Decoded: Why Delhi keeps choking every winter — role of traffic, regional pollution, firecrackers and stubble burning

Delhi-NCR’s winter air pollution is once again drawing attention, with the region remaining under a blanket of smog days after Diwali. While firecrackers added significantly to the pollution load during the festive period, they are only one part of a larger problem involving vehicle emissions, industrial activity, construction dust, regional pollution and weather conditions.

An assessment by the Indian Institute of Tropical Meteorology (IITM), Pune, found that firecrackers contributed 71.1% of the PM2.5 concentration in Delhi’s air between 1 am and 9 am on November 9, immediately after Diwali celebrations. The contribution was 43.4% during the same hours on November 8 and 5.6% on November 7, The Indian Express reported.

The figures highlight the immediate impact of firecrackers during the period analysed. However, Delhi’s recurring winter pollution cannot be attributed to one activity alone. The accumulation of pollutants from multiple sources, combined with weather conditions that restrict their dispersal, makes the problem particularly severe during the colder months.

So, what causes Delhi’s winter smog, how is it different from fog, and why does it persist?

What is smog, and how is it different from fog?

The word smog combines ‘smoke’ and ‘fog’. It refers to polluted air containing a mixture of particulate matter and gases, often resulting in reduced visibility.

These pollutants include fine particulate matter such as PM2.5 and PM10, along with gases released by vehicles, industries and other sources. Nitrogen oxides and volatile organic compounds can also react in sunlight to form ground-level ozone, producing what is known as photochemical smog.

Delhi’s winter smog, however, is particularly associated with the accumulation of fine particulate matter under weather conditions that prevent pollutants from dispersing.

PM10 refers to particles measuring 10 micrometres or less in diameter, while PM2.5 refers to finer particles measuring 2.5 micrometres or less. These particles can include dust, soot and smoke, as well as substances formed through chemical reactions involving gases in the atmosphere.

Fog, in contrast, is made up of tiny water droplets suspended close to the ground. It forms when moist air cools and water vapour condenses into droplets, reducing visibility.

The key distinction is that fog is a natural weather phenomenon, while smog contains air pollutants that can pose health risks. Both can also occur simultaneously.

Why does Delhi’s air pollution worsen during winter?

Delhi’s winter pollution is the result of a combination of emissions, geography and meteorological conditions. As temperatures fall and wind speeds decline, pollutants become less dispersed and can accumulate closer to the ground.

During warmer months, rising air and stronger winds help carry pollutants away and disperse them. In winter, weaker winds and reduced atmospheric mixing can limit this movement, allowing pollution to build up.

Temperature inversion is another important factor. Under normal conditions, warmer air near the surface rises and helps carry pollutants upwards. During a temperature inversion, a layer of warmer air sits above cooler air closer to the ground. This acts like a lid, restricting vertical movement and trapping pollutants near the surface.

Delhi’s location in the Indo-Gangetic Plain also adds to the challenge. Pollution from neighbouring states can contribute to the regional pollution load, while unfavourable weather conditions can allow pollutants to accumulate over several days.

Together, these factors help explain why winter smog can persist even when emissions from a particular source decline.

Where does Delhi’s air pollution come from?

Delhi’s pollution comes from several sources, including vehicle exhaust, industrial emissions, construction activity, road dust, waste burning and biomass burning. Their relative contributions vary depending on the season, location and prevailing weather conditions.

During the post-monsoon period, smoke from agricultural residue burning in neighbouring states can add to the existing pollution load in Delhi. However, local sources such as transport, industrial activity and dust also contribute to the city’s air pollution.

The contribution of individual sources can change significantly over short periods. Before Diwali, sources such as biomass and waste burning, vehicle emissions and secondary organic aerosols contributed substantially to Delhi’s PM2.5 pollution.

During the hours immediately following Diwali, firecrackers became a dominant contributor. According to IITM Pune’s analysis, they accounted for 71.1% of the PM2.5 concentration in Delhi’s air between 1 am and 9 am on November 9. The corresponding contribution was 43.4% during the same period on November 8 and 5.6% on November 7.

These figures relate to specific time windows and should not be interpreted as the share of firecrackers in Delhi’s overall pollution throughout the winter season.

Why does Delhi’s smog linger after Diwali?

The end of a temporary pollution source does not immediately clear the air. Even after firecracker emissions subside, pollutants from vehicles, industries, dust and other sources continue to enter the atmosphere.

When wind speeds remain low and temperature inversion restricts the movement of air, these pollutants can remain trapped close to the ground. This allows pollution levels to stay elevated over several days.

The persistence of smog is therefore not simply a question of how much pollution is emitted on a particular day. Weather conditions and the accumulated pollution load also determine how quickly air quality improves.

How dangerous is PM2.5 pollution?

Fine particulate matter, particularly PM2.5, is a major health concern because of its small size. Measuring 2.5 micrometres or less in diameter, these particles can penetrate deep into the lungs. Some may also enter the bloodstream and affect other parts of the body, the Indian Express reported.

Exposure to polluted air can cause eye and throat irritation, coughing and breathing difficulties. It can also aggravate asthma and other respiratory conditions.

Long-term exposure to fine particulate pollution is associated with a higher risk of cardiovascular disease, stroke, lung cancer and chronic respiratory illnesses.

Children, older people, pregnant women and individuals with pre-existing heart or lung conditions are particularly vulnerable. However, prolonged exposure to high levels of air pollution can also affect otherwise healthy people.