Environmental Issues Class 12 Notes Biology Chapter 16

Chapter at a Glance

  • Pollution is any undesirable change in physical, chemical or biological character of air, land, water or soil.
  • Pollutants are the elements, molecules and particles involved in pollution.
  • The major kinds of pollution, usually classified by environment, are air pollution, water pollution, and land pollution.


  • Air pollution refers to the undesirable change occurring in air causing harmful effects on man and domesticated species.
  • The chemical compounds that lower the air quality are usually referred to as air pollutants. These compounds may be found in the air in two major forms: in a gaseous form (as gases), and in a solid form (as particulate matter suspended in the air).
  • Primary air pollutant are those poisonous gases and undesirable substance which are released by burning fossil fuels.
  • The primary air pollutants are soot released from unburned fuel, sulphur dioxide (S02), benzopyrene (hydrocarbon) released from cigarette smoke, ammonia (NH3), oxides of nitrogen., carbon monoxide (CO), lead (Pb).
  • Secondary air pollutants are poisonous substance formed from primary air pollutants. In bright sun light nitrogen, nitrogen oxides, hydrocarbons and 02 interact to produce more powerful photochemical oxidants like ozone (03), peroxyacetyl nitrate (PAN), aldehydes, sulphuric acid, peroxides, etc. All these constitute photochemical smog, which retard photosynthesis in plants.
  • Causes of air pollution are
    • Industrialisation and automobile exhaustion.
    • Overpopulation, deforestation, nuclear explosion and explosives used in war and fireworks.
    • Volcanic eruption, electric storm and solarflares, forest fire, dust storms.
    • Pollen, spores, cyst, bacteria and marshy flares gases (methane, CH4).
  • Effect of CO: CO (carbon monoxide) is the most poisonous gas and is released from motor vehicles & industries. CO combines with haemoglobin and reduces oxygen carrying capacity of haemoglobin.
  • CO2 (carbon dioxide) causes greenhouse effects.
  • Effects of NO and NO2
    • Nitrogen monoxide (NO), like CO, lowers oxygen carrying capacity of blood.
    • Nitrogen dioxide (NO2), on contact with H2O vapour in air, forms a highly corrosive nitric acid (HNO3), which irritates eyes, nose, lungs.
    • Causes defoliation and necrosis.
  • Effects of SO2 and SO3
    • They causes respiratory diseases like asthma, bronchitis and emphysema.
    • SO2 causes chlorosis, necrosis of vegetation, metabolic inhibition of growth reduction.
    • SO2 and SO3 makes sulphurous acid (H2SO3) and sulphuric acid (H2SO4) respectively. It may precipitate as rain or snow, producing acid rain.
  • Hydrocarbons or volatile organic carbon are produced due to incomplete combustion of fuel in automobiles. They are also formed by bacterial decomposition of organic matter.
  • Methane, the most aboundant hydrocarbon in atmosphere is evolved from soil in flooded rice fields & swamps.
  • Benzene and its derivative such as formaldehyde are carcinogenic.
  • Particulate pollutants are added to air by industries and automobiles, operations like blasting, drilling, crushing, grinding and mixing. They can also be added by living organisms like pollen, spores, cysts and bacteria.
  • Control of air pollution
    • Good quality fuel like lead free or low sulphur should be used in motor vehicles.
    • Installation of equipments like gravity settling tanks or porous filters and electrostatic precipitator can minimise air pollution. The precipitator removes smaller particles.
    • Vehicles based on compressed natural gas (CNG) should be introduced. Delhi become the first city of the world to use CNG for public transport system.
    • In 1987, the Air (Prevention and Control of Pollution) Act was amended to include noise as a source of air pollution. In India this act came into force in 1981.


  • Water pollution is the deterioration of the quality of water due to addition of foreign substances, factors (heat) & deprivation that makes it health hazard, unfit for human use & aquatic organisms.
  • Sources of water pollution
    • Industrial discharge of chemical wastes & byproducts.
    • Discharge of poorly treated or untreated sewage.
    • Surface run off containing pesticides.
    • Slash & bum farming practice, which is often an element within shifting cultivation agricultural system.
    • Oil pollution: Oil is a source of pollution in sea-water. Oil pollution is due to ship accidents, loading and discharging of oil at the harbour, oil refineries and off-shore oil production. Degree of impurity of after due to organic matter is measured in terms of BOD (Biochemical Oxygen Demand). Oil cannot dissolve in water and forms a thick sludge in water. This suffocates fish, gets caught in the feathers of marine birds stopping them from flying and blocks light from photosynthetic aquatic plants.
  • BOD refers to amount of dissolved oxygen needed by bacteria in decomposing the organic waste of water. It is expressed as milligram of O/litre of waste in water. A pure water should have BOD< 1500 mg/litre.
  • When large amount of sewage is dumped into water, the BOD will increase.
  • Chemical oxygen demand (COD) test is aimed to determine the amount of oxygen needed to oxidise all pollution materials.
  • The value of COD is much higher than BOD.
  • Phosphorus and nitrates dissolved in H20 act as nutrients and accelerate the growth of algae (algal bloom) that forms a mat on water surface. This increased productivity is called eutrophication.
  • The phenonmenon through which certains pollutants gets accumulated in tissues in increasing concentration along the food chain is called biological magnification.
  • Polluted water is treated in effluent treatment plant before their release in the water bodies. Three main steps in sewage treatment are-
    • Primary treatment: It involves physical treatments such as sedimentation, floatation, fragmentation and filteration.
    • Secondary treatment: There are two ways – anoxic (through the action of anoxic micro-organisms and macromolecules) and aerobic (it occurs by two ways – trickling filter method and activated sludge methods).
    • Tertiary treatment: It includes removal of nitrates and phosphates.


  • Solid wastes, any discarded or abandoned materials, can be solid, liquid, semi-solid or containerized gaseous material.
  • Types of solid waste are: municipal solid waste, industrial solid waste and biomedical waste or hospital waste.
  • Modern landfill site is a disposal site for solid waste where the waste is buried between layers of soil, filling in low lying ground.
  • Electronic waste ore-waste, is a term for electronic products that have become unwanted, non-working or obsolete, and have essentially reached the end of their useful life.
  • E-waste can be treated or destroyed by incineration (burning in presence of oxygen), and landfills (in which the e-wastes are dumped in a trench or depression after compaction and covered with dirt every day). But this method can be dangerous as it can leach lead and other substances in soil and groundwater and contaminate them.
  • Polyblends is a fine powder of recycled modified plastics. It has been used in the development of roads in Bangalore.


  • Agrochemicals are chemical used in agriculture. It includes chemical fertilisers, herbicides, and insecticides.
  • Radioactive waste is a type of waste in gas, liquid or solid form that contains radioactive nuclear substance.
  • Organic farming is a form of agriculture, which avoids or largely excludes the use of synthetic fertilisers and pesticides, plant growth regulators & livestock feed additives.
  • Organic farmers rely on crop rotation, crop residues, animal manures & mechanical cultivation to maintain soil productivity, to supply plant nutrients & to control weeds etc.


  • Greenhouse gases such as CO2, CH4, N20 and CFC’s present in atmosphere radiate part of radio wave radiations emitted by earth back to the earth. This downflux is called greenhouse flux which keeps the earth warm and the phenomenon is called greenhouse effect.
  • Due to increased/enhanced green house effect there is increase in global mean temperature known as global warming. The global warming has many fold effects as – effect on climate, sea level change, effects on range of species distribution; food production.
  • Control of greenhouse effect
    • Less use of fossil fuel.
    • Improving efficiency of energy usage.
    • Reducing deforestation.
    • Increasing the vegetation cover, mainly the forests for photosynthesis.
    • Decreasing down growth of human population.
    • Developing substitutes for CFC.
  • Ozone layer (discovered by Charles fabry and Henri Buisson, french physicists in 1913) in stratosphere acts as ozone shield and protects earth biota from harmful effect of UV radiation.
  • Ozone (03) depletion refers to the phenomenon of reduction in the amount of ozone in the stratosphere.
  • Ozone hole is usually present from September to November each year.
  • CFC, nitrogen oxides & hydrocarbons are responsible for 03 depletion. Ozone layer depletion has various negative effects on atmosphere. UV radiation increases more on earth. In human it causes skin cancer cataract, reducing the functioning of immune systems.


  • Soil erosion may be defined as depletion of fertile top soil by over-cultivation, unrestricted grazing, deforestation and poor irrigation practices, resulting in arid patches of land. When large barren patches extend and meet over, the desert is created. Without soil and plants, the land becomes desert like and unable to support life. This process is called desertification. It is major problem now a days particularly due to increased urbanization.
  • Waterlogging and soil salinity retards growth of crop damaging agriculture. Irrigation without proper drainage of water leads to waterlogging in soil. Beside affecting crops, waterlogging draws salt to surface of soil. The salt is then deposited as thin crust on land surface or start collecting at roots of the plants.
  • Deforestation is the conversion of forested area to non-forested one.
  • Consequence of deforestation is greenhouse effect, loss of biodiversity due to habitat destruction, disturbance in hydrological cycles, soil erosion and desertification in extreme cases.
  • Conservation of forests aim at management of forests in such a way so as to maintain them at optimum form and derive optimum sustainable benefit for present as well as future generations.
  • Joint Forest Management Concept was adopted considering the extraordinary courage and dedication the local people showed in protecting the wildlife through the movements like Bishnoi’s is movement in Jodhpur and Chipko Movement in Garhwal Himalayas.

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