Environment Health & Air Pollution

Air pollution is a major modifiable determinant of non-communicable and respiratory disease, and a perennial favourite in NEET PG Community Medicine. This topic blends environmental science (pollutant sources, indices, standards) with clinical toxicology (CO poisoning, SO2 vs NO2). Master the pollutant–effect pairings, NAAQS cut-offs, and the classic smog episodes.

Definition & Scope

Air pollution is the presence in ambient air of one or more contaminants (gases, particulates, biological molecules) in quantities and durations that are injurious to human, animal, or plant life, or that interfere with the comfortable enjoyment of life and property.

Broadly classified by source and location:

• Outdoor (ambient) air pollution — from vehicular exhaust, industry, power plants, construction dust, crop-residue burning.
• Indoor air pollution (IAP) — predominantly from combustion of solid biomass fuels (wood, dung, crop residue, coal) for cooking and heating, plus tobacco smoke, radon, building materials.

Pollutants are also classed as:

Class	Definition	Examples
Primary pollutants	Emitted directly from a source	SO2, NO, CO, particulate matter, hydrocarbons, lead
Secondary pollutants	Formed in the atmosphere by chemical reactions of primaries (often photochemical)	Ozone (O3), PAN (peroxyacetyl nitrate), NO2 (partly), sulphuric acid, aldehydes

High-yield: Ozone is the indicator/index pollutant of photochemical smog. It is a secondary pollutant — it is NOT emitted directly but formed by sunlight acting on NO2 and hydrocarbons. This is one of the most repeated single-best-answer items.

The Major Outdoor Pollutants

Particulate Matter (PM10 and PM2.5)

Particulate matter is graded by aerodynamic diameter:

Particle	Diameter	Deposition site	Significance
PM10	≤10 µm ("respirable", coarse)	Upper airways, deposit down to bronchi	Thoracic fraction
PM2.5	≤2.5 µm (fine)	Reach alveoli / gas-exchange zone	Most strongly linked to cardiopulmonary mortality
Ultrafine	<0.1 µm	Cross alveolar membrane → systemic circulation	Emerging cardiovascular risk

High-yield: PM2.5 penetrates deepest (alveoli) and carries the greatest health risk — used as the primary metric for the WHO Air Quality Guidelines and Indian AQI. PM2.5 is the dominant driver of attributable cardiovascular and respiratory deaths.

PM2.5 effects: ischaemic heart disease, stroke, COPD exacerbation, lung cancer, low birth weight, increased all-cause mortality.

Gaseous Pollutants — the Examiner's Core Comparison

Pollutant	Source	Solubility / level of action	Key clinical effect
SO2	Coal/fuel combustion, smelting	Highly water-soluble → absorbed in upper airways	Upper-airway irritation, bronchoconstriction; chronic bronchitis; acid rain
NO2	High-temperature combustion, vehicles, gas stoves	Less soluble → reaches lower airways/alveoli	Bronchiolitis, deep lung injury, pulmonary oedema ("silo-filler's disease"); reduces resistance to infection
CO	Incomplete combustion	Binds haemoglobin	Tissue hypoxia (see below)
Ozone (O3)	Secondary, photochemical	Deep lung	Airway inflammation, ↓FEV1, asthma trigger

High-yield: SO2 = water-soluble = UPPER airway irritant; NO2 = less soluble = penetrates to LOWER airways/alveoli causing bronchiolitis and delayed pulmonary oedema. This SO2-vs-NO2 distinction is a classic discriminator question.

Silo-filler's disease is acute NO2 toxicity from fermenting silage producing nitrogen dioxide — causes acute and delayed (after a latent symptom-free interval of hours) pulmonary oedema and bronchiolitis obliterans. Eponym worth remembering.

Carbon Monoxide (CO)

A colourless, odourless gas from incomplete combustion (faulty heaters, vehicle exhaust, biomass smoke, fires).

Mechanism — multi-hit hypoxia:

1. CO binds haemoglobin with ~200–250× the affinity of oxygen → forms carboxyhaemoglobin (COHb).
2. Reduces oxygen-carrying capacity.
3. Left-shifts the oxyhaemoglobin dissociation curve → impairs release of the remaining O2 to tissues.
4. Binds cytochrome oxidase → blocks cellular respiration.

Clinical features by COHb level (approx):

COHb %	Features
<10%	Usually asymptomatic (smokers up to 5–10%)
10–20%	Headache, throbbing temples
20–30%	Headache, irritability, fatigue, impaired judgement
30–40%	Severe headache, nausea, vomiting, dizziness
40–50%	Confusion, syncope, tachycardia
>50%	Seizures, coma, cherry-red discolouration, death

High-yield: Cherry-red skin/mucosa and cherry-red lividity are classic of CO poisoning (also of cyanide poisoning). Pulse oximetry is falsely normal/high because standard oximeters cannot distinguish COHb from O2Hb — diagnosis needs co-oximetry / arterial COHb measurement.

Management of CO poisoning: Remove from source → 100% high-flow oxygen (reduces CO half-life from ~4–5 h on room air to ~60–90 min). Hyperbaric oxygen for severe cases (LOC, COHb >25%, pregnancy, neurological signs, cardiac involvement) — cuts CO half-life to ~20–30 min and reduces delayed neuropsychiatric sequelae.

Lead, Hydrocarbons, and Others

• Lead — historically tetraethyl lead in petrol (now banned in India); causes neurodevelopmental impairment, anaemia. Phase-out is a public-health success story.
• Hydrocarbons / VOCs — precursors of photochemical smog, some carcinogenic (benzene).

Smog: London Type vs Los Angeles Type

A central comparison question. "Smog" = smoke + fog.

Feature	London (Classical / Reducing) smog	Los Angeles (Photochemical / Oxidising) smog
Chief pollutants	SO2 + particulates (smoke)	Ozone, NO2, PAN, aldehydes
Temperature	Cold, humid, early morning, winter	Warm, dry, midday, sunlight, summer
Mechanism	Coal burning + fog	Sunlight + vehicular NOx + hydrocarbons
Chemistry	Reducing	Oxidising (photochemical)
Main effect	Bronchitis, respiratory deaths	Eye irritation, ↓FEV1, asthma, rubber cracking
Index pollutant	SO2	Ozone
Classic event	Great London Smog, December 1952 (~4,000+ deaths)	Los Angeles

Temperature inversion is the key meteorological enabler:

Normal lapse rate → air cools with altitude, warm surface air rises and disperses pollutants upward. Inversion → a warm layer sits above a cooler surface layer, acting as a lid: vertical mixing stops → pollutants are trapped near ground → concentrations rise → smog episode.

High-yield: Temperature inversion traps pollutants near the ground and is the meteorological trigger for severe smog episodes (e.g., London 1952). It occurs typically on cold, calm, clear nights/early mornings.

Indoor Air Pollution

A massive contributor to disease burden in India, especially among rural women and children.

Chief source: incomplete combustion of solid biomass fuels (wood, dung-cakes, crop residue, coal) on inefficient chulhas in poorly ventilated kitchens. Combustion releases PM2.5, CO, NO2, SO2, formaldehyde, polycyclic aromatic hydrocarbons (PAHs, carcinogenic), and benzopyrene.

Health effects of biomass smoke:

• Acute lower respiratory infections (ALRI / pneumonia) in under-5 children — a leading killer.
• COPD and chronic bronchitis in non-smoking women.
• Lung cancer (notably with coal smoke).
• Low birth weight, stillbirth (antenatal exposure).
• Cataract, tuberculosis association.

High-yield: Biomass fuel smoke is a major cause of COPD in non-smoking rural women and of ARI/pneumonia in under-5 children in India. The Indian policy response is the Pradhan Mantri Ujjwala Yojana (PMUY) providing LPG connections to BPL households to reduce IAP.

Other indoor pollutants: environmental tobacco smoke (passive smoking), radon (a radioactive gas seeping from soil/building stone — a recognised cause of lung cancer), asbestos, formaldehyde from furnishings, house-dust mites, moulds.

Sick Building Syndrome (SBS)

High-yield: Sick Building Syndrome = a constellation of non-specific symptoms (headache, mucous-membrane and eye irritation, fatigue, difficulty concentrating, dry throat) experienced by occupants of a (usually air-conditioned, poorly ventilated) building, that improve on leaving the building and have no identifiable specific cause/disease.

Contrast with Building-Related Illness (BRI) — a specific, diagnosable disease with an identifiable cause (e.g., Legionnaires' disease from contaminated cooling towers/AC, hypersensitivity pneumonitis, humidifier fever). Symptoms of BRI do not necessarily resolve on leaving the building.

National Ambient Air Quality Standards (NAAQS), India

Set by the Central Pollution Control Board (CPCB) under the Air (Prevention and Control of Pollution) Act, 1981; current standards notified 2009, covering 12 pollutants. Key annual values (industrial/residential):

Pollutant	Annual standard	24-h standard
PM2.5	40 µg/m³	60 µg/m³
PM10	60 µg/m³	100 µg/m³
SO2	50 µg/m³	80 µg/m³
NO2	40 µg/m³	80 µg/m³
CO	—	2 mg/m³ (8-h); 4 mg/m³ (1-h)
Ozone	—	100 µg/m³ (8-h); 180 µg/m³ (1-h)
Lead	0.5 µg/m³	1.0 µg/m³

High-yield: WHO 2021 air quality guideline for PM2.5 is 5 µg/m³ (annual) and 15 µg/m³ (24-h) — far stricter than the Indian NAAQS of 40 µg/m³. The downward WHO revision (from 10 to 5 µg/m³) is a frequently updated fact.

National Air Quality Index (AQI) — launched in India 2015 under the slogan "One Number – One Colour – One Description." It collapses 8 pollutants (PM2.5, PM10, NO2, SO2, CO, O3, NH3, Pb) into a single 0–500 scale:

AQI	Category	Colour
0–50	Good	Green
51–100	Satisfactory	Light green
101–200	Moderate	Yellow
201–300	Poor	Orange
301–400	Very Poor	Red
401–500	Severe	Maroon

National Clean Air Programme (NCAP), 2019 — national framework targeting reduction of PM2.5/PM10 concentrations (revised goal: up to 40% reduction by 2025–26 relative to 2017 levels) in non-attainment cities.

Health Effects: Stepwise Mechanistic Approach

Inhalation of pollutant → particle/gas deposition (depth governed by size & solubility) → local airway inflammation + oxidative stress → systemic inflammation → endothelial dysfunction → cardiovascular + respiratory + carcinogenic outcomes.

Major attributable conditions: ischaemic heart disease, stroke, COPD, lower respiratory infection, lung cancer, type 2 diabetes (emerging), adverse birth outcomes. Air pollution is consistently among the top global risk factors for death and DALYs, with PM2.5 the dominant component.

Diagnosis & Investigations (Clinical Toxicology Angle)

• CO poisoning: co-oximetry for COHb (not pulse oximetry, which reads falsely normal). ABG may show metabolic acidosis (lactate). ECG for ischaemia.
• Ambient monitoring: continuous ambient air-quality monitoring stations (CAAQMS) measure PM, SO2, NO2, O3, CO feeding the AQI.
• High-volume sampler — classic instrument for measuring suspended particulate matter (SPM/PM10).

Management & Prevention

Hierarchy of control (most → least effective):

1. Source control / substitution — clean fuels (LPG via Ujjwala), electric/CNG vehicles, BS-VI emission norms, banning crop-residue burning, leaded-petrol phase-out.
2. Engineering controls — scrubbers, electrostatic precipitators, catalytic converters, tall chimneys, improved chulha (smokeless stove) design, kitchen ventilation/smoke hoods.
3. Regulatory/administrative — Air Act 1981, NAAQS, NCAP, GRAP (Graded Response Action Plan in Delhi-NCR), odd-even vehicle schemes.
4. Personal protection — N95/respirator masks, air purifiers, limiting outdoor exertion on high-AQI days (least effective, last resort).

CO poisoning drug/intervention of choice: 100% normobaric oxygen, escalating to hyperbaric oxygen for severe/high-risk cases.

Complications

• Acute: smog-episode mortality surges, acute CO toxicity (death, delayed neuropsychiatric syndrome), asthma/COPD exacerbations, silo-filler's pulmonary oedema.
• Chronic: COPD, lung cancer, cardiovascular disease, stroke, reduced lung growth in children, low birth weight.

Key Differentials / Comparisons to Keep Straight

• CO vs cyanide poisoning — both cause cherry-red skin and inhibit cellular respiration; cyanide gives "bitter almond" odour and very high venous O2 saturation.
• SBS vs BRI — non-specific & resolves on leaving (SBS) vs specific diagnosable disease (BRI, e.g., Legionnaires').
• London vs LA smog — reducing/winter/SO2 vs oxidising/summer/ozone.
• SO2 vs NO2 — upper vs lower airway.

Recently asked / exam angle

• Index/indicator pollutant of photochemical (LA) smog → Ozone. (Repeatedly asked; ozone is a secondary pollutant.)
• Cherry-red colour of skin → carbon monoxide poisoning (and cyanide).
• Which pollutant penetrates deepest into the lung → PM2.5 / fine particulates reaching alveoli.
• SO2 affects upper airway (water-soluble); NO2 affects lower airway/alveoli (less soluble).
• Temperature inversion → traps pollutants → smog (London 1952 episode).
• Major source of indoor air pollution in rural India → biomass fuel combustion; causes ARI in children and COPD in women.
• Sick Building Syndrome definition and contrast with Legionnaires' disease (BRI).
• WHO PM2.5 annual guideline = 5 µg/m³ (post-2021 revision).
• AQI launched 2015; NCAP 2019; NAAQS under Air Act 1981 set by CPCB.
• Silo-filler's disease → NO2.

Rapid revision

1. Ozone = index pollutant of photochemical/LA smog and is a SECONDARY pollutant (sunlight + NO2 + hydrocarbons).
2. PM2.5 reaches alveoli and is the strongest predictor of cardiopulmonary mortality; PM10 is "respirable" coarse fraction.
3. SO2 = water-soluble = upper-airway irritant; NO2 = less soluble = lower airway/alveolar injury (silo-filler's disease, delayed pulmonary oedema).
4. CO affinity for Hb ~200–250× that of O2; left-shifts the O2 dissociation curve; cherry-red skin; pulse oximetry falsely normal — use co-oximetry.
5. Treat CO poisoning with 100% O2; hyperbaric O2 for severe cases (LOC, COHb >25%, pregnancy, cardiac/neuro signs).
6. London smog = reducing, winter, SO2 + smoke (1952, thousands dead); LA smog = oxidising, summer, ozone/PAN.
7. Temperature inversion (warm layer over cool) traps pollutants → smog episode.
8. Biomass fuel smoke is the chief indoor air pollutant in India → ARI in under-5s, COPD/lung cancer in non-smoking women; addressed by PMUY (Ujjwala LPG).
9. Sick Building Syndrome = non-specific symptoms relieved on leaving the building, no identifiable cause; Building-Related Illness (e.g., Legionnaires') is specific and diagnosable.
10. WHO 2021 PM2.5 guideline = 5 µg/m³ annual; Indian NAAQS PM2.5 = 40 µg/m³ annual.
11. NAAQS under Air Act 1981, set by CPCB (12 pollutants, 2009); AQI launched 2015 (8 pollutants, 0–500, six colour bands); NCAP launched 2019.
12. Radon = radioactive indoor pollutant causing lung cancer; lead phased out of petrol — a public-health success.