This May, Delhi recorded a monthly average AQI of 156, its cleanest in five years. The Central Pollution Control Board considers this ‘moderate’ air quality, but European standards would call it unhealthy. 

The improvement masks a complicated reality. In a city where the pollution debate is heated in winter, summer brings its own less visible hazards.

Over the last five years, PM10 levels in May have breached the National Ambient Air Quality Standards (NAAQS) limit on all but 13 days. Almost half of Delhi’s air quality monitoring stations recorded ozone levels beyond safe limits. SO2 levels, while within prescribed limits, have been rising. And experts say there are consequences. 

How is the air polluted even in May?

This year, May recorded 27 days on which the AQI was ‘moderate’ or ‘satisfactory’, with the rest falling in the ‘good’ category. But every summer, the National Capital Region is shrouded in an invisible layer of pollutants, with PM10 and ozone as the main culprits.

Stubble burning? Crackers? Biomass burning?

“Vehicular and industrial emissions,” answers Manoj Kumar, an analyst at the Centre for Research on Energy and Clean Air (CREA), speaking to Newslaundry. “You might see clear blue skies today, but the air is still very polluted,” he adds.

Road dust alone accounted for 56 percent of PM10 emissions, according to a report submitted by IIT Kanpur to the Delhi government in January 2016. This was followed by concrete batching, industrial point sources (such as thermal power plants), and vehicular emissions. The same industrial point sources accounted for more than 90 percent of total SO2 emissions.

PM10 in summers, PM2.5 in winters

In the summer, high temperatures dry out the soil, generating massive amounts of dust that contribute to PM10 levels. For the past 10 years, summers in Delhi have seen elevated levels of the pollutant. 

In May this year, Delhi’s PM10 levels were the highest on the last day of the month at 331 μg/m3, exceeding the 24-hour safe limit of 100 μg/m3 by over three times. While the World Health Organization (WHO) sets a 24-hour safe limit of 45 μg/m3 for PM10, the average daily PM10 concentration in Delhi in May 2026 is 172 μg/m3, according to CPCB data collated by Envirocatalyst, a research-driven firm based in Delhi.

Dust is only part of the summer story. The other culprit is invisible and odourless, and forms in the air above us.

Ozone: The invisible killer

Ozone is a secondary pollutant, meaning it isn’t emitted directly but forms in the atmosphere through chemical reactions. Nitrogen oxides react with volatile organic compounds – both of them primarily emitted by vehicular exhaust and industrial activity – in the presence of heat and sunlight to form ozone.

Almost half of Delhi’s 45 monitoring stations recorded at least one violation of ozone standards, according to a report by the Centre for Research on Energy and Clean Air (CREA).  

CREA found five major ozone hotspots in the city, with the highest concentration recorded at Pusa. The monitoring station in Pusa recorded an eight-hour average ozone concentration of 292 μg/m3, nearly three times the national standard of 100 μg/m3. The other four spots include NSUT Jaffarpur (229 μg/m3), Commonwealth Sports Complex (208 μg/m3), Delhi University North Campus (207 μg/m3) and Chandni Chowk (178 μg/m3). 

What makes these findings particularly concerning is that the monitoring stations aren’t restricted to areas with heavy traffic and industrial clusters, but also residential and institutional locations, according to this report in the Times of India. 

Further, when plotting ozone levels over the last three months alongside AQI data, another correlation emerges. On days when AQI spikes, ozone levels tend to rise too, reflecting the same underlying conditions of heat and emissions that drive both. Even within limits, ozone remains an important summer pollutant.

“Ozone is a stronger oxidant. It damages the respiratory system and may be more harmful for sensitive groups – when combined with heat stress – than other pollutants. Since it is an odourless, colourless gas, it has no visible imprint on our senses,” says Dr Abhishek Chakraborty, a professor at the Environmental Science and Engineering Department in IIT Bombay, speaking to Newslaundry. 

Delhi’s massive population and high emissions, combined with how the gas reacts to sunlight, make the ozone problem much worse. “A rise in sunny days, coupled with increased emissions, is a deadly combination,” says Manoj Kumar. 

The precursors to ozone – nitrogen oxides and volatile organic compounds – point back to vehicular and industrial emissions, the same sources driving Delhi’s pollution year-round. And the same industries are also responsible for a third pollutant that rarely makes headlines.

SO2: Doing silent damage

Sulphur dioxide, mainly emitted by coal-based power plants, is a precursor to fine particulate matter (PM2.5). In fact, according to CREA, 96 percent of the particulate pollution caused by these power plants in Delhi comprises secondary particles created when SO2 reacts with sunlight and humidity.

While CPCB data suggests that no Indian city exceeded established SO2 standards in 2024, levels of the gas are rising. The average SO2 level in May 2022 was 11 μg/m3. In 2026, the monthly average rose to 18μg/m3.

“The standard itself is too high,” says Dr Chakraborty. “Any city will be under permissible limits.” Emission standards for the gas in India are 80 mg/Nm3 (24-hour average), with European standards capping SO2 emissions at 20 mg/Nm3.

“Another factor is that sulphur dioxide is a highly reactive gas. On emission, it is immediately converted into sulphates, which are not included while measuring SO2 levels,” he adds. Since emissions are not measured at the source, most of the gas has changed its chemical composition before reaching monitoring stations. What is measured are the leftover crumbs.

A 2026 study published in npj Clean Air by researchers at IIT Delhi states that mitigating SO2 emissions from coal-fired power plants could prevent 1.24 lakh deaths. However, emission control technology isn’t being strictly enforced. SO2 emissions can be effectively reduced by installing Flue Gas Desulphurization units. All power plants were required to install FGD units by 2017. The environment ministry, however, has revised the deadline several times already. The current deadline for power plants in Delhi-NCR is December 2027.

SO2 emissions have steadily decreased globally, but India witnessed a rise from 2.36 thousand Kilotons in 2005 to 5.05 thousand Kilotons in 2021, according to a 2026 study by IIT Delhi.

And these higher emissions are here to stay. Most of Delhi is persistently heat-stressed. That means massive power demand, most of which will be supplied by coal-fired thermal power plants, resulting in higher SO2 emissions.

Add extreme summer heat to the mix, which accelerates the formation of these toxic secondary pollutants, creating a deadly double trap for vulnerable populations. This year’s weather only compounds the worry. India is locked in an El Niño cycle, bringing a harsher summer and a weaker monsoon that will do less to flush out accumulated pollutants before winter arrives. 

These are ideal conditions for both ozone and secondary pollutant formation.

‘Focus on the low-hanging fruit’

“What would you do?” asks Manoj Kumar, “Regulate a few hundred industries or the millions of vehicles in Delhi? We have to get the low-hanging fruit.” He points to strict regulation of industries and says people need to be given alternatives before regulations are slapped on private vehicles. 

The number of registered vehicles in Delhi exceeds 8 million. A 2016 report by the Ministry of Urban Development found that nearly 60 percent of trips in Delhi were under 4 km, and 80 percent were under 6 km, indicating that many trips in Delhi occur within neighbourhoods or surrounding areas. 

Still, as of 2025, according to a working paper published by the International Council of Clean Transportation, 31 percent of Delhi’s land area lies beyond the 500-meter walkable distance to public transit. 

Out of sight, out of mind?

Solving air pollution is not straightforward, experts say. 

“You cannot directly compare the air quality and regulatory environments of developed countries with those of developing countries like India. Air quality issues in India differ in terms of sources, nature, and meteorology from those in western countries with colder climates, and even from those in China. If we blindly ape Western regulations, it may have significant negative socio-economic impacts without much tangible improvement in air quality,” says Dr Chakraborty. 

Sunil Dahiya, lead analyst at Envirocatalysts, tells Newslaundry, “India has a dust-prone geography compared to Europe because of high temperatures, loose soil and other factors. Considering these, even if the PM2.5 safe standards in India cannot immediately move to 10 μg/m3 – equivalent to European annual standards – we should aim to move towards it. We need staggered national ambient air quality standards, meaning different standards for different geographical regions. For example, a target of 35 μg/m3 as the 24-hour standard for Delhi NCR, while limiting the annual PM2.5 standard to 25 μg/m3 for the next five years.” 

Sunil Dahiya adds that “GRAP and NCAP are reactive measures; we have to focus on preventing air quality from degrading in the first place.” The distinction matters more than it might seem. 

GRAP (Graded Response Action Plan) kicks in only after air quality has already deteriorated past a threshold. By then, the damage to the lungs has already begun. So experts call for dynamic and region-specific models rather than blanket policy decisions – ones that account for seasonal variation in pollutant sources, geography, and population vulnerability. 

“These (GRAP and odd-even vehicle restrictions) are noble ideas, but their framing and implementation may not be scientifically sound. If we don’t improve our regulatory bodies, there is no hope for clean air in the near future,” says Dr Chakraborty. 

Improvement would also mean better-placed monitoring stations, updated standards, and enforcement that reaches industrial source.

In 2026, the Delhi government established six new air quality monitoring stations. All of these have been placed in green areas, i.e. areas free of buildings, with widespread vegetation cover.

Asked about this, one air pollution expert, who wishes to remain anonymous, has a question of his own. “If I put up a monitoring station in a remote village in Tamil Nadu, and it shows the highest quality of air, will you believe that the air is clean around you?”

The authors are editorial interns with Newslaundry.

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