Urbanization and industrialization have often resulted in whole areas of forests being cleared to gain new land and to obtain timber for the various building projects. Large areas of fields and forests have disappeared to make way for concrete jungles many of which are fitted with huge plants and chimney stacks. Industrial growth has necessitated the increased demand for fuel oil to run the machines and in doing so produces industrial gases and fumes which belch through the chimney and pollute the atmosphere. The most evident elements in the contamination of the atmosphere are dust, sulphur dioxide, carbon monoxide and nitrous oxide.