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........

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