Environmental Pollution
Air, water, soil and noise pollution — key pollutants and indices (AQI, BOD), primary vs secondary pollutants, the two types of smog, and eutrophication.
Key Takeaways
- Air pollutants are primary (emitted directly) or secondary (formed in the atmosphere).
- The Air Quality Index (AQI) summarises air pollution on a 0–500 scale in six categories.
- High BOD indicates high water pollution; nutrient enrichment causes eutrophication.
Core concept
Pollution is the introduction of harmful substances (or energy) into the environment at a rate faster than it can be dispersed or absorbed. It degrades air, water, soil and the acoustic environment, harming health and ecosystems.
Static foundation — air pollutants
- Primary pollutants are emitted directly — CO, SO₂, NOₓ, particulate matter (PM10, PM2.5), and lead.
- Secondary pollutants are formed in the atmosphere by reactions — ground-level ozone and PAN (peroxyacetyl nitrate).
The Four Types of Pollution
| Type | Key pollutants / indicators | Main effects |
|---|---|---|
| Air | PM2.5/PM10, SO₂, NOₓ, CO, ground-level O₃ | Respiratory & heart disease; smog; acid rain |
| Water | Sewage, heavy metals, nutrients (high BOD/COD) | Waterborne disease; eutrophication; dead zones |
| Soil | Pesticides, industrial waste, plastics, heavy metals | Loss of fertility; entry of toxins into the food chain |
| Noise | Traffic, industry, loudspeakers (measured in decibels) | Hearing loss, stress, sleep disturbance |
Classical (London) vs Photochemical (Los Angeles) Smog
| Feature | Classical / London smog | Photochemical / LA smog |
|---|---|---|
| Main cause | SO₂ + smoke + fog (coal burning) | NOₓ + VOCs + sunlight |
| Nature | Reducing (sulphurous) | Oxidising (contains ozone, PAN) |
| Season / condition | Cool, humid winters | Warm, sunny days; heavy traffic |
| Colour | Grey/black | Brownish haze |
Key Pollution Terms
Tap to reveal.
Value addition — acid rain & the greenhouse link
SO₂ and NOₓ dissolve in rain to form acid rain (sulphuric and nitric acid), which damages forests, lakes, soils and monuments (e.g., the Taj Mahal). Note the difference: ground-level ozone is a pollutant, but stratospheric ozone protects us — 'good up high, bad nearby'.
Current affairs linkage
North India's winter air-pollution crisis (stubble burning, vehicles, dust, temperature inversion) drives policy — the National Clean Air Programme (NCAP), the Commission for Air Quality Management, and GRAP. (Add the latest AQI episode or NCAP target.)
Prelims trap zones
- Ground-level (tropospheric) ozone is a harmful SECONDARY pollutant; stratospheric ozone is protective.
- High BOD means MORE pollution (more oxygen demanded), not less.
- Eutrophication is caused by nutrients (N, P), leading to oxygen DEPLETION — not enrichment of oxygen.
Knowledge Check
2 questions · check your understanding
1. A high Biochemical Oxygen Demand (BOD) in a water body indicates:
2. Ground-level ozone and PAN are examples of:
Prelims Pointers
- Ground-level ozone and PAN are secondary pollutants formed in photochemical smog.
- PM2.5 (fine particulates) is the most harmful air pollutant for human health.
- BOD (Biochemical Oxygen Demand) rises with organic pollution; eutrophication depletes dissolved oxygen.
- The CPCB sets the National Ambient Air Quality Standards (NAAQS).
Mains Angle
- 'India's cities face an air-pollution public-health emergency.' Discuss the causes and solutions.
- Explain eutrophication and its consequences for aquatic ecosystems.
Practice this topic
Browse all Environment & Ecology MCQs →Ready for Mains? Write a full answer on this topic and get it graded by our AI examiner.
Write an answerRelated topics
Ecology & Ecosystems
The fundamentals of ecology — the levels of biological organisation, the abiotic and biotic components of an ecosystem, and its types and functions.
Food Chains, Energy Flow & Ecological Pyramids
How energy and matter move through an ecosystem — food chains and webs, trophic levels, the 10% law of energy transfer, and ecological pyramids.
Biogeochemical Cycles
How nutrients circulate through the living and non-living world — the carbon, nitrogen and phosphorus cycles, and the difference between gaseous and sedimentary cycles.
Biodiversity & Hotspots
The three levels of biodiversity, why it matters, the criteria for biodiversity hotspots, and India's four hotspots as a megadiverse nation.