Why Does Delhi's Air Pollution Get Worse In Winter? The Science Behind The Smog

Delhi's air pollution makes headlines every winter as a thick layer of smog settles over the national capital and adjoining areas. As temperatures begin to fall, Delhi and its neighbouring cities in the National Capital Region (NCR), including Noida, Gurugram, Ghaziabad and Faridabad, experience a sharp deterioration in air quality.

The problem is not limited to a few days of poor visibility. For weeks, residents are exposed to high concentrations of particulate matter and other pollutants, increasing the risk of respiratory and cardiovascular illnesses. Exposure to polluted air can also aggravate existing health conditions, particularly among children, older people and those with respiratory disorders.

While crop residue burning in neighbouring states often becomes the focus of public debate, it is only one part of a much larger problem. Vehicle emissions, industrial activity, construction dust and waste burning contribute to pollution throughout the year. Winter weather conditions, however, make it harder for these pollutants to disperse.

So, why does Delhi's air become particularly hazardous during winter? And is the pollution a seasonal problem or a year-round concern?

Why Does Delhi's Pollution Get Worse In Winter?

One of the main reasons behind the winter pollution crisis is a change in atmospheric conditions. During winter, a phenomenon known as temperature inversion can prevent pollutants from rising and dispersing into the atmosphere.

Under normal conditions, air temperature generally decreases with altitude. Warmer air near the ground rises, allowing pollutants to disperse upwards. During a temperature inversion, however, a layer of warmer air settles above colder air closer to the surface. This restricts vertical air movement and traps pollutants near the ground.

The situation becomes worse when wind speeds are low. Strong winds can carry pollutants away and help disperse them over a wider area. During winter, weaker winds reduce this movement, allowing smoke, dust and other emissions to accumulate.

Another important factor is the reduction in the atmospheric mixing height, also known as the boundary layer height. It refers to the vertical distance within which pollutants can mix with the surrounding air. During winter, this layer often becomes shallower, leaving pollutants concentrated in a smaller volume of air close to the ground.

Low temperatures and increased humidity also contribute to fog formation. When fog occurs alongside high concentrations of suspended pollutants, visibility deteriorates and the region experiences the thick haze commonly described as smog.

Delhi's Geography Adds To The Problem

Delhi's geographical location also plays a role in its winter pollution crisis. Situated in the Indo-Gangetic Plain, the national capital is part of a densely populated and heavily industrialised region.

The wider Indo-Gangetic Plain is an interconnected airshed, where pollutants can travel across state and city boundaries depending on wind direction and atmospheric conditions. During winter, unfavourable winds and limited atmospheric mixing can prevent pollutants from dispersing efficiently.

The pollution problem also extends beyond Delhi's administrative boundaries. Neighbouring cities such as Noida, Gurugram, Ghaziabad and Faridabad form part of an interconnected urban and industrial region. Emissions from one area can travel to another, making air pollution a regional problem rather than one confined to the national capital.

This means that measures taken within Delhi alone cannot address the entire problem. Pollution from surrounding districts and other parts of the wider airshed also needs to be considered.

What Causes Delhi's Air Pollution?

Although winter weather worsens the situation, several sources contribute to Delhi's air pollution throughout the year.

A February 2026 assessment by the Commission for Air Quality Management (CAQM), based on a meta-analysis of source-apportionment studies conducted between 2015 and 2025, identified secondary particulate matter as the largest contributor to Delhi's winter PM2.5 concentrations. The assessment also highlighted the role of transport, biomass burning, dust and industrial emissions.

The estimated average contributions during winter were secondary particulate matter (27%), transport (23%), biomass burning (20%), dust (15%) and industry, including thermal power plants (9%). The remaining six per cent was attributed to other sources. These are estimates from the studies analysed and should not be treated as fixed contributions on every winter day.

Secondary particulate matter: Unlike primary pollutants, which are released directly into the atmosphere, secondary particulate matter forms through chemical reactions involving gases already present in the air. Emissions from vehicles, industries, power plants and biomass burning release gases that can react with other atmospheric compounds to form fine particles.

These reactions produce pollutants such as sulphates, nitrates and secondary organic aerosols, which contribute to PM2.5 concentrations. Their formation depends on several factors, including temperature, humidity and the presence of precursor gases. The CAQM assessment estimated that secondary particulate matter accounted for 27% of Delhi's winter PM2.5 concentrations, making it the largest identified contributor.

Vehicular emissions: The large number of vehicles on Delhi's roads is a major source of air pollution. Exhaust emissions release particulate matter and gases such as nitrogen oxides, which also contribute to the formation of secondary pollutants. Traffic congestion, fuel combustion and emissions from vehicles entering the city from neighbouring areas add to the problem. The CAQM assessment estimated transport's contribution to winter PM2.5 concentrations at 23%.

Construction and road dust: Rapid urbanisation and infrastructure development generate substantial amounts of dust. Construction, demolition, excavation and the movement of vehicles on dusty roads release particulate matter into the air. These particles can remain suspended and contribute to higher PM10 concentrations. Road, soil and construction dust together accounted for an estimated 15% of winter PM2.5 concentrations in the CAQM assessment.

Industrial emissions: Factories, industrial units and power plants around Delhi-NCR release pollutants through their operations. The burning of coal and other fossil fuels contributes to emissions of particulate matter, sulphur dioxide and nitrogen oxides. These gases can also undergo chemical reactions in the atmosphere to form secondary particulate matter. Industrial sources, including thermal power plants, accounted for an estimated nine per cent of winter PM2.5 concentrations.

Crop residue and other biomass burning: The burning of paddy stubble in Punjab, Haryana and parts of Uttar Pradesh is an important seasonal contributor to pollution in Delhi-NCR. After harvesting paddy, some farmers burn the remaining crop residue to clear their fields before the next sowing season. The smoke contains particulate matter and gases that can travel over long distances, depending on wind direction and atmospheric conditions.

However, the CAQM's 20% estimate for biomass burning should not be attributed to stubble burning alone. This category also includes municipal solid waste burning and residential use of biomass. The contribution of crop residue burning varies from day to day, depending on the number of fires, wind patterns and prevailing weather. It is therefore an important part of the winter pollution problem, but cannot alone explain Delhi's deteriorating air quality.

Waste and biomass burning: The burning of municipal waste, leaves, wood and other biomass releases smoke and harmful pollutants. Such activities, particularly in densely populated areas, add to the existing pollution burden. They also contribute to the formation of secondary pollutants through the release of gases into the atmosphere.

How Do Smoke, Fog And Gases Combine To Form Smog?

Smog is a form of air pollution involving a mixture of particulate matter and gases that reduces visibility and affects air quality. Delhi's winter smog is influenced by both direct emissions and chemical reactions taking place in the atmosphere.

Particulate matter, particularly PM2.5 and PM10, remains suspended in the air. PM2.5 refers to particles with a diameter of 2.5 micrometres or less, while PM10 includes particles measuring up to 10 micrometres.

Gases such as nitrogen oxides, sulphur dioxide and volatile organic compounds can also react in the atmosphere to produce secondary pollutants. These reactions contribute to the formation of fine particulate matter, which can remain suspended in the air.

Fog and smog, however, are not the same. Fog consists of tiny water droplets suspended close to the ground, while smog refers to polluted air containing particulate matter and gases.

During winter, high humidity and low temperatures favour fog formation, while weak winds and temperature inversion allow pollutants to accumulate. When fog occurs alongside high concentrations of pollutants, the resulting haze can become particularly dense and visibility can deteriorate sharply.

What Does Winter Pollution Mean For Health?

Prolonged exposure to polluted air can affect several parts of the human body. Fine particles such as PM2.5 are particularly concerning because they can travel deep into the lungs and, in some cases, enter the bloodstream.

Exposure can aggravate asthma, bronchitis and other respiratory conditions. It can also increase the risk of cardiovascular problems, including heart disease and stroke. The World Health Organization (WHO) has linked exposure to fine particulate matter with a range of adverse health effects, including respiratory and cardiovascular diseases.

Children, older people and those with pre-existing respiratory or cardiac conditions are particularly vulnerable. Even otherwise healthy individuals can experience coughing, throat irritation, breathing difficulties and eye irritation during periods of severe pollution.

The effects are not restricted to winter. While unfavourable weather conditions make pollution more concentrated and severe during this season, the sources of emissions remain active throughout the year.