Environmental Issues
Welcome to HSLC Guru! In this lesson, we present a complete English-medium guide to Class 12 Biology Chapter 16 — Environmental Issues, designed strictly for ASSEB (Assam State School Education Board) students. This chapter explores the major environmental problems caused by human activities, their consequences, and the steps taken at the national and global level to control them. You will learn about air, water and soil pollution, greenhouse effect, ozone depletion, deforestation and waste management. The notes include a clear summary, short and long answer questions, MCQs, fill-in-the-blanks, true/false statements and a glossary of important terms — all crafted to help you score full marks in your HSLC and Higher Secondary examinations.
Summary
Air Pollution and its Control: Air pollution is the contamination of the atmosphere by harmful substances such as smoke, dust, fumes and gases. The main pollutants are particulate matter (PM) and gaseous pollutants like SO₂, NOx, CO and hydrocarbons. Particulate pollutants are removed using electrostatic precipitators (ESP), which can remove over 99% of particles by giving them a negative charge through electrode wires and collecting them on positively charged plates. Scrubbers remove gases like SO₂ by passing exhaust through a spray of water or lime. In automobiles, catalytic converters fitted in exhaust pipes convert unburnt hydrocarbons into CO₂ and water, and CO and NO into CO₂ and N₂. To curb pollution, the Government of India enacted the Air (Prevention and Control of Pollution) Act in 1981 and introduced Bharat Stage (BS) emission standards for vehicles. The famous Delhi smog case study shows how the conversion of public transport (buses, three-wheelers, taxis) to Compressed Natural Gas (CNG) drastically reduced air pollution in the capital.
Water Pollution and its Control: Water pollution is mainly caused by domestic sewage, industrial effluents, agricultural run-off and thermal discharge. Domestic sewage contains organic biodegradable wastes that increase the Biochemical Oxygen Demand (BOD) of water. High BOD indicates more organic load and lower dissolved oxygen, leading to fish kills. Excess nutrients (nitrates and phosphates) from sewage and fertilisers cause eutrophication — the natural ageing of lakes, accelerated by human activity. This produces dense algal blooms that release toxins and deplete oxygen. The Water (Prevention and Control of Pollution) Act was passed in 1974. To clean major rivers, the Ganga Action Plan (1985) and Yamuna Action Plan (1993) were launched to reduce sewage discharge and set up treatment plants.
Soil Pollution, Agro-chemicals and Solid Waste: Soil pollution is caused by excessive use of agro-chemicals (chemical fertilisers, pesticides, herbicides), industrial waste and improper waste disposal. These chemicals contaminate groundwater and enter the food chain, causing biomagnification. Radioactive wastes from nuclear plants are extremely hazardous and must be stored in shielded containers buried deep underground. Solid waste management is a serious challenge — wastes are classified as biodegradable (food, paper, agricultural waste) and non-biodegradable (plastics, glass, metals). E-waste (electronic waste) from computers and mobiles contains toxic metals like lead, cadmium and mercury and is often recycled in developing countries by burning, posing severe health risks. Sanitary landfills, composting and recycling are recommended methods for waste disposal.
Greenhouse Effect, Ozone Depletion and Deforestation: The greenhouse effect is the warming of Earth caused by greenhouse gases — CO₂ (60%), CH₄ (20%), CFCs (14%), N₂O (6%) and water vapour — which trap infrared radiation. Their increasing concentration is causing global warming, leading to melting glaciers, rising sea levels and erratic weather. Ozone depletion occurs in the stratosphere when CFCs (chlorofluorocarbons) release Cl atoms that destroy ozone (O₃) molecules, forming the “ozone hole” over Antarctica. The Montreal Protocol (1987) banned CFCs, and the Kyoto Protocol (1997) aimed to reduce greenhouse gas emissions. Deforestation — the clearing of forests for agriculture and timber — includes slash-and-burn (jhum) cultivation common in the North-East. To protect forests, the Chipko movement was started in 1974 in Garhwal, with leaders like Sundarlal Bahuguna hugging trees to prevent felling. The Government of India launched the Joint Forest Management (JFM) programme in 1980 to involve local communities in forest protection.
Very Short Answer Questions (1 Mark)
Q1. What is the full form of ESP?
Answer: ESP stands for Electrostatic Precipitator, used to remove particulate matter from industrial exhaust.
Q2. Expand BOD.
Answer: BOD stands for Biochemical Oxygen Demand, the amount of oxygen required by bacteria to decompose organic matter in water.
Q3. Name the gas mainly responsible for the greenhouse effect.
Answer: Carbon dioxide (CO₂) contributes about 60% to the greenhouse effect.
Q4. In which year was the Air Act passed?
Answer: The Air (Prevention and Control of Pollution) Act was passed in 1981.
Q5. Who led the Chipko movement?
Answer: Sundarlal Bahuguna led the Chipko movement, started in 1974 in Garhwal, Uttarakhand.
Q6. What is jhum cultivation?
Answer: Jhum is a slash-and-burn shifting cultivation practised in North-East India, where forest land is cleared, burnt and farmed for a few years.
Q7. Which protocol banned the use of CFCs?
Answer: The Montreal Protocol of 1987 banned the use of CFCs to protect the ozone layer.
Q8. Name a non-biodegradable waste.
Answer: Plastic, glass and metals are non-biodegradable wastes that do not decompose naturally.
Q9. What is e-waste?
Answer: E-waste is electronic waste from discarded computers, mobile phones and other electronic devices, containing toxic metals.
Q10. What is the layer of the atmosphere where ozone depletion occurs?
Answer: Ozone depletion occurs in the stratosphere, about 15-35 km above the Earth’s surface.
Short Answer Questions (2-3 Marks)
Q1. Explain how an electrostatic precipitator works.
Answer: An electrostatic precipitator (ESP) removes more than 99% of particulate matter from exhaust gases of thermal power plants. It contains electrode wires that are maintained at several thousand volts, producing a corona that releases electrons. These electrons attach to dust particles, giving them a negative charge. The collecting plates, which are grounded and positively charged, attract the charged particles. The particles are then removed by mechanical rapping. A blower controls the flow of air between the plates so that pollutants can be effectively trapped.
Q2. Differentiate between biodegradable and non-biodegradable waste with examples.
Answer: Biodegradable wastes are materials that can be broken down into simple substances by the action of microorganisms. Examples include food waste, paper, leaves, agricultural residues and cow dung. Non-biodegradable wastes cannot be decomposed naturally and persist in the environment for a long time. Examples are plastics, glass, polythene, metals, aluminium cans and synthetic fibres. Biodegradable wastes can be composted, while non-biodegradable wastes must be recycled or sent to sanitary landfills.
Q3. What is eutrophication? How does it harm aquatic life?
Answer: Eutrophication is the natural ageing of a lake by nutrient enrichment of its water, which is greatly accelerated by human activities such as discharge of sewage, fertilisers and detergents. Excess nutrients (mainly nitrates and phosphates) cause rapid growth of algae, forming algal blooms that cover the water surface. When these algae die, decomposing bacteria consume large amounts of dissolved oxygen, leading to oxygen depletion. Aquatic animals such as fish suffocate and die, and the lake gradually turns into a marsh or land.
Q4. Discuss the Delhi smog and the role of CNG in reducing air pollution.
Answer: Delhi was once one of the most polluted cities in the world due to vehicular emissions, smoke and dust. To control this, the Supreme Court of India directed the conversion of all public transport — buses, three-wheelers and taxis — to run on Compressed Natural Gas (CNG) by the end of 2002. CNG burns more efficiently than petrol or diesel, leaves no unburnt residue and produces very little harmful gas. As a result, the levels of CO₂ and SO₂ dropped significantly. Other measures included phasing out old vehicles, applying Bharat Stage emission standards and encouraging use of unleaded petrol.
Q5. What is the greenhouse effect? Name the major greenhouse gases.
Answer: The greenhouse effect is the natural process by which certain gases in the atmosphere trap the heat radiated by Earth’s surface, keeping the planet warm enough to support life. However, increased emission of greenhouse gases due to human activity is intensifying this effect, leading to global warming. The major greenhouse gases are: carbon dioxide (CO₂) — 60%, methane (CH₄) — 20%, chlorofluorocarbons (CFCs) — 14%, nitrous oxide (N₂O) — 6%, and water vapour. CO₂ is the principal contributor.
Q6. Write a short note on the Chipko movement.
Answer: The Chipko movement was a non-violent forest protection movement that began in 1974 in the Garhwal region of Uttarakhand. Local women and villagers, led by Sundarlal Bahuguna, hugged trees to prevent contractors from cutting them down. The word “chipko” means “to hug” or “to embrace”. The movement raised national awareness about deforestation and led the Government of India to ban tree felling in the Himalayas above 1000 m for 15 years. It inspired similar movements such as Appiko in Karnataka and laid the foundation of community forestry in India.
Long Answer Questions (5-7 Marks)
Q1. Describe in detail the causes, effects and control measures of air pollution.
Answer: Air pollution is the contamination of the atmosphere by harmful gases, smoke and particles that affect the quality of air. Causes: burning of fossil fuels in vehicles, thermal power plants and factories; emissions from industries (SO₂, NOx, CO); domestic combustion; agricultural burning; and natural sources such as forest fires and volcanic eruptions. Effects: respiratory diseases (asthma, bronchitis, lung cancer); reduced visibility (smog); acid rain that damages soil, lakes and monuments; depletion of ozone layer; and global warming. Control Measures: (i) Particulate pollutants are removed using electrostatic precipitators and scrubbers in industries. (ii) Vehicles use catalytic converters containing platinum-palladium-rhodium that convert unburnt hydrocarbons, CO and NO into harmless gases. (iii) Use of unleaded petrol and CNG. (iv) Planting trees to absorb CO₂. (v) Implementation of Air Act 1981 and Bharat Stage emission norms. The Delhi CNG conversion is a successful example of pollution control.
Q2. Explain water pollution. What measures have been taken in India to prevent it?
Answer: Water pollution refers to the contamination of water bodies by harmful substances. Sources: (i) Domestic sewage — contains pathogens, organic waste, soaps and detergents; raises BOD of water. (ii) Industrial effluents — contain toxic chemicals, heavy metals (Hg, Pb, Cd) and dyes. (iii) Agricultural run-off — fertilisers and pesticides leach into rivers. (iv) Thermal pollution — hot water from power plants reduces dissolved oxygen. (v) Oil spills from ships. Effects: spread of diseases (cholera, typhoid, jaundice), eutrophication, biomagnification of toxins, fish kills and destruction of aquatic ecosystems. Indian Measures: (a) Water (Prevention and Control of Pollution) Act, 1974, which established the Central Pollution Control Board (CPCB). (b) Ganga Action Plan (1985) for cleaning the river Ganga by treating sewage from major cities. (c) Yamuna Action Plan (1993) with similar objectives. (d) Construction of effluent treatment plants by industries. (e) Use of bioremediation and constructed wetlands.
Q3. What is ozone depletion? Discuss the cause, effects and steps taken to control it.
Answer: The ozone layer is present in the stratosphere (15-35 km above Earth) and absorbs harmful UV radiation from the sun. The thickness of ozone is measured in Dobson Units (DU). Ozone depletion is the thinning of this layer. Cause: Chlorofluorocarbons (CFCs) released from refrigerators, air conditioners, aerosol sprays and foam packaging rise to the stratosphere where UV radiation breaks them, releasing chlorine atoms. One Cl atom can destroy 1,00,000 ozone molecules through a chain reaction: Cl + O₃ → ClO + O₂; ClO + O → Cl + O₂. The “ozone hole” over Antarctica is a serious example. Effects: increased UV-B radiation causes skin cancer, cataract, suppression of immunity, damage to phytoplankton and reduced crop yield. Control: The Montreal Protocol (1987) banned the production and use of CFCs. Substitutes such as HFCs and HCFCs are being used. India has also signed and implemented the protocol.
Q4. Write a detailed note on solid waste management including e-waste.
Answer: Solid wastes are unwanted solid materials produced from human activities. They are classified as biodegradable (food remains, paper, leaves, agricultural waste) and non-biodegradable (plastics, glass, metals, polythene). Hospital waste includes infectious materials, syringes and pathological waste, while e-waste consists of discarded computers, mobile phones, televisions and other electronic items. Methods of Management: (i) Sanitary landfills — wastes are dumped in low-lying areas, compacted and covered with soil. (ii) Composting — biodegradable waste is converted into manure by microbes. (iii) Vermicomposting using earthworms. (iv) Incineration for hospital waste. (v) Recycling of plastics, glass and metals. (vi) 3R principle — Reduce, Reuse, Recycle. E-waste: contains lead, cadmium, mercury and brominated flame retardants. It is shipped from developed countries to India, China and Pakistan, where it is recycled by burning, releasing toxic fumes. Proper recycling under the E-Waste (Management) Rules, 2016 is essential to recover precious metals safely.
Q5. Discuss deforestation. What are its causes, consequences and prevention measures?
Answer: Deforestation is the conversion of forested areas to non-forested ones. India has lost a major part of its forest cover in the last century. Causes: (i) Slash-and-burn (jhum) cultivation in tribal areas of the North-East. (ii) Conversion of forests to agricultural land. (iii) Timber harvesting and fuelwood collection. (iv) Industrialisation, mining and dam construction. (v) Urbanisation and road building. (vi) Forest fires. Consequences: enhanced CO₂ in the atmosphere causing global warming; loss of biodiversity; soil erosion and desertification; disturbance of the hydrological cycle; reduction in rainfall; and displacement of tribal communities. Prevention Measures: (i) Reforestation and afforestation programmes. (ii) Joint Forest Management (JFM) programme (1980) involving local communities in protecting and managing forest resources. (iii) The Chipko movement (1974, Sundarlal Bahuguna) in Garhwal, where villagers hugged trees. (iv) Strict implementation of the Forest Conservation Act 1980. (v) Use of alternative fuels like biogas and LPG. (vi) Encouraging agroforestry and social forestry. These steps help maintain the ecological balance for present and future generations.
Multiple Choice Questions (MCQs)
Q1. Which of the following devices is used to remove particulate matter?
(a) Catalytic converter
(b) Electrostatic precipitator
(c) Scrubber
(d) Compressor
Answer: (b) Electrostatic precipitator
Q2. The Air (Prevention and Control of Pollution) Act was passed in:
(a) 1972
(b) 1974
(c) 1981
(d) 1986
Answer: (c) 1981
Q3. BOD stands for:
(a) Biological Oxygen Density
(b) Biochemical Oxygen Demand
(c) Biotic Oxygen Distribution
(d) Basic Oxygen Demand
Answer: (b) Biochemical Oxygen Demand
Q4. Which gas contributes the most to the greenhouse effect?
(a) CH₄
(b) N₂O
(c) CO₂
(d) CFC
Answer: (c) CO₂
Q5. The Montreal Protocol is associated with:
(a) Greenhouse gas reduction
(b) Ozone layer protection
(c) Water conservation
(d) Wildlife protection
Answer: (b) Ozone layer protection
Q6. The Chipko movement was started in:
(a) 1970
(b) 1972
(c) 1974
(d) 1980
Answer: (c) 1974
Q7. Catalytic converters are fitted in:
(a) Power plants
(b) Automobiles
(c) Refrigerators
(d) Sewage plants
Answer: (b) Automobiles
Q8. The Ganga Action Plan was launched in:
(a) 1981
(b) 1985
(c) 1987
(d) 1993
Answer: (b) 1985
Q9. Eutrophication is mainly caused by:
(a) Acid rain
(b) Excess nutrients
(c) Heavy metals
(d) Radioactive waste
Answer: (b) Excess nutrients
Q10. The Kyoto Protocol was adopted in:
(a) 1985
(b) 1987
(c) 1992
(d) 1997
Answer: (d) 1997
Fill in the Blanks
Q1. The full form of CNG is __________.
Answer: Compressed Natural Gas
Q2. Ozone depletion takes place in the __________ layer of the atmosphere.
Answer: stratosphere
Q3. The Water Act was passed in __________.
Answer: 1974
Q4. __________ are used in vehicles to reduce gaseous pollutants.
Answer: Catalytic converters
Q5. The thickness of the ozone layer is measured in __________ units.
Answer: Dobson
True or False
Q1. CFCs are responsible for ozone layer depletion.
Answer: True
Q2. High BOD indicates clean water.
Answer: False (High BOD indicates more organic pollution)
Q3. The Chipko movement was led by Sundarlal Bahuguna.
Answer: True
Q4. Plastic is a biodegradable waste.
Answer: False (Plastic is non-biodegradable)
Q5. The Montreal Protocol was signed in 1987.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Pollution | Undesirable change in the physical, chemical or biological characteristics of air, water and soil |
| Particulate Matter (PM) | Tiny solid or liquid particles suspended in air, harmful to lungs |
| Electrostatic Precipitator (ESP) | Device used to remove particulate matter from industrial flue gases |
| Scrubber | Device that removes gaseous pollutants like SO₂ by passing them through a spray of water or lime |
| Catalytic Converter | Device fitted in automobile exhaust to convert harmful gases into harmless ones |
| CNG | Compressed Natural Gas, a clean fuel used in vehicles |
| BOD | Biochemical Oxygen Demand — oxygen required to decompose organic matter in water |
| Eutrophication | Nutrient enrichment of water leading to algal bloom and oxygen depletion |
| Algal Bloom | Rapid growth of algae on water surface due to excess nutrients |
| Biomagnification | Increase in concentration of toxic substances at successive trophic levels |
| E-waste | Electronic waste from discarded electronic devices |
| Greenhouse Effect | Trapping of heat by atmospheric gases like CO₂, CH₄, CFCs and N₂O |
| Global Warming | Rise in Earth’s average surface temperature due to greenhouse gases |
| Ozone Hole | Region of severely depleted ozone over Antarctica |
| CFC | Chlorofluorocarbon, the major cause of ozone depletion |
| Dobson Unit (DU) | Unit used to measure thickness of ozone layer |
| Montreal Protocol (1987) | International treaty to phase out ozone-depleting substances |
| Kyoto Protocol (1997) | International agreement to reduce greenhouse gas emissions |
| Deforestation | Removal of forest cover for agriculture, timber or development |
| Jhum | Slash-and-burn shifting cultivation practised in North-East India |
| Chipko Movement | 1974 forest protection movement led by Sundarlal Bahuguna in Garhwal |
| Joint Forest Management (JFM) | Programme involving local communities in protecting forests |
| Ganga Action Plan | Plan launched in 1985 to clean the river Ganga |
| Yamuna Action Plan | Plan launched in 1993 to clean the river Yamuna |
| Bharat Stage Norms | Indian emission standards for motor vehicles |
| Air Act 1981 | Indian law for prevention and control of air pollution |
| Water Act 1974 | Indian law for prevention and control of water pollution |
| Radioactive Waste | Hazardous waste from nuclear plants requiring deep underground storage |
| Sanitary Landfill | Engineered disposal site for non-recyclable solid wastes |
| Composting | Process of converting biodegradable waste into manure by microbes |
| Smog | Mixture of smoke and fog containing harmful pollutants |
| Acid Rain | Rainfall with high acidity due to dissolved SO₂ and NOx |
This chapter is essential for ASSEB Class 12 Biology students preparing for board examinations. Mastering Environmental Issues helps you understand the urgent need for sustainable living and conservation of our natural resources.
Keep practising the questions above and revise the glossary regularly to score full marks in the HSLC and Higher Secondary examinations conducted by ASSEB. Stay connected with HSLC Guru for chapter-wise notes, MCQs and previous year solutions for all subjects.