Air pollution

Topic: Geographical Issues: Pollution, Urban Waste, Slums and Land Degradation · NCERT: Class 10, Ch 6 "Manufacturing Industries"; Class 12, Ch 9 "Geographical Perspective on Selected Issues and Problems"

Meaning

Air pollution means harmful things are added to the air. These are dust, fumes, gas, fog, odour, smoke or vapour. They are added in amounts large enough, and for long enough, to harm plants, animals and property. Two things decide how much harm is done: how much is added (quantity) and how long it stays (duration).

It matters because 99% of the world's people breathe air that is above WHO limits. Air pollution causes about 7 million preventable deaths each year [8]. Many of the world's most polluted cities for PM2.5 are in India's Indo-Gangetic plain.

Explanation

Sources and pollutants

• Particulate matter (PM) means tiny solid or liquid bits floating in the air.
• PM10 bits are 10 micrometres across or smaller. They can enter the nose and throat.

• PM2.5 bits are 2.5 micrometres across or smaller. They go deep into the lungs and the blood.

• Main sources and what they release:

Source	What it releases
Burning fossil fuels (coal, oil, gas)	Oxides of sulphur (SOₓ) and nitrogen (NOₓ), CO₂, CO
Mining	Dust, asbestos
Industry (refineries, smelters, cement plants)	SOₓ, NOₓ, hydrocarbons, lead, asbestos
Vehicles	NOₓ, CO, hydrocarbons, PM2.5 (and lead before leaded petrol was phased out)
Construction and road dust	PM10
Burning biomass and waste	PM2.5, CO, black carbon
Brick kilns	PM, SO₂, black carbon

• Hydrocarbons are compounds of hydrogen and carbon that come from unburnt fuel.
• In sunlight they react with NOₓ.

• This forms ground-level ozone.

• CO (carbon monoxide) is a poisonous gas that comes from incomplete burning. It stops the blood from carrying oxygen.

Effects

• Three body systems are harmed:
• Respiratory system (breathing): asthma, bronchitis, lung cancer.
• Nervous system: lead and CO damage the brain and nerves.

• Circulatory system (heart and blood): heart attacks and stroke.

• WHO adds heart disease, stroke, COPD (chronic obstructive pulmonary disease), cancer and pneumonia to this list [8].

• Air pollution also damages crops, forests and buildings.
• WHO Air Quality Guidelines 2021 [8]:
• PM2.5: 5 µg/m³ as a yearly average and 15 µg/m³ as a 24-hour average. The old yearly guideline was 10.
• PM10: 15 µg/m³ yearly and 45 µg/m³ over 24 hours.
• NO₂: 10 µg/m³ yearly and 25 µg/m³ over 24 hours.

Two results: smog and acid rain

• Smog (smoke + fog) is a smoky fog that hangs over cities.
• Smoke and particles mix with water droplets in cool, damp air.
• Each particle becomes a seed for a fog droplet, so the fog turns thick and dark.

• If the wind is calm, the smog stays over the city for days.

• Two types of smog:

• Classical (sulphurous) smog forms from coal smoke (SO₂ + soot) and fog in cold, damp winters. Examples: London in 1952, and Delhi in winter.

• Photochemical smog forms when sunlight acts on NOₓ and hydrocarbons, making ozone. It forms on warm, sunny days. Examples: Los Angeles, and Indian cities in summer.

• Acid rain is rain with a pH below about 5.6. pH is a scale of acidity from 0 to 14, and anything below 7 is acidic.

• Clean rain already has a pH of about 5.6. It dissolves a little CO₂ and forms weak carbonic acid.
• SO₂ and NOₓ from coal, oil and vehicles mix with water vapour and oxygen in the air.

• They turn into sulphuric acid and nitric acid, which fall with rain, snow or fog.

• The first rain after summer is the most acidic (NCERT):

• Pollutants pile up in the dry summer air.
• The first shower washes them all out at once, so it has the lowest pH.

• Later rains fall through cleaner air.

• Damage from acid rain: soils lose calcium and magnesium, lakes turn acidic and fish die, forest leaves are damaged, and marble and limestone monuments wear away.

Conditions that trap pollution (Delhi's winter, November–January)

• Temperature inversion is when a layer of cold air sits near the ground under a layer of warmer air.
• Normally, air gets cooler as it rises, so warm polluted air near the ground rises and spreads out.
• On long, clear winter nights the ground loses heat fast, and the air touching it becomes cold and heavy.

• The warmer layer above acts like a lid, so the pollution cannot rise.

• Mixing height is the height up to which air near the ground mixes freely. In winter it falls to a few hundred metres, so the same pollution is packed into much less air.

• Calm winds do not blow the pollution away. Fog and high humidity help particles grow into smog.
• Outside smoke comes in:
• North-westerly winds carry smoke from paddy stubble burning (October–November) in Punjab and Haryana down the Indo-Gangetic plain.

• Diwali firecrackers add a sharp spike.

• Geography makes it worse:

• Delhi lies in a land-locked plain, so there is no sea breeze to clean the air.
• The Himalaya to the north acts like a wall. Polluted air collects over the plain like water in a bowl.

Where it occurs

• World:
• More than 6,000 cities in 117 countries now check their air quality [8].
• Fewer than 1% of monitored cities in low- and middle-income countries meet the WHO PM guidelines. In high-income countries, 17% do [8].

• London (classical smog, 1952) and Los Angeles (photochemical smog) are the classic examples.

• Indo-Gangetic plain: most of India's PM2.5 hotspots are here. Brick kilns ring its cities.

• Delhi-NCR: has severe winter smog, worst in November–January.
• Punjab, Haryana and western Uttar Pradesh: farmers burn paddy stubble in October–November.
• 2024: Punjab had 9,655 fires and Haryana 1,118 [7].

• 2025: Punjab had 5,114 fires. This was 53% lower than 2024 and 93% lower than 2021. Haryana's count was 91% lower than 2021 [6].

• Chota Nagpur plateau: dust and asbestos come from the coal and iron ore mining belts.

• Agra, Uttar Pradesh:
• SO₂ from the Mathura refinery and from foundries turned into acid. It yellowed and pitted the Taj Mahal's marble ("marble cancer").

• The Supreme Court created the Taj Trapezium Zone (TTZ) in the M.C. Mehta case (1996).

• Big cities such as Delhi, Mumbai, Kolkata and Bengaluru have heavy vehicle pollution.

• National Clean Air Programme (NCAP):
• Run by MoEFCC, launched in January 2019 [2].
• Covers 131 cities in 24 States/UTs. These are non-attainment cities (cities that failed the national air standards for five years) and million-plus cities [2].

• Target: cut PM10 by up to 40%, or meet the NAAQS, by 2025-26 [3].

• CAQM (Commission for Air Quality Management in NCR and Adjoining Areas):

• Set up by an Act of Parliament in 2021 [5].
• It coordinates action on stubble burning in Punjab, Haryana, Rajasthan and UP [6].

Don't confuse with

• Classical smog vs photochemical smog: classical smog comes from coal smoke and fog in cold, damp winters. Photochemical smog needs sunlight, NOₓ and hydrocarbons, so it forms on warm, sunny days.
• Acid rain vs normal rain: normal rain is already slightly acidic (pH about 5.6) because of CO₂. Acid rain is below 5.6, and its acids come from SOₓ and NOₓ, not CO₂.
• Ground-level ozone vs a direct pollutant: ground-level ozone is not released straight from chimneys or exhausts. It forms in sunlight from NOₓ and hydrocarbons, so it is a secondary pollutant.
• PM10 vs PM2.5: PM10 (mostly dust) reaches only the nose and throat. PM2.5 goes deep into the lungs and blood and is more harmful. NCAP targets PM10.

Prelims Hooks

• Acid rain has a pH below about 5.6. In cities, the first rain after summer has the lowest pH.
• Taj Trapezium Zone: created by the Supreme Court in the M.C. Mehta case, 1996. The polluter was the Mathura refinery.
• Temperature inversion means cold air near the ground under warm air. It forms on calm, clear winter nights and traps smog over Delhi.
• Stubble smoke reaches Delhi on north-westerly winds from Punjab–Haryana, along the Indo-Gangetic plain.
• NCAP: launched in 2019 for 131 cities, with a target of up to a 40% PM10 cut by 2025-26 [2][3]. CAQM was created by statute in 2021 [5].
• WHO 2021 yearly PM2.5 guideline: 5 µg/m³ (the old one was 10) [8]. Trap: the Happy Seeder (sows wheat straight through the stubble) is in-situ management, while straw pellets burnt in power plants are ex-situ management.

Mains Points

• Delhi's smog is caused by geography as much as by emissions (GS-I/III):
• Winter inversion, low mixing height, calm winds, a land-locked plain and the Himalayan barrier all trap pollution.
• Stubble smoke and firecracker smoke are added on top of this.

• So action inside one city is not enough. All the states that share the same air need to act together, through CAQM.

• Stubble burning is a farm policy problem:

• Paddy is grown because of MSP. The Punjab Preservation of Subsoil Water Act, 2009 delays transplanting, which leaves only 2–3 weeks before wheat sowing. Combine harvesters leave tall stubble.
• Machines (the Happy Seeder, Super SMS and the PUSA bio-decomposer) and markets for straw cut fires by about 90% between 2021 and 2025 [6].

• Lasting change needs crop diversification away from paddy, towards maize, pulses and cotton.

• NCAP's record is mixed:

• 108 cities had lower PM10 than in 2017-18. 72 cut it by more than 20%, and 26 by more than 40% [5].
• But only 14 cities met the NAAQS [5], and the programme targets PM10, not the more harmful PM2.5.
• 49 cities funded by the XV Finance Commission must cut PM10 by 15% a year, so their funding depends on results [4].

Related concepts

• Smog
• Acid rain
• Delhi winter smog
• Stubble burning

Read more

• Air pollution: sources, smog, acid rain and Delhi's winter crisis