Air pollution

1. Pollutants, sources and classification

Air pollution arises from both natural and human sources. Dust storms, wildfires, volcanoes and pollen contribute naturally. Major human sources include transport, thermal power plants, industries, construction, waste burning, agricultural residue burning and household combustion. Emissions are pollutants released by a source; ambient concentration is the amount present in surrounding air after transport, dilution and chemical transformation. Consequently, identical emissions can produce different exposure levels under different weather conditions.

Primary pollutants are emitted directly: examples include carbon monoxide, sulphur dioxide, nitric oxide, soot and mineral dust. Secondary pollutants form through atmospheric reactions. Examples include ground-level ozone, sulphuric and nitric acids, ammonium sulphate, ammonium nitrate and secondary organic aerosols. Particulate matter can therefore be either primary or secondary. Sources also differ spatially: vehicle exhaust is a mobile source, a chimney is a point source, and dispersed construction activity constitutes an area source.

• Particulate matter: PM10 can enter the respiratory system; PM2.5 penetrates deep into the lungs and contributes to systemic cardiovascular effects. Composition, concentration and exposure duration all influence toxicity.
• Sulphur dioxide: principally associated with sulphur-containing fuels and industrial processes; contributes to respiratory irritation, sulphate aerosols and acid deposition.
• Nitrogen oxides: largely associated with high-temperature combustion; contribute to ozone, nitrate particles and acid deposition.
• Carbon monoxide: produced by incomplete combustion; forms carboxyhaemoglobin, reducing the blood’s oxygen-carrying capacity.
• Volatile organic compounds, ammonia and lead: VOCs participate in photochemical reactions; ammonia from fertilisers and livestock helps form secondary particles; lead is a potent neurotoxicant.

2. Atmospheric processes and transport

Pollution depends strongly on wind speed, mixing height, rainfall, topography and atmospheric stability. During a temperature inversion, warmer air overlies cooler near-surface air, restricting convection and trapping pollutants below. Weak winds and a shallow winter boundary layer favour accumulation over the Indo-Gangetic Plain. Valleys can similarly retain polluted air. Rain scavenges many particles and soluble gases, although the effectiveness varies with pollutant properties and rainfall intensity.

Classical sulphurous smog is associated with smoke, sulphur dioxide, fog and cool, humid conditions, exemplified by London in 1952. Photochemical smog develops when sunlight drives reactions involving nitrogen oxides and VOCs. It contains ozone, peroxyacetyl nitrate and other oxidants. Ozone is not emitted directly from vehicle exhaust, and concentrations can peak downwind rather than beside a busy road. Its response to precursor reductions is chemically complex.

Acid deposition includes wet deposition through rain, snow or fog and dry deposition of acidic gases and particles. Sulphur dioxide and nitrogen oxides are important precursors. It can acidify water bodies, deplete soil nutrients and damage buildings. Pollution also crosses administrative boundaries, making the airshed, a region sharing air circulation and pollutant transport, a more useful management unit than a municipal boundary alone.

• Black carbon is a light-absorbing component of particulate matter from incomplete combustion. It warms the atmosphere and can accelerate snow and ice melting after deposition.
• Air pollution and climate change overlap but are not identical: carbon dioxide is chiefly a greenhouse gas, whereas sulphate particles generally exert a cooling influence while harming health.

3. Health, ecosystems and household exposure

Air pollution increases risks of ischaemic heart disease, stroke, chronic obstructive pulmonary disease, lung cancer and respiratory infections. Children, older persons, pregnant women, outdoor workers and people with existing illness are especially vulnerable. Exposure is determined not just by the nearest monitoring station but also by time spent outdoors, workplace conditions, household fuels and building ventilation. Short pollution episodes can trigger acute illness, while sustained exposure creates substantial chronic disease burdens.

Household air pollution commonly arises from burning wood, dung, crop residues, coal or kerosene in poorly ventilated spaces. WHO identifies household air pollution as a major preventable health risk. Pradhan Mantri Ujjwala Yojana, launched in 2016, expands LPG access, but sustained health gains require affordable refills and consistent clean-fuel use. Fuel stacking, where households continue using biomass alongside LPG, can limit exposure reductions.

Ground-level ozone damages leaf tissue, impairs photosynthesis and reduces crop productivity. Nitrogen deposition can fertilise ecosystems initially but also cause nutrient imbalance and eutrophication. Particles reduce visibility, while acidic deposition and soot damage cultural heritage. Pollution control therefore supports public health, agricultural productivity, biodiversity conservation and protection of monuments.

4. Monitoring, standards and interpretation

The Central Pollution Control Board coordinates air-quality monitoring with State Pollution Control Boards and Pollution Control Committees. Manual stations and continuous ambient air-quality monitoring stations provide complementary information. The 2009 National Ambient Air Quality Standards specify twelve pollutants: PM10, PM2.5, sulphur dioxide, nitrogen dioxide, ozone, carbon monoxide, lead, ammonia, benzene, benzo(a)pyrene, arsenic and nickel. Averaging periods differ, so annual and short-term limits must not be interchanged.

The National Air Quality Index translates concentrations into six categories: Good, Satisfactory, Moderately Polluted, Poor, Very Poor and Severe. It uses eight pollutants: PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. The overall AQI is governed by the highest available pollutant sub-index under prescribed data-availability rules, not a simple average. AQI communicates health risk but does not identify the contributing sources.

WHO’s 2021 guideline for annual PM2.5 is 5 micrograms per cubic metre, compared with India’s annual standard of 40. Comparing values requires matching the pollutant, unit and averaging period. Emission inventories estimate releases by source; source-apportionment studies estimate contributions to ambient pollution; satellite observations provide wider spatial coverage but do not directly substitute for ground-level exposure measurements.

5. Indian governance and prevention

The Air (Prevention and Control of Pollution) Act, 1981 provides the principal statutory framework for air-pollution prevention and control. Pollution control boards administer consent requirements, emission standards and enforcement. The Environment (Protection) Act, 1986 provides additional powers for environmental regulation. The Commission for Air Quality Management Act, 2021 created a regional institution for the National Capital Region and adjoining areas relevant to NCR air quality.

The National Clean Air Programme, launched in 2019, supports city action plans and monitoring. Its revised target envisaged up to a 40% reduction in particulate pollution, or achievement of national standards, by 2025–26 against the 2017 baseline. Bharat Stage VI vehicle-emission norms took effect nationwide in April 2020. Delhi-NCR’s Graded Response Action Plan uses escalating measures according to air-quality severity and forecasts.

Durable improvement requires cleaner fuels, reliable public transport, industrial emission control, dust management, alternatives to residue burning and prevention of open waste burning. Electrostatic precipitators and bag filters capture particles; flue-gas desulphurisation controls sulphur dioxide; selective catalytic reduction controls nitrogen oxides. Tall chimneys mainly disperse pollution rather than eliminate it. Emergency restrictions must complement, not replace, year-round regional emission reductions.

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