In the atmosphere, about 30 km above the surface of the earth, the ozone molecules (O3) form an umbrella. It prevents the penetration of harmful ultra violet radiation from the sun and thus protects the life of the earth. It is now feared that there is danger of depletion of the ozone umbrella, which may occur by the use of freons and other CFC-compounds in refrigerants, coolants in domestric refrigenrators and cold storage facilities, and as filling agents in the form of plastics and in aerosol packages. On reaching the ozone umbrella, they destroy ozone molecules as a result of photochemical reactions. Over the past 16 years, the density of the ozone layer has been diminishing at an average rate of 3%. It is calculated that the depletion of ozone layer by 1% results in an increase in the incidence of skin cancer by 5% to 7%. A hole in O3 layer has been discovered in Antarctica.