Why this chapter matters for UPSC: Air pollution and the ozone layer are standing GS3 environment topics, and Mains has asked how the WHO's 2021 air-quality guidelines differ from the earlier ones. Every such question builds on this chapter's simple facts: what air is made of, which gas supports burning, why mountaineers carry oxygen, and how plants keep the oxygen in the air topped up. In NCERT's rationalised 2023-24 edition this is Chapter 11, "Air Around Us", the last chapter of the book.

🧠 First Principles — Read This First

We cannot see air, but we feel it: leaves rustle, clothes on a line sway, the pages of a book flutter under a fan, a kite flies. Air in motion is wind. A firki spins in moving air, and a weather cock shows which way the wind blows.

NCERT's chapter proves four things with simple activities:

  1. Air occupies space and is present everywhere, even in an "empty" bottle.
  2. Air is a mixture of nitrogen, oxygen, a little carbon dioxide, water vapour and other gases, with dust particles.
  3. Air is dissolved in water and present in soil, where aquatic animals and soil organisms use it.
  4. Plants replace the oxygen that living things use up, because photosynthesis gives out more oxygen than plants consume.

The envelope of air around the Earth is the atmosphere. As we go higher, the air gets rarer, which is why mountaineers carry oxygen cylinders.

PART 1 — Quick Reference

What Is Air Made Of? (NCERT)

ComponentHow NCERT shows itKey point
Water vapourDrops form when air touches a cool surfaceImportant for the water cycle
OxygenA candle covered by a glass tumbler goes out; an uncovered one keeps burningSupports burning; needed for respiration
NitrogenAir remains in the tumbler after the candle goes outThe major part of air; does not support burning
Carbon dioxideSuffocation in a closed room where something is burningA small component; produced by respiration and burning
Dust and smokeParticles shining in a beam of sunlight through a slitSmoke from burning fuel contains gases and fine dust; nose hair and mucus keep dust out

Nitrogen and oxygen together make up 99% of the air; the remaining 1% is carbon dioxide, a few other gases and water vapour (NCERT VI). By volume, about 78% is nitrogen and about 21% oxygen (NCERT VIII), with carbon dioxide, argon, methane, ozone and water vapour in very small amounts.

Carbon dioxide today: NOAA measured a global average of 425.62 ppm for 2025, and a record 432.34 ppm at Mauna Loa in May 2026.

Uses of Moving Air (NCERT)

UseExamples
WindmillsDraw water from tubewells, run flour mills, generate electricity
Flight and sailingSailing yachts, gliders, parachutes, aeroplanes; birds, bats and insects fly
PlantsDisperses seeds and pollen
NaturePlays an important role in the water cycle
FarmWinnowing works better in moving air

PART 2 — Concepts & Narrative

Air Occupies Space

Push an "empty" bottle mouth-down into a bucket of water and no water enters: the bottle is full of air, which has nowhere to escape. Tilt it, and air bubbles out as water fills the space it occupied. Air occupies space; it is colourless and transparent. The atmosphere extends many kilometres above the surface, and the air gets rarer with height.

Air in Water and Soil

NCERT heats water slowly: well before it boils, tiny bubbles appear on the inside of the container. These are air dissolved in water escaping as it warms. Aquatic animals use this dissolved oxygen. Pour water on a dry lump of soil and bubbles rise: soil contains air, which soil organisms and plant roots use for respiration. Burrows made by soil animals let air move in and out. In heavy rain, water fills these spaces, so animals such as earthworms come out to breathe.

How Oxygen Is Replaced

If so many organisms use oxygen, why does the atmosphere not run out? Plants make food by photosynthesis and give out oxygen. They also use oxygen for respiration, but produce more than they consume. The balance of oxygen and carbon dioxide is kept by respiration in plants and animals and photosynthesis in plants: plants and animals depend on each other.

Key Term

Layers of the atmosphere (NCERT Class XI):

LayerExtent (NCERT XI)Key feature
TroposphereAverage height 13 km: about 8 km at the poles, 18 km at the equatorAll weather and climate changes; contains dust and water vapour; temperature falls with height
StratosphereAbove the tropopause, up to 50 kmContains the ozone layer, which absorbs ultraviolet radiation
MesosphereUp to 80 kmTemperature falls to about minus 100 °C at 80 km
Ionosphere (thermosphere)80 to 400 kmIons reflect radio waves back to Earth; temperature rises with height
ExosphereAbove the thermosphereExtremely rarefied; merges with outer space; "very little is known about it"

NCERT XI places ozone "between 10 and 50 km" above the surface; oxygen becomes negligible at about 120 km, and carbon dioxide and water vapour are found only up to 90 km.

When Air Is Polluted (NCERT Class VIII)

Air pollution is the contamination of air by unwanted substances that harm living and non-living things. Pollutants come from natural sources (smoke and dust from forest fires and volcanoes) and from human activity: factories, power plants, vehicle exhaust and burning firewood and dung cakes. NCERT VI already advises against burning dry leaves and crop remains.

Explainer
PollutantMain sources (NCERT VIII)Effect
Carbon monoxideIncomplete burning of petrol and dieselPoisonous; reduces the blood's oxygen-carrying capacity
Nitrogen oxidesVehicles; refineriesWith other pollutants and fog they form smog (smoke + fog): asthma, cough, wheezing
Sulphur dioxideBurning coal in power plants; refineriesRespiratory problems including permanent lung damage
CFCsRefrigerators, air conditioners, aerosol spraysDamage the ozone layer
Suspended particlesDiesel and petrol vehicles, steel making, mining, ash from power plantsReduce visibility; cause disease when inhaled

Acid rain and the Taj Mahal: sulphur dioxide and nitrogen dioxide from industries around Agra, especially the Mathura oil refinery, react with water vapour to form sulphuric and nitric acids that fall as acid rain and corrode the marble ("marble cancer"); soot has yellowed it. The Supreme Court ordered industries in the Taj zone to switch to CNG and LPG and vehicles to unleaded petrol.

Greenhouse effect: the atmosphere traps part of the heat radiated by the Earth, as a greenhouse traps the sun's heat. Without it, life would not be possible; but excess carbon dioxide, from burning fuels and from deforestation, traps more heat and raises the average temperature: global warming. Methane, nitrous oxide and water vapour are also greenhouse gases.

India's Clean-Air Framework

UPSC Connect

Standards: the National Ambient Air Quality Standards were notified by CPCB on 18 November 2009 for 12 pollutants. For particulate matter: PM10 60 µg/m³ (annual) and 100 µg/m³ (24-hour); PM2.5 40 µg/m³ (annual) and 60 µg/m³ (24-hour). The WHO Global Air Quality Guidelines (2021) are far stricter: PM2.5 annual 5 µg/m³ and PM10 annual 15 µg/m³, with interim targets on the way (PM2.5: 35, 25, 15, 10).

National Air Quality Index: launched on 17 October 2014 under Swachh Bharat. Six categories: Good, Satisfactory, Moderately polluted, Poor, Very Poor, Severe; eight pollutants: PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃ and lead.

National Clean Air Programme (NCAP): launched in January 2019 for 131 cities (non-attainment and million-plus cities) in 24 States and UTs. The target was revised to up to 40% reduction in PM10, or meeting the national standard, by 2025-26.

Cleaner fuels and vehicles:

  • Unleaded petrol throughout the country from 1 February 2000.
  • BS-VI emission norms for vehicles throughout India from 1 April 2020 (G.S.R. 889(E), 16 September 2016); the government decided in January 2016 to skip BS-V altogether.
  • Pradhan Mantri Ujjwala Yojana (deposit-free LPG connections for adult women from poor households; launched May 2016): about 10.33 crore connections as on 1 November 2025.

The Ozone Layer and the Montreal Protocol

UPSC Connect

The ozone shield: ozone in the stratosphere absorbs harmful ultraviolet radiation and breaks down to oxygen (NCERT VII asks students whether that is a chemical change; see Changes Around Us). CFCs from refrigerators, air conditioners and aerosols damage it.

  • Parties: 198 to both the Vienna Convention and the Montreal Protocol (1987), as of 10 August 2026 (UNEP Ozone Secretariat). India acceded to the Vienna Convention on 18 March 1991 and the Montreal Protocol on 19 June 1992.
  • India's phase-out: production and consumption of CFCs stopped from 1 August 2008, except for metered-dose inhalers for asthma and COPD, which were phased out in December 2012; carbon tetrachloride and halons were completely phased out by 1 January 2010, the global phase-out date for developing countries under the Protocol. India is now phasing out HCFCs.
  • Kigali Amendment (2016) on HFCs, gases that do not harm ozone but are powerful greenhouse gases: the Union Cabinet approved ratification on 18 August 2021, and India ratified it on 27 September 2021. With 2024-2026 as baseline years, India will cut HFCs in four steps: 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047.

PART 3 — UPSC Integration

The chapter's activities explain three things UPSC returns to. The composition of air (nitrogen and oxygen 99%, a growing trace of carbon dioxide) underlies the greenhouse effect and global warming. Oxygen in water and soil explains why polluted rivers lose fish and why waterlogged soil kills roots. The oxygen-carbon dioxide balance kept by plants is the base of the carbon cycle and the case for forests. Pollution policy then layers on: national standards (NAAQS 2009) far looser than WHO's 2021 guidelines; an index to tell people how bad the air is; a city-level programme (NCAP); cleaner fuels (unleaded petrol, BS-VI, LPG). The ozone story is the model of a treaty that worked, now extended through Kigali to climate. Answers should give the standard or target, its date and its basis.

Exam Strategy

Prelims traps:

  • Oxygen supports burning; nitrogen does not. Nitrogen + oxygen = 99% of air; by volume ~78% N₂, ~21% O₂.
  • Air occupies space, is present in water and soil; earthworms surface in heavy rain because water fills the air spaces.
  • Plants produce more oxygen than they use.
  • Troposphere: weather layer, avg 13 km (8 at poles, 18 at equator); ozone mainly in the stratosphere; ionosphere (80-400 km) reflects radio waves.
  • NAAQS (2009): PM2.5 40 annual / 60 daily; PM10 60 / 100. WHO 2021: PM2.5 5, PM10 15 (annual).
  • AQI: launched 17-Oct-2014; 6 categories; 8 pollutants.
  • BS-VI from 1-Apr-2020; BS-V skipped. Unleaded petrol nationwide from 1-Feb-2000.
  • Montreal Protocol: 198 parties; India's CTC and halons gone by 1-Jan-2010 (on the deadline, not ahead of it).
  • Kigali: India ratified 27-Sep-2021; phase-down steps 2032/2037/2042/2047.

Mains angle: Compare India's air-quality standards with the WHO's 2021 guidelines and assess NCAP; or explain why the Montreal Protocol is considered a successful environmental treaty.

Practice Questions

Practice (UPSC-pattern, not past papers). Real UPSC questions on this chapter are listed in the PYQ box above.

Prelims:

  1. A burning candle covered by a glass tumbler goes out after a while. This shows that air contains a component that:
    (a) Is used up in respiration only
    (b) Supports burning and gets used up
    (c) Puts out fires
    (d) Is present only in moving air

  2. Which two gases together make up about 99% of the air?
    (a) Oxygen and carbon dioxide
    (b) Nitrogen and oxygen
    (c) Nitrogen and argon
    (d) Oxygen and water vapour

  3. Earthworms come out of the soil during heavy rain mainly because:
    (a) They need to feed on surface leaves
    (b) Water fills the air spaces in the soil, so they come out to breathe
    (c) Rainwater is warmer than the soil
    (d) They are carried out by run-off

  4. With reference to air-quality standards, consider the following statements:

    1. India's annual NAAQS for PM2.5 is 40 µg/m³.
    2. The WHO's 2021 annual guideline for PM2.5 is 5 µg/m³.
    3. India's National Air Quality Index uses six categories.
      Which of the statements given above are correct?
      (a) 1 and 2 only
      (b) 2 and 3 only
      (c) 1 and 3 only
      (d) 1, 2 and 3

  5. Radio waves transmitted from the Earth are reflected back by the:
    (a) Troposphere
    (b) Stratosphere
    (c) Ionosphere
    (d) Exosphere

  6. BS-VI emission norms for vehicles came into force across India on:
    (a) 1 April 2017
    (b) 1 April 2020
    (c) 1 April 2022
    (d) 1 April 2023

  7. Under the Kigali Amendment, India's commitment concerns the phase-down of:
    (a) CFCs
    (b) Halons
    (c) HFCs
    (d) Carbon tetrachloride

Mains (UPSC-pattern):

  1. "India's air-quality standards lag behind the WHO's guidelines." Discuss with reference to particulate matter and the National Clean Air Programme. (150 words)
  2. Explain how acid rain forms and how it damages monuments such as the Taj Mahal. What remedies were ordered? (150 words)

📦 Revision Capsule

Revision Capsule

Hard Facts

  • Air occupies space; air in motion = wind; atmosphere gets rarer with height
  • N₂ + O₂ = 99% (NCERT VI); ~78% N₂, ~21% O₂ by volume (NCERT VIII)
  • Oxygen supports burning; nitrogen does not; CO₂ a small part (425.62 ppm global, 2025)
  • Air dissolved in water; air in soil; plants produce more O₂ than they use
  • Troposphere avg 13 km; stratosphere to 50 km (ozone); mesosphere to 80 km (−100 °C); ionosphere 80-400 km
  • NAAQS 18-Nov-2009: PM2.5 40/60; PM10 60/100 µg/m³. WHO 2021: PM2.5 5, PM10 15
  • AQI 17-Oct-2014 (6 categories, 8 pollutants); NCAP January 2019 (131 cities); BS-VI 1-Apr-2020; unleaded petrol 1-Feb-2000
  • Montreal Protocol: 198 parties; India acceded 19-Jun-1992; Kigali ratified 27-Sep-2021

Core Concepts

  • Simple activities reveal each component of air
  • The oxygen-carbon dioxide balance links plants and animals
  • Pollutants harm health and monuments; excess CO₂ drives global warming
  • Standards, an index, city plans and cleaner fuels make up the clean-air toolkit

Confused Pairs

  • Oxygen (supports burning) vs nitrogen (does not)
  • Troposphere (weather) vs stratosphere (ozone) vs ionosphere (radio waves)
  • NAAQS (Indian legal standard) vs WHO AQG (health guideline)
  • CFCs (ozone-depleting; Montreal) vs HFCs (greenhouse gases; Kigali)
  • Smog (smoke + fog) vs acid rain (SO₂/NO₂ + water vapour)

PYQ Pattern

  • Mains 2021 GS3 asked for the key points of the WHO's revised Global Air Quality Guidelines and how they differ from the 2005 update. Prelims asks composition, atmospheric layers, pollutants and the ozone treaties.

Sources

Sources