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A pollutant is a substance present in the environment in concentrations that is harmful to both the environment and living organisms. Pollutants include substances (e.g. smoke), chemicals (e.g. gases, metals and salts) or factors (e.g. heat, noise and radiation).

Types of Pollutants

Based on degradability, pollutants are categorised into biodegradable and non-biodegradable pollutants.

  • Biodegradable pollutants such as market garbage, livestock wastes, municipal sewage etc. can be decomposed efficiently by the decomposers. Therefore, biodegradable pollutants are easily manageable by natural processes or in engineered systems such as the waste treatment plants. If managed properly, biodegradable wastes can be turned into useful resources.
  • Non-biodegradable pollutants such as chlorinated hydrocarbon pesticides (dichlorodiphenyl trichloroethane or DDT, benzene hexachloride or BHC etc.), waste plastic bottles, polyethylene bags, used soft drink cans, etc. are either not degraded or degraded very slowly by decomposers in nature. Therefore, non-biodegradable pollutants are difficult to manage and in most cases there is no treatment process to handle the anthropogenic input of such materials in the ecosystem.

Types of Pollution

There are five major kinds of pollution, namely

  1. air pollution,
  2. water pollution,
  3. soil or land pollution,
  4. thermal pollution and
  5. noise pollution.

Air pollution

Air pollution Air pollution is a problem particularly for the people living in large congested industrialised cities. The natural sources of air pollution are forest fires, ash from smoking volcanoes, dust storm and decay of organic matters. Pollen grains floating in air are also a natural source of pollution. Man-made sources are population explosion, deforestation, urbanisation and industrialisation. Certain activities of human beings release several pollutants to air which include oxides of nitrogen, lead, carbon monoxide, sulphur dioxide, hydrocarbons, arsenic, asbestos, radioactive matter and dust.

Air pollutants
Air pollutants can be classified into two categories, namely the primary and secondary air pollutants.
  • Primary pollutants enter into the atmosphere directly from various sources. Major air pollutants are particulate matter, carbon monoxide (CO), hydrocarbons, sulphur dioxide (SO2) and nitrogen oxides (NOx).
  • Secondary pollutants are formed during chemical reactions between primary air pollutants and other atmospheric constituents, such as water vapour. Commonly, these reactions occur in the presence of sunlight.
Effects of air pollution
  • Photochemical smog (smoke + fog = smog) occurs in urban areas receiving a large amount of sunlight and caused by photochemical reactions among nitrogen oxides, hydrocarbons and other components of polluted air that produce photochemical oxidants. Photochemical smog is composed mainly of ozone (O3), peroxyacetyl nitrate (PAN) and NOx.
  • Ozone, an effective oxidant, aggravates lung diseases in humans. It corrodes the heritage building surfaces. PAN damages chloroplasts, and thus the photosynthetic efficiency and growth of plants are reduced. In humans, PAN causes acute irritation of eyes.
  • Sulphur dioxide is oxidised to sulphur trioxide which readily dissolves in water to form sulphuric acid. Sulphuric acid washed down with rain is called acid rain. This damages field crops, buildings and marble monuments etc.
  • The two major greenhouse gases are water vapour and carbon dioxide (CO2). Other greenhouse gases include methane (CH4), ozone, chlorofluorocarbons (CFCs) and nitrous oxide (N2O).
  • The atmospheric cover around the earth acts like a window glass pane. It allows most of the solar radiations to enter right up to the earth’s surface, but does not allow a significant amount of the long-wave radiation emitted by the earth to escape in space. The greenhouse gases absorb and reflect the incoming solar radiation and also absorb and re-radiate the outgoing infrared radiation. The absorption and re-radiation of infrared radiation is called greenhouse effect.

Measures to control air pollution

  • Lead-free and sulphur-free petrol should be used.
  • Automobiles should be fitted with exhaust emission controls such as catalytic converters.
  • Harmful gases coming out from industries should be passed through filters.
  • Industries and refineries should adopt waste control measures and should be located away from residential areas.
  • Forestation should be encouraged.
  • Large-scale tree plantation along the road sides and elsewhere in the cities and towns (social forestry) reduces air pollution.
  • People should be educated about the consequences of air pollution.
  • A variety of mechanical devices are used to separate particulate matter from a moving gas stream, which works on the principle that particles are heavier than gas molecules. Some of the devices are gravity settling chambers, fabric filters, wet scrubbers and electrostatic precipitators.

Water pollution

Water pollution Water pollution may be defined as the presence of foreign organic, biological, radiological or physical substances in water that tends to lower its quality, and either constitutes a health hazard or decreases the utility of water. There are two types of sources of water pollution, namely point sources and non-point sources. Point sources include factories, power plants, underground coal mines and oil wells situated close to water source. They discharge pollutants directly into the water source. But it is generally possible to treat the pollutants before they enter the water body. Non-point sources are scattered and do not have any specific location for discharging pollutants into a particular water body. Non-points sources include agricultural fields, roads, construction sites etc. and these are difficult to monitor, regulate and treat.

Water pollutants 
The major pollutants of water are as follows:

  • Domestic wastes and sewage, including human and animal excreta.
  • Industrial wastes and effluents, including radioactive wastes.
  • Surface runoff carrying fertilisers and synthetic organic chemicals (pesticides, herbicides).
  • Metallic ions produced by mining.
  • Hot water released after cooling power plants and nuclear power stations.
  • Minerals, oils and slits.

Effects of water pollution

  • Availability of excess nutrients causes profuse growth of algae (algal bloom), especially the blue-green algae. Such algal blooms may totally cover the water surface, often release toxins in water, and sometimes cause deficiency of oxygen in the water. Thus, in bloom-infested water body, the growth of other algae may be inhibited due to toxins and aquatic animals (e.g. fish) may die due to toxicity or lack of oxygen. The process of nutrient enrichment of water and consequent loss of species diversity is called eutrophication.
  • Domestic sewage contains pathogens such as virus, bacteria, parasitic protozoa and worms. Contaminated water, therefore, can carry the germs of water-borne diseases such as cholera, typhoid, amoebiasis, jaundice etc. Such contamination may make the water unsuitable for drinking, bathing, swimming and even for irrigation.
  • Organochlorine pesticides such as DDT in the agricultural wastes are non-degradable and move along the food chains. It may lead to biological magnification—the phenomenon through which certain pollutants get accumulated in tissues in increasing concentrations along the food chain.
  • Heavy metal contamination of water can cause serious health hazards. Mercury compounds in waste water are converted by bacterial action into extremely toxic methyl mercury which can cause numbness of limbs, lips and tongue, deafness, blurring of vision and mental derangement (minamata).
  • Excess fluoride in drinking water causes teeth deformity, hardened bones and stiff and painful joints (skeletal fluorosis).
  • At many places in India, groundwater is contaminated with arsenic, mostly from naturally occurring arsenic in bedrocks. Over-exploitation of groundwater perhaps initiates leaching of arsenic from soil and rock sources and contaminates groundwater. Chronic exposure of arsenic causes black-foot disease. Arsenic causes diarrhoea, peripheral neuritis, hyperkeratosis, and also lung and skin cancer.

Measure to control water pollution

  • Sewage treatment: It is done in three steps. First, suspended materials are removed. Second, waste water is aerated, which promotes bacterial decomposition of organic compounds. This is followed by chlorination. Finally, nitrates and phosphates are removed and the water is purified.
  • Effluent treatment: Industrial effluents are suitably treated to eliminate the pollutants. The steps involve the removal of toxic chemicals, the neutralisation of acids and alkalis, the precipitation of metallic compounds and reducing the temperature of waste waters from power plants.
  • Rivers and lakes should not be used for bathing or washing as it contaminates water.
  • In towns where sewage facilities are not available, septic tanks should be made in the houses.
  • The pesticides and fertilisers should be judiciously used.
  • Water hyacinth (Eichhornia crassipes) is an aquatic weed. This plant filters out heavy metals such as cadmium, mercury, lead and nickel as well as other toxic substances.

Soil or land pollution

Soil or land pollution Any unfavourable alteration in soil by the addition or removal of substances and factors which decreases soil fertility is called soil or land pollution.

Causes of soil pollution

  • Natural agents such as water and wind constantly tend to remove the top soil and cause erosion. Rain, falling upon the unprotected top soil, washes it down into the streams. Due to the absence of plant covering, eroded soil cannot hold water. Water rushes into the rivers and overflows as flood.
  • Human beings also cause soil erosion. The growing human habitation and expansion of urban areas lead to the removal of vegetation. Once vegetation is removed, the naked soil gets exposed to wind and water.
  • Improper tillage is another cause of soil erosion. Farmers often loosen the top soil for removing weeds and preparing seed beds. They also leave agricultural fields lying fallow for a long time. These practices expose the top soil to the wind and cause erosion.
  • Radioactive wastes from nuclear testing laboratories, nuclear power plants and the radioactive fallout from nuclear explosions also contaminate the soil. Radioactive materials thrive in the soil for long periods because they usually have a long half-life.
    • Indiscriminate use of pesticides.
    • Addition of non-degradable domestic wastes such as plastic.
    • Residues from the factories.

Effects of soil pollution

  • Pesticides used in agriculture cause cancer, genetic mutations and birth defects.
  • Fertilisers used in agriculture cause bronchial problems and kidney damage.
  • Radioactive matter used in nuclear reactor leads to cancer.

Measures to control soil pollution

  • Minimising the usage of pesticides.
  • Periodic change of crops to increase the soil fertility.
  • Proper disposal of unwanted garbage created by clinics and hospitals either by burning or by burying into the soil.
  • Minimising the usage of plastics.
  • Recycling of wastes: Some of the biodegradable waste can be recycled to restore our natural resources and ecological balance. The products of recycled materials may be used again for our day-to-day activities. Recycling is a better option than either dumping or burning.

Thermal pollution

Thermal pollution It is the release of any kind of heated gases or heated water into the environment.



  • Hot water released after cooling power plants and nuclear power stations.
  • Heated gases from industries and factories.


  • The envelope of warmer air over cities adversely affects their climate.
  • The heated water released by the thermal power station or other industrial establishments into rivers and lakes causes a decrease in the oxygen dissolved in water which kills aquatic life including fish.

Measures to control thermal pollution Thermal pollution can be reduced by employing techniques such as use of cooling towers, spray ponds so as to cool the water before discharging into the water bodies.

Noise pollution

Noise pollution Noise is usually defined as any unwanted sound that produces several adverse effects on humans and other animals’ life. Improvement in human civilisation has resulted in the use of various kinds of vehicles for transport purpose. Today, such vehicles have become a major source of noise pollution. In addition, the use of microphones and other audio equipments in public places have added to the intensity of noise pollution which has a direct effect on human health. Noise is measured in terms of decibels.


Effects of noise pollution

  • It lowers the efficiency of work.
  • It disturbs sleep and leads to nervous irritability.
  • It affects human nervous system, leading to deafness, headache, blood pressure, brain disorders etc.
  • Harsh noise leads to uneasy feelings, short temper, heart disease and other related problems.

Measures to control noise pollution

  • Replacing very old machines by new ones.
  • Providing labourers in industries with ear plugs and improved sound proof instruments.
  • To interrupt the path of transmission: The rows of trees by the sides of roads, the curtains and doors in the houses etc., to some extent, reduce the noise reaching our ears.
  • Prohibiting blowing of horns near schools and hospitals in particular.
  • Parking of high-sounding vehicles should be only outside the city limits.
  • Regulation of loud speakers used in functions.

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