In the atmosphere (stratosphere), about 30 km above the surface of the earth, the ozone molecules $\left(\mathrm{O}_3\right)$ form an umbrella. It prevents the penetration of harmful ultra violet radiation from the sun and thus protects the life on earth. It is now feared that there is danger of depletion of the ozone layer. The prime suspects of ozone thining or depletion are chlorofluoro carbons (CFCs). Because it is odourless, invisible, that is why it is widely used in domestic refrigerators and cold storage facilities. It is calculated that the depletion of ozone layer by $1 \%$ results in an increase in the incidence of skin cancer by $5 \%$ to $7 \%$.