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Ice Breaker: Sikander loves flying kites in the open space. His mother Rani is concerned about his asthma and doesn't allow him to go out . They live near Chandini Chowk which is a part of Old Delhi and is a crowded area. Discuss the circumstances in which Sikander can start flying kites again.  
 A boy carrying a kite
Air is a mixture of various gases like Oxygen, Nitrogen, Carbon Dioxide and gases like argon, methane and ozone and water vapour are found in trace amounts. In this mixture \(78\%\) of the volume is Nitrogen , \(21\%\) is Oxygen and the remaining gases consists of the remaining \(1\%\).  If the air is contaminated by other unwanted substances called pollutants then it leads to air pollutions. These unwanted substances or pollutants cause harm to the health of living beings and also disturbs the environment. Air pollutions is excessive in places where there is heavy traffic. The other places where air pollution is high is industrial areas and densely populated areas.
Assume you are walking in your neighbourhood along with your parents. You come across an old car which is broken down. The owner of the car tries to start the car again. A thick black smoke is emitted from the car's exhaust pipe.
What can be the various gases present in the thick black smoke?
Solution: The thick black smoke is mostly carbon dioxide, carbon monoxide, nitrogen oxides and  other gases in trace amounts.
How does air get polluted?
There are numerous substances which can contaminate the air. These are called air pollutants. The air pollutants can be from smoke and dust arising natural sources like forest fires and volcanic eruptions or due to human activities. Factories, exhaust from vehicles, burning of wood and coal power plants are major sources of air pollution caused by humans. With the growing population the number of vehicles plying on the roads has increased which has caused increase in air pollution. In winter season this smoke is combined with fog and forms smog which is a thick fog like layer in the atmosphere. The smog may contain oxides of nitrogen along with air pollutants and is the leading cause of respiratory disorder like asthma in children. Carbon Monoxide is harmful gas which is produced by unburnt fossil fuels. This gas  reduces the blood's capacity to carry oxygen which can lead to death.
 India Gate
Depletion of the Ozone layer:
Chemicals like Chlorofluorocarbons (CFCs) deplete the Ozone layer in the atmosphere which protects us from the harmful ultra-violet (UV) rays from the sun. Nowadays humans have started using alternate gases instead of CFCs which is a significant development in protecting the ozone layer.
Major causes of air pollutions:
  1. Factories, petroleum refineries and power plants are major causes of air pollution as they emit gaseous pollutants like sulphur dioxide and nitrogen oxides which can cause respiratory problems and permanent lung damage.
  2. Exhaust smoke from the vehicles and the burning of wood or dung cakes are also major causes of pollution.
  3. Natural causes like forest fires and volcanic eruptions will increase the air pollution .
  4. Chloroflurorcarbons (CFCs) which are used in refrigerators, air conditioners and aerosol sprays also damage the ozone layer.