Environmental biology is the applied chapter that bridges ecology and public policy. CBSE Class 12 and NEET test pollution types, greenhouse gases, ozone depletion and Indian case studies. A good answer includes the cause, the effect and the mitigation.
This is one of the most scoring chapters in biology because it is entirely conceptual — no numerical problems, no biochemical pathways. Just clear definitions, specific examples and the ability to distinguish between related concepts (primary vs secondary pollutants, BOD vs COD, ozone depletion vs global warming). If you structure your knowledge as cause-effect-solution triplets, every question writes itself.
Core Concepts
Air pollution
Sources: Vehicle exhaust, industrial emissions, stubble burning, coal power plants, brick kilns. India has some of the most polluted cities globally, with Delhi’s winter smog making international headlines.
Primary pollutants — emitted directly: CO, SO, NO, particulate matter (PM10, PM2.5), lead, volatile organic compounds (VOCs).
Secondary pollutants — formed by reactions in the atmosphere: ground-level ozone (), PAN (peroxyacetyl nitrate), photochemical smog, acid rain.
| Pollutant | Source | Health effect |
|---|---|---|
| PM2.5 | Combustion, dust | Penetrates deep into lungs, cardiovascular disease |
| CO | Incomplete combustion | Binds haemoglobin (carboxyhaemoglobin), reduces O transport |
| SO | Coal/oil burning | Respiratory irritation, acid rain |
| NO | Vehicle exhaust | Smog formation, acid rain |
| Lead | Leaded fuel, batteries | Neurological damage, especially in children |
Photochemical smog: NO + sunlight → O + PAN. Brown haze over cities in warm, sunny conditions. Causes eye irritation, respiratory problems, plant damage. Los Angeles-type smog.
Classical smog: Smoke + fog. Cold, humid conditions. Contains SO and soot. Causes bronchitis. London-type smog (Great Smog of 1952 killed ~4000 people).
Acid rain: SO and NO dissolve in rainwater to form and . Rain with pH below 5.6 (normal rain is slightly acidic at 5.6 due to dissolved CO). Effects: corrodes marble buildings (Taj Mahal yellowing), kills fish in acidified lakes, damages crops and forests.
For NEET: distinguish photochemical smog (warm + sunny, oxidising, has O and PAN) from classical smog (cold + foggy, reducing, has SO and soot). Also know that acid rain pH is below 5.6, not below 7.
Water pollution
Sources: Untreated sewage, industrial effluent, agricultural runoff (fertilisers, pesticides), oil spills.
BOD (Biochemical Oxygen Demand) — the amount of dissolved oxygen consumed by bacteria to decompose organic matter in a water sample. Higher BOD = more organic pollution.
COD (Chemical Oxygen Demand) — oxygen needed to chemically oxidise all organic and inorganic matter. COD is always BOD because it includes non-biodegradable pollutants too.
Eutrophication: Excessive nutrients (nitrates, phosphates from fertilisers and detergents) cause algal bloom → algae die → bacteria decompose them, consuming dissolved oxygen → fish and other organisms die due to oxygen depletion. This creates dead zones.
Biomagnification: Persistent, fat-soluble pollutants (DDT, mercury, PCBs) accumulate and concentrate at each trophic level. Top predators end up with concentrations thousands of times higher than the water. DDT biomagnification caused eggshell thinning in raptors (peregrine falcon, bald eagle) in the 1960s-70s.
Indian cases: Ganga pollution (industrial + sewage), Yamuna (one of the most polluted rivers globally), arsenic contamination in West Bengal groundwater, fluorosis in Rajasthan.
Solid waste and plastic pollution
India generates over 62 million tonnes of municipal solid waste per year, with only about 20% being properly treated. Plastics are the biggest challenge:
- Non-biodegradable — persist for hundreds of years
- Microplastics (<5 mm) contaminate oceans, enter food chains
- Solutions: Reduce, Reuse, Recycle (the 3 Rs), extended producer responsibility, plastic bans
E-waste is a growing problem — discarded electronics contain toxic heavy metals (lead, mercury, cadmium) that leach into soil and groundwater.
Climate change and greenhouse effect
Greenhouse gases: CO, CH, NO, water vapour, CFCs, ozone. They absorb outgoing infrared radiation and re-emit it back toward the surface, warming the planet. The natural greenhouse effect is essential (without it, Earth would be about -18°C). The problem is the enhanced greenhouse effect from excess emissions.
| Gas | Source | GWP (relative to CO) |
|---|---|---|
| CO | Fossil fuels, deforestation | 1 |
| CH | Rice paddies, cattle, landfills | ~25 |
| NO | Fertilisers, combustion | ~298 |
| CFCs | Refrigerants (now banned) | ~1000-10000 |
Global average temperature has risen about 1.1°C since pre-industrial times. Consequences: sea level rise, more extreme weather events, shifting rainfall patterns, coral bleaching, glacier retreat, species range shifts.
Paris Agreement (2015): Aims to limit warming to well below 2°C, preferably 1.5°C. Countries submit nationally determined contributions (NDCs). India pledged to reach 50% non-fossil energy capacity by 2030 and net-zero by 2070.
Ozone depletion
Covered in detail in the ozone topic hub. Key facts for this chapter: CFCs release chlorine in the stratosphere → catalytic destruction of O → Antarctic ozone hole → Montreal Protocol (1987) → recovery underway.
Sustainable development
Meeting present needs without compromising the ability of future generations to meet their own. Three pillars — economic growth, social equity, environmental protection.
Indian examples:
- Joint Forest Management (JFM) — government and local communities co-manage forests
- Chipko Movement (1970s, Uttarakhand) — villagers hugged trees to prevent logging
- Bishnoi community (Rajasthan) — traditional conservation of Khejri trees and wildlife for centuries
Radioactive pollution
Nuclear waste from power plants, weapons testing and medical procedures. Radioactive isotopes contaminate soil and water, enter food chains, and cause mutations, cancer and birth defects. Examples: Chernobyl (1986), Fukushima (2011).
Worked Examples
CFCs are inert at ground level but reach the stratosphere intact, where UV splits off chlorine. A single chlorine atom acts as a catalyst, destroying about 100,000 ozone molecules. CO does not interact with ozone at all — it is a greenhouse gas but not an ozone-depleting substance. Different pollutants, different problems.
DDT is fat-soluble and persistent. At each trophic level, it concentrates about 10-fold. Water may have 0.003 ppm DDT. Zooplankton: 0.04 ppm. Small fish: 0.5 ppm. Large fish: 2 ppm. Osprey (top predator): 25 ppm — over 8000 times the water concentration. At high levels, DDT interferes with calcium metabolism, causing thin eggshells that break during incubation.
Agricultural runoff adds nitrates and phosphates → algal population explodes (bloom) → surface becomes covered, blocking light to submerged plants → algae and plants die → bacteria decompose the dead organic matter, consuming dissolved oxygen → oxygen drops below levels needed by fish and invertebrates → mass die-off → the lake becomes a dead zone. The process can take months to years.
Industries in the Mathura refinery area release SO, which combines with moisture to form sulfuric acid. Acid rain reacts with the marble (CaCO) of the Taj Mahal: . The calcium sulphate is softer and flaky, causing the marble surface to yellow and corrode. This is sometimes called “marble cancer.”
Common Mistakes
Calling the ozone hole climate change. They are separate problems. Ozone depletion = stratospheric chemistry, caused by CFCs. Climate change = tropospheric heat trapping, caused by CO and methane. Different causes, different layers, different treaties (Montreal vs Paris).
Saying CO is the only greenhouse gas. Methane is about 25 times more potent per molecule, and NO about 298 times. Water vapour is actually the most abundant greenhouse gas but is not directly driven by human emissions.
Confusing primary and secondary pollutants. Primary = emitted directly (CO, SO, PM). Secondary = formed in the atmosphere (O, PAN, acid rain). Ground-level ozone is a secondary pollutant; stratospheric ozone is natural and protective.
Writing that sustainable development means no economic growth. It means growth that does not compromise future generations. The three pillars are economic, social and environmental — all three must be balanced.
Confusing BOD and COD. BOD = biological (bacterial) oxygen demand. COD = chemical oxygen demand. COD BOD always, because COD measures both biodegradable and non-biodegradable pollutants.
Exam Weightage and Strategy
Environmental Issues carries 4-5 marks in CBSE Class 12 boards and 1-2 NEET questions per year. The chapter is purely conceptual — no numericals. Focus on: pollution types and their effects, biomagnification, eutrophication, ozone depletion mechanism, greenhouse effect, and Indian case studies.
Build one table — pollutant, source, effect, mitigation — with about ten rows covering all pollution types. Add rows for ozone depletion, greenhouse effect and biomagnification. That single table covers most PYQs on this chapter.
Practice Questions
Q1. Distinguish between biodegradable and non-biodegradable pollutants with examples.
Biodegradable pollutants are decomposed by microorganisms and do not persist — examples: sewage, food waste, paper, animal dung. Non-biodegradable pollutants resist biological breakdown and persist in the environment — examples: DDT, plastic, heavy metals (mercury, lead), PCBs. Non-biodegradable pollutants are more dangerous because they accumulate in ecosystems (biomagnification).
Q2. What is photochemical smog? How does it differ from classical smog?
Photochemical smog forms in warm, sunny conditions when NO from vehicle exhaust reacts with sunlight and hydrocarbons to produce O, PAN and formaldehyde. It is oxidising in nature. Classical (London-type) smog forms in cold, foggy conditions from SO and soot — it is reducing in nature. Photochemical smog causes eye irritation and plant damage; classical smog causes bronchitis and respiratory issues.
Q3. What are the consequences of global warming?
(1) Sea level rise from thermal expansion and ice melt — threatens coastal cities. (2) More frequent extreme weather events (cyclones, droughts, floods). (3) Coral bleaching from ocean warming. (4) Shifting rainfall patterns affecting agriculture. (5) Species range shifts — organisms moving poleward or to higher altitudes. (6) Food and water insecurity. (7) Spread of tropical diseases to new regions.
Q4. What is the Chipko Movement? Why is it significant?
The Chipko Movement began in the 1970s in the Garhwal Himalayas (Uttarakhand) when villagers, led by Sunderlal Bahuguna and others, hugged trees to prevent commercial logging. “Chipko” means “to hug.” It was one of the earliest environmental movements in India, combining ecological awareness with livelihood protection. It led to a 15-year ban on tree felling in the Himalayan region and inspired community-based conservation movements globally.
FAQs
What is the greenhouse effect and is it bad?
The natural greenhouse effect is essential — without it, Earth’s average temperature would be about -18°C instead of +15°C. The problem is the enhanced greenhouse effect caused by human emissions of CO, methane and other greenhouse gases, which are trapping more heat than natural and causing global warming.
What is the difference between global warming and climate change?
Global warming refers specifically to the rise in average global temperature. Climate change is broader — it includes changes in precipitation, wind patterns, ocean currents, and frequency of extreme events. Global warming drives climate change, but they are not synonymous.
Is nuclear energy clean?
Nuclear power produces no CO during operation, making it low-carbon. However, it produces radioactive waste that remains hazardous for thousands of years, and accidents (Chernobyl, Fukushima) can have catastrophic consequences. Whether nuclear is “clean” depends on how you weigh carbon savings against waste and accident risks.
What is e-waste and why is it dangerous?
Electronic waste (discarded phones, computers, batteries, circuit boards) contains toxic heavy metals — lead (solder), mercury (switches), cadmium (batteries), beryllium (motherboards). When e-waste is dumped in landfills or burned, these metals leach into soil and groundwater or become airborne particles. India generates about 3 million tonnes of e-waste per year, much of it processed in informal recycling units without safety measures.
What is the difference between primary and secondary treatment of sewage?
Primary treatment is physical — screening and sedimentation remove large debris and settleable solids. Secondary treatment is biological — activated sludge process or trickling filters use aerobic bacteria to decompose dissolved organic matter, reducing BOD by 80-90%. Tertiary treatment (if done) removes specific contaminants like nitrogen, phosphorus and pathogens through chemical or advanced biological processes.
Environmental chapters reward structure — cause, effect, response. Use that template on any case study and your answer writes itself.