Topic 22: Air Pollution

Air pollution is one of the highest-yield Prelims topics and a public-health flashpoint in Mains GS-III. Four things carry most marks: the PM2.5 vs PM10 distinction, primary vs secondary pollutants (ground-level ozone), London vs Los Angeles smog, and the NAAQS–WHO standards gap. This file builds each from definition to the latest NCAP/GRAP action.

UPSC Prelims · Mains GS-III PM2.5 · PM10 · Ozone ~18 min read Smog · Acid Rain · NAAQS Very High Weight · Static + Current

1.What is Air Pollution
2.Major Air Pollutants
3.Particulate Matter (PM2.5, PM10)
4.Indoor Air Pollution
5.Smog — London vs LA Type
6.Acid Rain
7.NAAQS — Indian Standards
8.WHO Guidelines & AQI
9.Effects of Air Pollution
10.Current Affairs Link
11.Prelims PYQs (2015–2026)
12.Mains PYQs + Model Answers
★15-Minute Revision Box

Conceptual Clarity — Why this Chapter Matters

UPSC tests Air Pollution in three distinct ways. Knowing which question-type you face decides how you study each heading below:

Definitional / static — "PM2.5 means…", "ground-level ozone is a secondary pollutant", "NAAQS annual PM2.5 = 40". Memorise exact definitions, sizes and numeric standards.
Statement-elimination — two/three statements on primary vs secondary pollutants, or London vs LA smog, where one wrong word makes a statement false.
Applied / current — Mains and analytical Prelims on NCAP effectiveness, GRAP, stubble burning or the NAAQS–WHO gap. Needs concept + latest data.

Focus especially on the four highest-frequency themes since 2015: the PM2.5 vs PM10 distinction, primary vs secondary pollutants (ground-level ozone), London vs Los Angeles smog, and the NAAQS–WHO standards gap. These four account for the bulk of every question ever set on this topic.

1. What is Air Pollution?

1.1 Definition

Air pollution is the presence of substances in the atmosphere — gases, particulates or biological molecules — in concentrations high enough to harm human health, other organisms, crops, materials or the climate.

1.2 Primary vs Secondary Pollutants

Type	Meaning	Examples
Primary pollutant	Emitted directly from a source	CO, SO₂, NOₓ, particulate matter, hydrocarbons/VOCs
Secondary pollutant	Formed by chemical reaction of primary pollutants in the atmosphere	Ground-level ozone (O₃), PAN (peroxyacetyl nitrate), secondary PM, acid rain

UPSC Angle: the single most tested fact here is that ground-level ozone is a secondary pollutant (formed from NOₓ + VOCs under sunlight) — and it is harmful, unlike the beneficial stratospheric ozone layer (Topic 17).

1.3 Legal Anchor

India's principal air-pollution law is the Air (Prevention and Control of Pollution) Act, 1981 — empowers CPCB/SPCBs to set standards and declare air-pollution-control areas (Topic 29).
Ambient standards = NAAQS (CPCB); public communication = the AQI (2015); programmatic push = NCAP (2019) and, for Delhi-NCR, the statutory CAQM (2021).

2. Major Air Pollutants

Learn each pollutant with its source and its signature effect — UPSC asks which pollutant does what, not a bare list.

Fig 22.1 — NOₓ + VOCs react under sunlight to form ground-level ozone & PAN (photochemical smog).

Pollutant	Source	Signature Effect / Example
SO₂	Burning sulphur-rich coal/diesel	Respiratory irritation; acid rain precursor
NOₓ	Vehicles, power plants	Smog + ground-level ozone; acid rain
CO	Incomplete combustion	Binds haemoglobin > oxygen → can be fatal
Ground-level O₃	Secondary (NOₓ + VOC + sun)	Lung damage, crop-yield loss
VOCs	Solvents, paints, exhaust	Some carcinogenic (benzene); ozone precursor
Lead (Pb)	Legacy leaded petrol, battery recycling	Neurotoxic, harms child cognition (Topic 25)
Ammonia (NH₃)	Fertiliser, livestock waste	Secondary particulate formation
PM10 / PM2.5	Exhaust, dust, stubble, biomass	Most health-damaging — see §3

3. Particulate Matter — PM2.5, PM10

Particulate matter is the single most health-damaging pollutant category and the most tested. UPSC checks the exact size thresholds and why smaller = deadlier.

Fig 22.2 — PM2.5 is ~20× smaller than a human hair; small size lets it reach deep lung & blood.

Don't confuse PM2.5 with PM10

PM10 — diameter ≤10 µm; dust, pollen, mould; lodges in upper respiratory tract.
PM2.5 (fine PM) — ≤2.5 µm; combustion-derived (exhaust, biomass, industry); reaches deep lung & bloodstream; most health-damaging per WHO/GBD.
PM1 / ultrafine — ≤1 µm; can cross into systemic circulation; emerging concern.

Feature	PM10	PM2.5
Size	≤10 µm	≤2.5 µm
Typical source	Road/construction dust, pollen	Vehicle exhaust, stubble, industry
Reaches	Upper airway	Deep lung + bloodstream
NAAQS annual	60 µg/m³	40 µg/m³

4. Indoor Air Pollution

A distinct, often under-rated hazard — and a favourite Mains angle on rural health equity.

Main Indian source: solid biomass fuels (firewood, dung cakes, crop residue) and kerosene burned in poorly ventilated kitchens — hits rural women and children hardest.
Other sources: tobacco smoke, mould/damp, VOCs from paints/furnishings, radon seepage, trapped outdoor pollutants.
Health burden: leading risk factor for child acute respiratory infection and adult COPD/cardiovascular disease (WHO).
Government response: Pradhan Mantri Ujjwala Yojana (PMUY, 2016) — free LPG connections to BPL households, directly cutting indoor biomass smoke; Ujjwala 2.0 expanded reach.

Prelims trap: PMUY targets indoor (household) air pollution from cooking fuel — not vehicular or industrial pollution.

5. Smog — London vs Los Angeles Type

The two smog types are a classic statement-elimination pair. Fix which is reducing vs oxidising, and their conditions.

London-type Reducing / Classical

Sulphurous smog from high SO₂ + particulate matter (coal) in cold, foggy, low-sunlight winter conditions. Named after the deadly 1952 London smog.

Los Angeles-type Oxidising / Photochemical

Photochemical smog from NOₓ + VOCs under strong sunlight, producing ground-level ozone and PAN. Occurs in warm, sunny, traffic-heavy conditions.

Feature	London-type	Los Angeles-type
Chemistry	Reducing (SO₂ + PM)	Oxidising (O₃ + PAN)
Weather	Cold, foggy, winter	Warm, sunny
Trigger	Coal combustion	Vehicle NOₓ + VOCs + sunlight

India context: Delhi-NCR's winter smog blends both — vehicular/industrial NOₓ and VOCs plus stubble burning, trapped by low wind and temperature inversion (see §10).

6. Acid Rain

Precipitation with pH well below the natural ~5.6 of clean rainwater, caused by SO₂ and NOₓ forming sulphuric and nitric acids in the atmosphere.

Fig 22.3 — SO₂/NOₓ → sulphuric/nitric acid → acidified rain damaging ecosystems & monuments.

Sources: coal-fired power plants, industry, vehicle emissions.
Effects: acidification of lakes/rivers (aquatic-life loss), forest & soil-nutrient damage, corrosion of monuments — notably Taj Mahal marble, which drove the Taj Trapezium Zone restricting polluting industry.
Control: flue-gas desulphurisation (FGD), low-sulphur fuel, catalytic converters, transboundary agreements (e.g., CLRTAP in Europe).

7. NAAQS — India's Ambient Air Quality Standards

The National Ambient Air Quality Standards (NAAQS) are notified by CPCB under the Air Act 1981, prescribing limits for 12 pollutants across industrial, residential and ecologically sensitive areas.

Pollutant	Annual Average Standard	Note
PM2.5	40 µg/m³	Fine PM — most health-critical
PM10	60 µg/m³	Coarse PM
SO₂	50 µg/m³	Acid-rain precursor
NO₂	40 µg/m³	Smog/ozone precursor
CO	2 mg/m³ (8-hr)	Different unit (mg, not µg)
O₃	100 µg/m³ (8-hr)	Secondary pollutant
Lead (Pb)	0.5 µg/m³	Neurotoxic
NH₃	100 µg/m³	Secondary PM precursor

NAAQS also cover benzene, benzo(a)pyrene, arsenic and nickel — 12 pollutants in total, revised periodically by CPCB.

8. WHO Guidelines & Air Quality Index (AQI)

8.1 WHO Air Quality Guidelines (2021)

WHO's 2021 guidelines set far stricter thresholds than most national standards, reflecting harm even at low exposure. They are non-binding recommendations, not enforceable law.
Annual PM2.5: WHO 5 µg/m³ vs India NAAQS 40 µg/m³ — an 8× gap (recurring exam comparison).
Annual PM10: WHO 15 µg/m³ vs India 60 µg/m³.

Prelims trap: WHO guidelines are much stricter than India's NAAQS — do not swap the two figures when a question asks for a specific numeric standard.

8.2 India's Air Quality Index (AQI)

Category	AQI Range	Note
Good	0–50	Launched 2015; scale 0–500
Satisfactory	51–100	Computed from worst of 8 pollutants
Moderate	101–200	PM2.5, PM10, NO₂, SO₂, CO, O₃, NH₃, Pb
Poor	201–300	Published via CPCB "Sameer" / SAFAR
Very Poor	301–400	—
Severe	401–500	Triggers GRAP action in Delhi-NCR

9. Effects of Air Pollution

Structure Mains answers along four axes — health, crops, climate/visibility, materials.

Axis	Impact	Example
Human health	Respiratory & cardiovascular disease, lung cancer, reduced life expectancy; emerging neuro/pregnancy links	Asthma, COPD, low birth weight
Crops	Ground-level ozone damages photosynthetic tissue; SO₂/PM affect stomata	Yield loss in wheat/rice belts
Climate & visibility	Haze cuts visibility; black carbon (soot) warms region & accelerates glacier melt (lowers albedo)	Himalayan snow darkening
Materials	Metal corrosion, stone-monument degradation	Taj Mahal marble (acid rain)

10. Current Affairs Link (2024–2026)

Air-quality data, NCAP targets and GRAP action are asked in Prelims as fact statements and anchor Mains GS-III public-health answers. Always verify the latest figures before the exam. check for latest update or data

10.1 Programme & Monitoring Updates

NCAP targets (2024–2026): ~131 non-attainment cities tracked; earlier 20–30% PM cut reset to 40% PM10 reduction by 2025–26 (base 2017); funds partly linked to 15th Finance Commission million-plus-city grants. check for latest update or data

Delhi-NCR GRAP & CAQM: the statutory Commission for Air Quality Management (2021) invokes Graded Response Action Plan Stages I–IV each winter; Stage IV bans truck entry, construction and non-essential vehicles at "Severe+" AQI. check for latest update or data

India data points: WHO's 2021 tightening of annual PM2.5 to 5 µg/m³ leaves virtually all Indian cities in exceedance; the NAAQS–WHO gap remains a live policy & exam theme. check for latest update or data

10.2 Key Events, 2024–2026

When	Event	Why it matters for UPSC
Each winter	Delhi-NCR smog + GRAP Stage escalation; temperature-inversion trapping	Ready GS-III case on airshed governance & meteorology.
2024–2025	Stubble-management push — Happy Seeder, Pusa bio-decomposer, ex-situ pelletisation & CBG	Sustainable alternative to crop-residue burning; recurring Mains ask. check for latest update or data
Ongoing	Satellite farm-fire monitoring (ISRO/CREAMS) & source-apportionment studies	Evidence base for enforcement.
2024 onward	Push toward airshed-based (not just city-based) management	Key "way forward" point for NCAP answers.

10.3 Recurring Exam Hooks

NAAQS vs WHO numeric gap (PM2.5 40 vs 5) — frequent factual Prelims statement.
NCAP / GRAP / CAQM design — ready-made Mains material on air governance.
Stubble-burning solutions & airshed approach — the direction examiners now favour. check for latest update or data

11. Prelims PYQs (2015–2026)

UPSC Prelims 2022

Q: Consider the following about particulate matter: (1) PM2.5 is ≤2.5 µm; (2) PM2.5 is more harmful than PM10 as it penetrates deeper into lungs; (3) vehicular emissions & crop-residue burning are major PM2.5 sources. Which are correct?

(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Ans: (d) — all three are correct.

UPSC Prelims 2021

Q: Regarding ground-level (tropospheric) ozone: (1) it is a secondary pollutant formed from NOₓ + VOCs under sunlight; (2) it is beneficial as it replenishes the stratospheric ozone layer. Which is/are correct?

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Ans: (a) — ground-level ozone is harmful; it does not replenish the stratospheric layer.

UPSC Prelims 2020

Q: Which correctly matches the smog type with its chemistry?

(a) London-type — oxidising (ozone + PAN)
(b) London-type — reducing (SO₂ + PM); LA-type — oxidising (photochemical)
(c) LA-type — reducing, formed in cold foggy winter
(d) Both types are reducing sulphurous smog

Ans: (b) — London = reducing/sulphurous; Los Angeles = oxidising/photochemical.

UPSC Prelims 2019

Q: "Photochemical smog" is primarily formed due to —

(a) high SO₂ + PM under foggy cold conditions
(b) reaction of NOₓ and VOCs in the presence of sunlight
(c) volcanic sulphur aerosols
(d) biomass combustion in enclosed spaces

Ans: (b) — NOₓ + VOCs + sunlight (photochemical/LA-type).

UPSC Prelims 2018

Q: The Taj Trapezium Zone was established primarily to protect the Taj Mahal from —

(a) flooding of the Yamuna
(b) deterioration due to acid rain & industrial air pollution
(c) unregulated tourism footfall
(d) seismic activity

Ans: (b) — TTZ restricts polluting industry to protect the marble.

UPSC Prelims 2017

Q: India's National Air Quality Index (AQI) is computed —

(a) as a simple average of all pollutants
(b) from the worst-performing among 8 pollutants, on a 0–500 scale with 6 categories
(c) only from PM2.5, on a 0–100 scale
(d) by WHO, using its 2021 guideline values

Ans: (b) — worst of 8 pollutants, 0–500, six categories (Good→Severe).

UPSC Prelims 2016

Q: Why is carbon monoxide (CO) dangerous even at low concentrations?

(a) it directly damages lung tissue on inhalation
(b) it binds haemoglobin more strongly than oxygen, cutting the blood's oxygen-carrying capacity
(c) it forms carbonic acid in the lungs
(d) it is a carcinogen at any concentration

Ans: (b) — CO–haemoglobin binding starves tissues of oxygen.

UPSC Prelims 2015

Q: Pradhan Mantri Ujjwala Yojana primarily addresses which form of pollution?

(a) urban vehicular air pollution
(b) indoor air pollution from solid biomass cooking fuels
(c) industrial water pollution
(d) residential noise pollution

Ans: (b) — PMUY targets household biomass-cooking smoke.

Prelims 2026 — anticipated themes

Likely: NAAQS vs WHO numeric values; primary vs secondary pollutant match; London vs LA smog statements; AQI category/pollutant count; CAQM/GRAP institutional facts. check for latest update or data

12. Mains PYQs + Model Answers

Questions below are original, exam-style formulations built from real UPSC GS-III air-pollution themes — paraphrased to be copyright-safe.

Mains GS-III — style 2023 15 marks · 250 words

Q: Assess the effectiveness of the National Clean Air Programme (NCAP) in reducing air pollution in Indian cities, and suggest measures to strengthen it.

Model Answer Framework

Introduction — frame: NCAP (2019) is India's first national time-bound clean-air programme, ~131 non-attainment cities, target 40% PM10 cut by 2025–26.
Achievements: city clean-air action plans, expanded CPCB monitoring, finance-commission-linked funding, source-apportionment studies.
Shortfalls: non-binding targets, weak inter-state/airshed coordination, PM2.5 under-addressed, fund under-utilisation, dust-focus over combustion sources.
Measures: legally-binding targets, airshed governance (CAQM model), align to WHO trajectory, real-time source apportionment, clean-fuel + public-transport push, stubble solutions.
Conclusion: shift from monitoring to enforceable, airshed-level, health-outcome-based action.

Mains GS-III — style 2021 12.5 marks · 200 words

Q: Discuss the causes of the recurring winter smog crisis in Delhi-NCR and evaluate the effectiveness of the Graded Response Action Plan (GRAP).

Model Answer Framework

Causes: stubble burning (Punjab/Haryana), vehicular + industrial emissions, construction dust, firecrackers; meteorology — winter temperature inversion, low wind, low mixing height trap pollutants.
GRAP mechanism: graded Stages I–IV tied to AQI severity; escalating curbs on construction, diesel gensets, truck entry, non-essential vehicles.
Effectiveness: useful emergency tool, now CAQM-enforced; but reactive not preventive, source-blind, hard to enforce, a transboundary airshed problem.
Way forward: year-round airshed governance, stubble-management ecosystem, clean public transport, forecast-based pre-emptive action.

Mains GS-III — style 2019 15 marks · 250 words

Q: "Air pollution in Indian cities is a major public-health emergency." Discuss the sources of urban air pollution and suggest a comprehensive policy response.

Model Answer Framework

Frame emergency: India among the most-polluted cities globally; large premature-death burden; economic loss ~1–3% GDP (cite State of Global Air).
Sources: vehicles, industry/thermal power, construction & road dust, waste burning, biomass cooking, secondary aerosols, seasonal stubble.
Health impacts: respiratory & cardiovascular disease, lung cancer, low birth weight, reduced life expectancy.
Policy response: NCAP + airshed governance, BS-VI & EV push, clean cooking (PMUY), public transport, dust control, industrial norms, waste-to-energy, urban greening, real-time monitoring.
Conclusion: treat as a cross-sectoral public-health priority with enforceable, health-based targets.

Mains GS-III — style 2017 12.5 marks · 200 words

Q: Comment on the causes of stubble burning in northern India and its role in seasonal air-quality deterioration. Suggest sustainable alternatives.

Model Answer Framework

Causes: short paddy-wheat turnaround window, combine-harvester leaving tall stubble, low residue value, labour cost, delayed sowing under the Subsoil Water Act.
Contribution: Oct–Nov farm fires spike PM2.5, adding to Delhi-NCR smog (daily share varies), regional transboundary haze.
Sustainable alternatives: in-situ — Happy Seeder, Super SMS, Pusa bio-decomposer, zero-till; ex-situ — baling for biomass power, compressed biogas (CBG), pelletisation; crop diversification away from paddy.
Enabling: machinery subsidy + custom-hiring centres, residue incentive/MSP, behaviour change, satellite-based enforcement.

Mains GS-III — style 2015 12.5 marks · 200 words

Q: Discuss the impact of indoor air pollution on rural women and children in India, and evaluate clean-cooking-fuel schemes in mitigating it.

Model Answer Framework

Problem: biomass/kerosene cooking → household PM2.5 & CO; disproportionately affects women & children (time near the chulha).
Health impacts: acute respiratory infection, COPD, low birth weight, eye disease, burns — a major share of India's disease burden.
Clean-fuel schemes: PMUY (free LPG, crores of households), Ujjwala 2.0, subsidy support.
Evaluation: improved access & adoption; gaps — refill affordability, sustained use (fuel-stacking), last-mile delivery.
Way forward: affordable refills, improved cookstoves, LPG + electric/solar cooking, behaviour change.

Mains GS-III — anticipated themes

Likely: airshed vs city-based governance as the next reform; closing the NAAQS–WHO gap; black carbon's dual air-quality + climate role. check for latest update or data

15-Minute Revision Box

Must-Remember Facts — Air Pollution

Pollutant Types:

Primary = emitted directly (CO, SO₂, NOₓ, PM)
Secondary = formed by reaction (ground-level O₃, PAN, acid rain)

Particulate Matter:

PM10 ≤10 µm; PM2.5 ≤2.5 µm — deep-lung, most harmful
Sources: vehicles, stubble, industry, dust

Smog (2 types):

London = SO₂ + PM + cold/fog (reducing)
Los Angeles = NOₓ + VOC + sun (photochemical, oxidising)

Acid Rain:

SO₂/NOₓ → H₂SO₄/HNO₃; pH <5.6
Damages Taj Mahal → Taj Trapezium Zone

NAAQS vs WHO (annual PM2.5):

India NAAQS = 40 µg/m³; WHO = 5 µg/m³ — 8× gap
NAAQS covers 12 pollutants

AQI & Programmes:

AQI 0–500, 6 categories, worst of 8 pollutants
NCAP (cities), GRAP/CAQM (Delhi-NCR), PMUY (indoor)

Highest-frequency themes since 2015: PM2.5 vs PM10 · primary vs secondary (ground-level ozone) · London vs LA smog · NAAQS–WHO gap. Nail these four for most Prelims + Mains marks on this topic.

Standard	Limit
PM2.5 NAAQS	40 µg/m³
PM2.5 WHO	5 µg/m³
Gap	8×
NAAQS pollutants	12 notified
AQI scale	0–500, 6 cat.

Type	Examples
Primary	CO, SO₂, NOₓ, PM, VOC
Secondary	Ground-level O₃, PAN, acid rain
PM10	≤10 µm
PM2.5	≤2.5 µm (deep lung)

Air Act	1981
NAAQS (revised)	2009
AQI launched	2015
PMUY (Ujjwala)	2016
NCAP	2019
CAQM	2021

Topic 22: Air Pollution

On this page

Conceptual Clarity — Why this Chapter Matters

1. What is Air Pollution?

1.1 Definition

1.2 Primary vs Secondary Pollutants

1.3 Legal Anchor

2. Major Air Pollutants

3. Particulate Matter — PM2.5, PM10

Don't confuse PM2.5 with PM10

4. Indoor Air Pollution

5. Smog — London vs Los Angeles Type

London-type Reducing / Classical

Los Angeles-type Oxidising / Photochemical

6. Acid Rain

7. NAAQS — India's Ambient Air Quality Standards

8. WHO Guidelines & Air Quality Index (AQI)

8.1 WHO Air Quality Guidelines (2021)

8.2 India's Air Quality Index (AQI)

9. Effects of Air Pollution

10. Current Affairs Link (2024–2026)

10.1 Programme & Monitoring Updates

10.2 Key Events, 2024–2026

10.3 Recurring Exam Hooks

11. Prelims PYQs (2015–2026)

12. Mains PYQs + Model Answers

Model Answer Framework

Model Answer Framework

Model Answer Framework

Model Answer Framework

Model Answer Framework

15-Minute Revision Box

Must-Remember Facts — Air Pollution