Class 7 Biology Forests: Our Lifeline Air pollution and its control


  • Air pollution is any undesirable change in the physical, chemical or biological characteristics of air.

Harmful effects of air pollution

  • Air pollutants cause injury to all living organisms.
  • They reduce growth and yield of crops and cause premature death of plants.
  • Air pollutants also deleteriously affect the respiratory system of humans and of animals.

Cause of air pollution

Automobiles are a major cause for atmospheric pollution.

  • The use of lead-free petrol or diesel can reduce the pollutants they emit.
  • Smokestacks of thermal power plants, smelters release particulate and gaseous air pollutants with harmless gases, such as nitrogen, oxygen, etc. which must be separated before releasing the harmless gases into the atmosphere.
  • Fine particulates can be inhaled deep into the lungs which cause breathing and respiratory symptoms, irritation, inflammations and damage to the lungs and premature deaths.


Fig. smoke from thermal power plants 

Control of air pollution

  • Air pollution can be controlled by
  • Catalytic converters
  • Electrostatic precipitator
  • Scrubber
  • Catalytic convertershaving expensive metals namely platinum-palladium and rhodium as the catalysts, are fitted into automobiles for reducing emission of poisonous gases and when the exhaust passes through the catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water, and carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas, respectively.
  • Electrostatic precipitator
  • Electrostatic precipitator can remove over 99 per cent particulate matter present in the exhaust from a thermal power plant.
  • It has electrode wires that are maintained at several thousand volts, which produce a corona that releases electrons.
  • The electrons attach to dust particles giving them a net negative charge.
  • The collecting plates are grounded and attract the charged dust particles.
  • The velocity of air between the plates must be low enough to allow the dust to fall.


Fig. electrostatic precipitator 

  • Scrubber
  • A scrubber can remove gases like sulphur dioxide.
  • In a scrubber, the exhaust is passed through a spray of water or lime.


Fig. a scrubber 

  • In Delhi, Entire fleet of public transport switched over to compressed natural gas (CNG)because
  • CNG burns most efficiently
  • very little of it is left unburnt
  • CNG is cheaper than petrol or diesel
  • cannot be siphoned
  • Cannot be adulterated like petrol or diesel.
  • The problem of using CNG is the difficulty of laying down pipelines to deliver CNG through distribution points/pumps and ensuring uninterrupted supply.
  • parallel steps taken in Delhi for reducing vehicular pollution include
  • phasing out of old vehicles
  • use of unleaded petrol
  • use of low-sulphur petrol and diesel
  • use of catalytic converters in vehicles
  • Application of stringent pollution level norms for vehicles, etc.


Fig. catalytic converters

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