Notes Environmental Issues Class 12 Biology

Notes Class 12

Please refer to the Environmental Issues Notes Class 12 Biology given below. These revision notes have been designed as per the latest NCERT, CBSE, and KVS books issued for the current academic year. Students will be able to understand the entire chapter in your class 12th Biology book. We have provided chapter-wise Notes for Class 12 Biology as per the latest examination pattern.

Revision Notes Chapter 16 Environmental Issues Class 12 Biology

Students of Class 12 Biology will be able to revise the entire chapter and also learn all important concepts based on the topic-wise notes given below. Our best teachers for Grade 12 have prepared these to help you get better marks in upcoming examinations. These revision notes cover all important topics given in this chapter.

  • Odum defined pollution as, ‘undesirable change in physical, chemical and biological properties of air, water and soil, which directly or indirectly affect human beings’. 
  • The basic cause of pollution is ever increasing rise in human population that is putting an equally increasing demand for more food, water supply, roads, transportation, dwelling  units, schools, hospitals, electricity, more industrial products and a large number of other commodities. This is exerting pressure on natural resources which are, hence, undergoing depletion and degradation.
  • A pollutant is any substance, chemical or other factor that changes the natural balance of environment.

Classiffcation of pollutants

  • On the basis of their existence in nature, form (in which the pollutants persist after release), and nature of disposal, pollutants are of various types.
  • They various classes of pollutants on the basis of:

(a) Existence
(i) Quantitative: Those components which become pollutants when their concentration reaches beyond a threshold value, e.g., CO, CO2, NO2.
(ii) Qualitative: Do not occur naturally in nature, but are passed through human activities, e.g., DDT and other pesticides.

(b) Persistence
(i) Primary pollutants: Persist in form in which they are added to the environment, e.g., plastic ware, DDT.
(ii) Secondary pollutants: Formed by interactions among primary pollutants; more toxic than primary pollutants, e.g., peroxyacyl nitrates (PAN) and ozone.

(c) Nature of disposal
(i) Biodegradable: Degrades by natural means, such as heat or microorganisms e.g., sewage, domestic wastes.
(ii) Non-biodegradable: Either do not degrade or degrade very slowly in nature, e.g., mostly inorganic compounds (like salts of heavy metals, radioactive materials etc.) DDT, polythene bags, etc.

Air pollution is the occurrence or addition of foreign particles, gases or pollutants in the air which have an adverse effect on human beings, animals, vegetation etc.

Causes of Air Pollution

Notes Environmental Issues Class 12 Biology
Notes Environmental Issues Class 12 Biology
Notes Environmental Issues Class 12 Biology

Acid rain
Acid rain is the rainfall and other forms of precipitation with a pH of less than 5. pH of normal rain is 5.6 – 6.5. Acid rain is caused by large scale emission of acidic gases into the atmosphere. Oxides of sulphur and oxides of nitrogen may react with water vapour to formsulphuric acid (H2SO4) and nitric acid (HNO3) respectively. Acid rain causes reduced rates of photosynthesis and growth, and increased sensitivity to drought and disease. Marble statues and buildings are corroded by acid rain e.g., Taj Mahal in India is affected by the acid rain.

Water pollution is the deterioration of the quality of water due to addition of foreign substances (inorganic, organic, biological, radiological), factors (heat) and deprivation that makes it health hazard, unfit for human use and aquatic organisms. Water pollutants may be of three types – biological (various pathogens e.g., viruses, bacteria, protozoa,  algae, etc.); chemical (organic wastes, organic biocides, inorganic chemicals); and physical (hot water, oil spills, etc).

Sources of Water Pollution

Notes Environmental Issues Class 12 Biology

Effects of Water Pollution
(i) Increase in Biological oxygen demand (BOD): BOD is the oxygen in milligrams required for five days in one litre of water at 20°C for the microorganisms to metabolise organic waste. When large amount of sewage is dumped into water, the BOD will increase and thus, dissolved oxygen (DO) will decrease.
(ii) Algal bloom: They excess growth of planktonic algae that causes colouration of water is called algal bloom. It deteriorates water quality. 
(iii) Eutrophication: Phosphorus and nitrates of fertilisers and detergents dissolve in water and accelerate growth of algae which form mat on the water surface. They algal growth deoxygenates water which is responsible for death of fishes and other aquatic organisms. They increased productivity of lake and pond etc. due to nutrient enrichment is called eutrophication.
Eutrophication causes reduction in dissolved oxygen.
1. Natural eutrophication occurs slowly at a rate which may not be detectable in human life time.
2. Accelerated or cultural eutrophication occurs due to human activities like passage off sewage and run of from fertilised fields into ponds, lakes and other water bodies.
(iv) Biomagniffcation: They phenomenon, through which certain pollutants, such as pesticide, run of, get accumulated in tissues in increasing concentration along the food chains, and produce fatal effect is called biomagniffcation or biological magniffcation.

 Harmful effects on human health

  • Water-borne infectious diseases like cholera, dysentery, typhoid, jaundice and worm infection are the major public health problems in developing countries.
  • Excess quantities of heavy metals such as mercury, lead, cadmium, nitrate, fkuoride and chlorine are harmful to health in many ways as discussed in the table.

Harmful effects of metal contamination of water

Notes Environmental Issues Class 12 Biology
Notes Environmental Issues Class 12 Biology

Polluted water is treated in effluent treatment plants before its release into water bodies. Theyre are three steps in sewage treatment :

Sewage/Efluent treatment plant
(i) Primary treatment : Physical treatments such as sedimentation, floatation, fragmentation and filtration are involved in primary treatment, to remove floating and large suspended solids.
(ii) Secondary treatment : Theyre are two ways in secondary treatment : anoxic (it is through the action of anoxic microorganisms and macromolecules) and aerobic (it is by two ways : trickling filter
method and activated sludge method).
(iii) Tertiary treatment : Cleared water is chlorinated or irradiated with UV rays to kill pathogens and for removal of nitrates and Phosphates

Notes Environmental Issues Class 12 Biology


  • Discharge of hot waste water produced by industries, thermal power plants and oil refineries into water bodies.
  • As a consequence of thermal pollution, temperature of water bodies rises and dissolved oxygen content decreases.
  • It affects aquatic life, as organisms sensitive to high temperature get killed. 

Pollution of air, water and soil with radioactive material is called as radioactive pollution. It affects all the organisms including humans.

Notes Environmental Issues Class 12 Biology
  • Control of radioactive pollution : Leakageof radioactive elements from nuclear reactors, laboratories and industries producing, processing or using them should be totally checked. All safety measures for this purpose should be strictly enforced. Radioactive wastes should be changed into harmless form or stored in safe place where they may gradually decay in a harmless manner.


  • Soil pollution is the alteration in soil caused by removal or addition of substances and factors which decrease its productivity or quality. Pesticides, fertilisers, chemicals and radioactive wastes are the main source of soil pollution. Pesticides include insecticides, fungicides, algicides, rodenticides and weedicides. Alongwith target organisms, they harm nontarget organisms as well this destroys soil ecosystem. 
  • Control of soil pollution : Soil pollution can be checked by reducing the disposal wastes, appropriate use of chemical fertilisers and use of biological pest control. They most important measure to check land degradation is restoration of forest, crop rotation, improved drainage, etc.


  • Solid wastes are discarded or left over solid materials. They various sources of solid wastes are municipal wastes, industrial wastes, mining wastes, hazardous wastes, defunct ships and electronic wastes.
  • Control of solid wastes : Disposal of waste consists of recovery and recycling, source reduction, burning and dumping .Burning is combustion of solid waste containing organic compounds in open places. Incineration is controlled aerobic combustion of wastes inside chambers at temperature of 900-1300°C. Pyrolysis is combustion inside chambers in the absence of oxygen at a temperature of 1650°C. It does not yield pollutants. In source reduction, garbage and other organic wastes are taken out of urban areas and used for formation of compost, biogas and manure. 

Three Rs of Waste Management
They three Rs of waste management are reduce, reuse and recycle. They first R relates to reduction of waste, second R relates to the reuse of various articles and the third R relates to recycling by collecting the waste articles and transforming them into the new products.


  • Unpleasant loud sound is called as noise (also called slow killer) and disturbing level of noise is known as noise pollution. Noise pollution is measured in decibels. Generally sound above 80 dB is termed as noise. A sound more than 115 dB is harmful to the ears. Various sources of noise pollution include industries, social events, transportation, construction activities, etc.
  • A number of temporary physiological changes occur in the human body as a direct result of noise exposure. Theyse are a rise in blood pressure, a rise in intracranial pressure, an increase in heart rate and breathing and an increase in sweating. A sudden loud noise of 150dB or more may cause permanent loss of hearing.
  • Control of noise pollution : Green muffler scheme involves growing green plants along roadsides to reduce noise pollution. Specific legislation and regulations should be proposed for designing and operation of machines, vibrations control, sound-proof cabins and use of sound-absorbing materials.


  • Stratosphere has a thick layer of ozone (good ozone) called ozonosphere which protects life on earth from harmful effects of UV radiations. Thickness of ozone is measured in Dobson units (D.U).
  • Ozone in the earth’s atmosphere is generally created by ultraviolet light striking oxygen molecules, which consist of two oxygen atoms (O2), creating two single oxygen atoms, known as atomic oxygen. They atomic oxygen then combines with a molecule of Oto create ozone, O3. In stratosphere, ozone is formed and photodissociated. It dissipates the energy of UV radiations.
  • Ozone absorbs most of the ultraviolet radiation, so it shields earth against biologically harmful  solar radiations. 
  • Ozone depleting substances (ODS) are substances which react with ozone present in the stratosphere and destroy the same. They ozone layer is destroyed by aerosols which are certain chemicals released into the air with force in the form of mist or vapour. Major aerosol pollutant present in jet plane emissions is fluorocarbon. Besides chloroflourocarbons  or CFCs (CCl2F2 and CCl3F; used as coolants in air conditioners and refrigerators, cleaning solvents, aerosol propellants and foam insulation), nitrogen oxides (coming from fertilisers) and hydrocarbons are also responsible for O3 depletion.
Notes Environmental Issues Class 12 Biology

International Initiatives for Mitigating Ozone Depletion

  • Montreal Protocol – They Montreal Protocol was a convention signed in 1987 by 27 industrialised countries. It is a landmark international agreement to protect the stratospheric ozone and to limit the production and use of ozonedepleting substances.
  • In 1999, an amendment named as Beijing amendment was passed on substances that deplete the ozone layer. They amendment lays down steps to reduce emission of HCFCs in developing and developed countries.
  • Earth Summit (United Nations Conference on Environment and Development, 1992) – It was held in Rio-de-Janerio (Brazil) and adopted the recommendations of CCC (Convention on Climate Change) for reducing greenhouse gases. In 2012, a second Earth Summit (Rio + 20) was also held in Rio-de-Janerio. This conference focused on two themes :
    (i) green economy in the context of sustainable development and poverty eradication
    (ii) the institutional framework for sustainable development.
  • Kyoto Protocol (Dec. 1997) – International conference held in Kyoto, Japan obtained commitments from different countries for reducing overall greenhouse gas emissions to an average of 5% against 1990 level in the period from 2008-2012. According to new commitments ,  parties committed to reduce greenhouse gas emission by atleast 18% below 1990 levels in the eight-year period from 2013 to 2020.


  • A greenhouse is a glass house that is used for growing plants specially during winter. They glass panels let the light in but do not allow the heat to escape, warming up the greenhouse. 
  • Similar warming effect is observed in atmosphere, where certain gases like carbon dioxide trap the heat and does not let it escape.  These gases are termed as greenhouse gases.
  • They various greenhouse gases are CO2 (warming effect 60%), CH4 (20%), chlorofluorocarbons or CFCs (14%) and N2O (6%). Others of minor significance are water vapours and ozone. 
  • Greenhouse gases (GHGs) or radiatively active gases are essential for keeping the earth warm and hospitable. Theyy prevent a substantial part of long wave radiations emitted by earth to escape into space. Rather greenhouse gases radiate a part of this energy back to the earth. They phenomenon is called greenhouse flux. Greenhouse flux maintains the mean annual temperature of earth nearly 15°C. In its absence it will fall to –18°C.
  • Scientists believe that increased input of COand other greenhouse gases into the atmosphere will increase the earth’s natural greenhouse effect and raise the average global temperature of the atmosphere near the earth’s surface. This enhanced greenhouse effect is called as global warming.
  • Some strategies should be followed to deal with global warming : Vegetation cover should  be increased for photosynthetic utilisation of carbon dioxide; chlorofluorocarbon should be replaced with some other substitute having little effect on global warming; reducing the use of nitrogen fertilisers to reduce nitrous oxide emission; minimising the use of fossil fuel to reduce the greenhouse gas emission, etc.


  • Deforestation is removal, decrease or deterioration of forest cover of an area. It is caused due to jhum cultivation, hydrolectric projects, forest fires, human establishments, road construction, overgrazing, mining and for timber production.
  • Its results in soil erosion, desertification, drought, loss of plant and animal habitats, ecosystem disturbance, global warming etc.
  • A Bishnoi woman Amrita Devi showed exemplary courage by hugging a tree and daring king’s men to cut her first before cutting the tree. In her honour, the Government of India instituted the Amrita Devi Bishnoi Wildlife Protection Award for individuals or communities from rural areas that have shown extraordinary courage and dedication in protecting wildlife. During Chipko Movement of Garhwal Himalayas, local women showed enormous bravery in protecting trees from the axe of contractors by hugging them.
  • They Government of India in 1980s has introduced the concept of Joint Forest Management (JFM) so as to work closely with the local communities for protecting and managing forests. In return for their services to the forest, the communities get benefit of various forest products (e.g., fruits, gum, rubber, medicine, etc.) and thus, the forest can conserved in a sustainable manner. 
Notes Environmental Issues Class 12 Biology