Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 12 min read
Air and air pollution
Clean dry air is about 78% nitrogen, 21% oxygen, 0.93% argon, about 0.04% carbon dioxide and traces of other gases, plus variable water vapour.
Air pollution is the presence in the atmosphere of substances in concentrations and durations that cause harm to human health, living organisms, materials or the environment. In India it is legally defined under the Air (Prevention and Control of Pollution) Act, 1981.
Sources and classification
| Basis | Classes |
|---|---|
| Origin | Natural (volcanoes, dust storms, forest fires, pollen, sea spray) and anthropogenic (industries, vehicles, power plants, domestic fuel, burning of waste and crop residue, construction dust) |
| Source geometry | Point (stacks), line (highways), area (cities, industrial estates) |
| Mobility | Stationary (industries, power plants) and mobile (vehicles, aircraft, ships) |
| Formation | Primary pollutants — emitted directly (SO₂, NOx, CO, particulates, hydrocarbons); secondary pollutants — formed in the atmosphere by reactions (ozone, PAN — peroxyacetyl nitrate, sulphuric acid mist, secondary nitrate and sulphate aerosols) |
| Physical state | Particulate (dust, fumes, smoke, mist, fog, fly ash, aerosols) and gaseous |
Particulate matter terms: dust (solid particles from mechanical processes), fumes (fine solids from condensation of vapours, e.g. metal fumes), smoke (fine particles from incomplete combustion), mist (liquid droplets), fly ash (fine ash from combustion), PM₁₀ (aerodynamic diameter ≤ 10 µm — inhalable) and PM₂.₅ (≤ 2.5 µm — fine, penetrates deep into the lungs).
Major pollutants and their effects
| Pollutant | Main sources | Effects |
|---|---|---|
| Particulate matter (PM₁₀, PM₂.₅) | Vehicles, road and construction dust, combustion, industries, biomass burning | Respiratory and cardiovascular diseases, reduced visibility, soiling |
| Sulphur dioxide (SO₂) | Burning of coal and oil (power plants, smelters) | Respiratory irritation; acid rain; damage to vegetation (chlorosis and necrosis of leaves); corrosion of materials |
| Oxides of nitrogen (NOx) | High-temperature combustion — vehicles, power plants | Respiratory effects; acid rain; precursor of photochemical smog and ozone |
| Carbon monoxide (CO) | Incomplete combustion — vehicles, stoves | Combines with haemoglobin (about 200 times more strongly than oxygen) forming carboxyhaemoglobin — reduces oxygen delivery; headache, dizziness, death at high levels |
| Ozone (O₃) (ground-level, secondary) | NOx + VOCs + sunlight | Eye and lung irritation, crop damage, rubber cracking |
| Hydrocarbons / VOCs (e.g. benzene) | Vehicles, solvents, fuel evaporation | Smog precursors; benzene is carcinogenic |
| Lead | Earlier from leaded petrol; smelters, batteries | Neurotoxic — especially harmful to children |
| Ammonia | Agriculture, fertiliser plants | Secondary particle formation, irritation |
| Fluorides | Aluminium smelters, phosphate fertiliser plants | Fluorosis in cattle, damage to vegetation |
| Benzo(a)pyrene, arsenic, nickel | Combustion, industries | Carcinogenic |
Indoor air pollution — from cooking with biomass and kerosene, tobacco smoke, radon, formaldehyde and VOCs — is a major health risk, especially for women and children in rural households.
Bhopal gas tragedy (1984) — leak of methyl isocyanate (MIC) from a pesticide plant; one of the worst industrial disasters, which led to stronger environmental and liability laws in India.
Global and regional issues
Acid rain
SO₂ and NOx oxidise to sulphuric and nitric acids that fall with rain (pH below about 5.6). Effects: acidification of lakes and fish kills, forest damage, leaching of soil nutrients, corrosion of buildings and monuments (e.g. marble cancer concerns for the Taj Mahal).
Greenhouse effect and global warming
Greenhouse gases — CO₂, CH₄, N₂O, CFCs/HFCs, water vapour, tropospheric ozone — absorb outgoing infrared radiation and warm the earth. Increasing concentrations cause global warming, sea-level rise, changing rainfall patterns and extreme events. Gases differ in global warming potential (GWP) relative to CO₂.
Ozone layer depletion
Stratospheric ozone absorbs harmful UV-B radiation. CFCs, halons, carbon tetrachloride, methyl chloroform release chlorine and bromine that destroy ozone (the Antarctic ozone hole). Controlled under the Montreal Protocol (1987); the Kigali Amendment (2016) phases down HFCs (potent greenhouse gases). Ozone thickness is measured in Dobson units.
Smog
| Classical (London / sulphurous) smog | Photochemical (Los Angeles) smog |
|---|---|
| Smoke + fog + SO₂ | NOx + hydrocarbons + sunlight → O₃, PAN, aldehydes |
| Cool, humid winter conditions; reducing | Warm, sunny conditions; oxidising |
| Respiratory distress (London, 1952) | Eye irritation, respiratory effects, crop damage |
National Ambient Air Quality Standards (NAAQS, 2009)
Notified by CPCB for 12 pollutants (values for industrial, residential, rural and other areas):
| Pollutant | Annual average | 24-hour average |
|---|---|---|
| SO₂ (µg/m³) | 50 | 80 |
| NO₂ (µg/m³) | 40 | 80 |
| PM₁₀ (µg/m³) | 60 | 100 |
| PM₂.₅ (µg/m³) | 40 | 60 |
| Ozone (µg/m³) | — | 100 (8-hour), 180 (1-hour) |
| Lead (µg/m³) | 0.5 | 1.0 |
| CO (mg/m³) | — | 2 (8-hour), 4 (1-hour) |
| Ammonia (µg/m³) | 100 | 400 |
| Benzene (µg/m³) | 5 | — |
| Benzo(a)pyrene (ng/m³) | 1 | — |
| Arsenic (ng/m³) | 6 | — |
| Nickel (ng/m³) | 20 | — |
(Stricter annual limits apply to SO₂ and NO₂ in ecologically sensitive areas.)
Air Quality Index (AQI)
India's National AQI (launched 2014) combines up to eight pollutants (PM₁₀, PM₂.₅, NO₂, SO₂, CO, O₃, NH₃, Pb) into one number; the overall AQI is the highest sub-index.
| AQI | Category |
|---|---|
| 0–50 | Good |
| 51–100 | Satisfactory |
| 101–200 | Moderately polluted |
| 201–300 | Poor |
| 301–400 | Very poor |
| 401–500 | Severe |
National Clean Air Programme (NCAP) — launched in January 2019 to reduce particulate pollution in non-attainment cities through city action plans (targets have been revised over time; check current MoEFCC documents).
Air pollution meteorology
Lapse rates and stability
- Environmental lapse rate (ELR) — actual rate of temperature decrease with height.
- Dry adiabatic lapse rate (DALR) — cooling of a rising dry air parcel ≈ 9.8 °C per km (about 1 °C per 100 m).
- Wet adiabatic lapse rate — lower (about 6 °C per km) due to latent heat release.
| Condition | Atmosphere |
|---|---|
| ELR > DALR (super-adiabatic) | Unstable — strong vertical mixing (good dispersion) |
| ELR = DALR | Neutral |
| ELR < DALR (sub-adiabatic) | Stable — little vertical mixing |
| Temperature increases with height (negative lapse rate) | Inversion — very stable; traps pollutants |
Inversions: radiation inversion (clear calm nights — ground cools rapidly; common in winter mornings), subsidence inversion (sinking air in high-pressure systems; persistent, at elevation).
Maximum mixing depth (height) — the height up to which convective mixing occurs; low mixing heights (winter nights and mornings) cause high pollution levels.
Plume behaviour
| Plume | Atmospheric condition | Remarks |
|---|---|---|
| Looping | Super-adiabatic (highly unstable) | Large eddies bring the plume to the ground intermittently — high short-term ground concentrations near the stack |
| Coning | Neutral or slightly stable | Cone-shaped spread; reaches ground farther away |
| Fanning | Strong inversion/stable (at stack height) | Spreads horizontally, little vertical spread; low ground concentration unless terrain rises |
| Lofting | Inversion below the stack, unstable above | Pollutants dispersed upwards — most favourable |
| Fumigation | Inversion above the stack, unstable below | Pollutants pushed down to the ground — most unfavourable (morning break-up of a night inversion) |
| Trapping | Inversions both above and below the stack | Plume confined between layers |
Dispersion and stack height
= emission rate; = wind speed at stack height; , = dispersion coefficients (depend on distance and stability class); = effective stack height (physical height + plume rise).
Maximum ground-level concentration on the centreline occurs approximately where .
Plume rise increases with exit velocity and gas temperature (buoyancy) — e.g. Holland's and Briggs' formulas.
- Based on SO₂ emission: — in kg/h of SO₂
- Based on particulate emission: — in tonnes/h of particulates
- The larger value is adopted, subject to a minimum stack height (commonly 30 m).