Why are Indian cities polluted in summer? | Explained

Story so far:

In March, the Commission for Air Quality Management in the National Capital Region and Adjacent Areas (CAQM) lifted all curbs under the Graded Response Action Plan (GRAP), signaling the end of Delhi’s winter air pollution.

A month later, as temperatures rose, it reintroduced Stage 1 GRAP to combat summer air pollution. It was briefly withdrawn and reinstated in May while northern India reeled under heat waves.

Why do cities witness pollution episodes in summer?

Delhi and the Indo-Gangetic plain are notorious for winter smog because cold temperatures, low wind speeds and their basin-like topography trap pollutants close to the ground.

Summer brings stronger winds, occasional thunderstorms that wash out pollutants, and warmer temperatures that allow pollutants to mix higher in the atmosphere. Despite such meteorological cheerleaders, Delhi has already witnessed 54 days between April 1 and May 31, 2026, when daily average PM10 levels exceeded the 24-hour National Air Quality Standard (NAAQS) of 100 ug/m3. At the same time, in 40 days, at least one continuous air quality monitoring station (CAAQMS) in the city recorded a violation of the hourly ozone standard of 180 ug/m3.

Unlike most headlines, this is not just a Delhi story. Other major cities such as Mumbai, Chennai, Hyderabad, Bengaluru and Kolkata also saw spikes in pollution over the same period this summer, with PM10 and ozone levels exceeding national norms to varying degrees. These peaks were shaped by local sources such as automobile emissions, road dust, construction activity, industrial emissions and dust from local storms. For example, Mumbai has been experiencing high levels of PM10 and ozone over the past few years due to construction activity, dust and traffic. While PM10 intrusions in Chennai are occasional, the high vehicular density and hot weather also make it an ozone hotspot.

How does air pollution in summer differ from that in winter?

While winter pollution is dominated by finer PM2.5 particles, summer air pollution is dominated by coarser PM10 and ozone.

Vehicles, industry, waste incineration, agricultural residue burning, construction sites and broken roads remain year-round sources of pollution. Winter adds biomass burning for heating. Summer brings dust storms that increase PM10 levels, while heat and sunlight catalyze ozone formation.

Why does ozone rise in hot weather?

Ozone is not emitted directly from the exhaust or chimney. It is formed when nitrogen oxides (NOx), mainly from vehicles and volatile organic compounds (VOCs) from industrial emissions, vehicle exhaust, paint and other sources, react in strong sunlight. Warmer and sunnier days thus create favorable conditions for the formation of ozone, which together with solid particles could cause diseases of the respiratory tract.

What is causing the increase in PM10 in India?

Hot conditions over the Indian subcontinent create a low pressure area that stretches towards Iran. Its interaction with surrounding high pressure areas creates hot and windy conditions. These winds could trigger dust storms, including hot winds called loos that carry dust from western Asia and the Thar Desert across India towards the Bay of Bengal. Such episodes could raise PM10 levels for several days, as was seen during severe North Indian dust storms in 2018.

The Indian subcontinent also experiences shorter, localized dust storms known as ‘andhis’, which usually subside within a few hours. These are formed when strong downdraft air associated with thunderstorms hits the ground, picks up loose dust and carries it away at high speed. While loo-type dust storms are common in northern India, cities like Mumbai and Hyderabad generally face dust episodes due to localized thunderstorms.

Human activity worsens this natural dust load. Construction and demolition work often resumes after GRAP’s stricter winter restrictions are lifted. Without adequate site-level dust control, these activities increase PM10 levels. Vehicles traveling on broken roads also further resuspend the released dry dust into the air during the dry summer months.

What can cities do to combat summer air pollution?

While natural sources of dust cannot be controlled, they can be predicted. Delhi’s Air Quality Early Warning System (AQEWS) was created in response to severe dust storms in 2018 and smog episodes in previous years and now runs year-round. It has since been expanded to other cities such as Jaipur and Mumbai and provides multi-pollutant forecasts several days in advance. Its bulletin provides detailed weather information for Delhi and three-day air quality index forecasts for 140 Indian cities.

The India Meteorological Department (IMD) also issues national weather forecast bulletins several times a day. Authorities should use these systems to issue local warnings about dust storms, ozone and poor air quality so that citizens can reduce exposure.

For controllable non-natural resources, enforcement matters. Construction sites need active dusting even outside of winter.

A study by the Council for Energy, Environment and Water (CEEW) found that simply restricting the movement of heavy vehicles on construction sites can reduce local levels of particulate matter. Brihanmumbai Municipal Corporation’s (BMC) Air Quality Decision Support System (AQDSS), also developed with CEEW, for example, helps in monitoring construction sites. As of October 2025 in Mumbai, it has already helped authorities take action against more than 1,000 locations.

Reducing ozone requires reducing NOx and VOC emissions from vehicles and industries through cleaner transportation, better compliance, and attention to solvents, paints, and fuel combustion. Even simple behavioral measures, such as the Delhi government’s “Red Light On, Gaadi Off” campaign, which encourages drivers to switch off their vehicles while waiting at intersections to reduce idling emissions, can reduce ozone formation. But Indian cities need more sustained action.

Delhi has a summer action plan from 2022. Other cities need similar plans that combine forecasts, public health recommendations, construction dust control, road dust management and measures against ozone-forming emissions. Summer can disperse some pollutants better than winter, but heat and sunlight create their own pollution. Indian cities need to plan for both seasons and treat them with equal seriousness.

(Mohammad Rafiuddin is the Program Leader and Sneha Maria Ignatious is the Program Associate at the Energy, Environment and Water Council)

Published – 02 Jun 2026 08:30 IST