4. Population:
Population can be defined as an irreversible change in the physical, chemical or biological characteristics of air, water and land that may harmfully affect human life directly or indirectly. The substances, which pollute the environment,are called pollutants. Air pollution is mainly caused by the burning of fuels like coal, petrol, wood, kerosene in homes, smoke from cars, buses, trucks, etc., and the smoke emitted from powerhouses and industries.