1 per cent per Kelvin of global warming) and more frequent high-pollution events. A reduction in the frequency and intensity of western disturbances with increasing CO2 may contribute to the reduction in the surface wind in IGP. This is a double whammy.
Of late, short-lived climate pollutants (SLCPs) have been under debate from a global warming perspective. Black carbon is the most prominent of them. The Intergovernmental Panel on Climate Change (IPCC) has long recognised methane, nitrous oxide and hydrofluorocarbons as greenhouse gases, but in the mid-2000s it also included black carbon, which is a product of incomplete combustion of carbonaceous fuels due to low temperature. Black carbon is the solid carbonaceous fraction of PM10 or PM2.5, which strongly absorbs light and converts that energy to heat. Black carbon is short-lived, lasting up to minutes, hours, and a week or a little more in the atmosphere. In contrast, CO2 emitted today can impact future climate for a range of 30 to more than 100 years. Black carbon is emitted from combustion processes, dust-generating activities and secondary particulates like nitrates and sulphates. Some of the early evidence on the characterisation of diesel and petrol emissions shows a much higher share of the black carbon fraction in finer particulate emitted from diesel vehicles than in petrol vehicles, and more in pre-Euro VI vintage vehicles (manufactured before the implementation of the Euro VI emission standards). The composition of black carbon varies with the type of fuel used, combustion process, and emission control technologies or practices.
IPCC’s Sixth Assessment Report (AR6) provides details on a range of impacts of black carbon, including warming, snow melt and effect on precipitation, among others. As per literature, black carbon absorbs light, converts it into heat and warms up the surrounding atmosphere. Scientists calculate the potential to cause global warming in terms of ‘radiative forcing’, which is the difference of sunlight absorbed and energy radiated back in watts per square metre of surface. The IPCC AR5 report, while quantifying the warming potential of each pollutant, states that black carbon can be 900 times more warming than CO2 in a 20-year time horizon. IPCC’s estimates of radiative forcing are said to be conservative compared to others in the published literature. Nonetheless, SLCPs such as black carbon absorb substantially more heat than CO2 to spike the global warming curve in the near term. As emissions and concentration of black carbon are not uniform across all regions, their effect is also more regional compared to the global impact of the more ubiquitous CO2. As long as they are in the atmosphere, their effects on the climate can be strong. Scientists point out that there is uncertainty in the emission metrics such as Global Warming Potential (GWP) and Global Temperature Change Potential (GTP) of black carbon and its potential to cause climate change. This may lead to variability in quantification of effects of black carbon on climate systems in different regions of the world. But overall, its impacts are undeniable.
Black carbon can also accelerate ice melt when it settles on snow. Bright snow surfaces reflect a high amount of solar energy back into space. But black carbon absorbs a substantial fraction of this energy and re-emits it as heat. The Arctic and the Himalayas are therefore hugely vulnerable. Black carbon on glacial snow can alter the melt cycle of glaciers and affect the water balance and water supply through seasons. These regional impacts affected by the local trend of pollution and movement of pollution are a matter of concern. Black carbon is also known to interfere with cloud formation and rainfall patterns, and may change precipitation and surface visibility. Scientists explain that emissions can suppress convection and stabilise the atmosphere in ways that may impede normal precipitation patterns. Scientists describe this as dimming of the earth’s surface, which reduces patterns of evaporation that make clouds. If black carbon heats up the layer of the atmosphere where clouds are forming, for example, the clouds will evaporate. Not being able to reflect sunlight back into space, soot-laced clouds end up warming the atmosphere. But black carbon that hangs above low-lying clouds stabilises the layer of air on top of the clouds, promoting their growth. These clouds, like shields, block incoming sunlight. As a result, black carbon also ends up cooling the planet.
There are now several studies and evidence that provide insight into the varied impacts of black carbon. Regional level impacts on cloud formation, rainfall pattern and weather, snow melt and water systems can be high and varied. As IPCC AR6 points out, in the northern hemisphere black carbon is likely to lead to early springtime snow melt but the magnitude is uncertain. In South Asia, absorbing particles may be influencing precipitation patterns. In the Tibetan Plateau, it may cause changes in circulation and darkening of snow and contribute towards glacier melting, though the magnitude is not clear. All these effects will require locally appropriate action. The US-based Scripps Institution of Oceanography has further pointed towards trans-boundary movement of black carbon on the basis of aircraft-based studies and modelling. It finds that as altitude increases, the fraction of total black carbon that originates in Asia also increases. When pollution reaches the boundary layer, it becomes stable and travels long distances. At ground level, black carbon is more from local sources.
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