**3.9 Air pollution**

Air pollution can be defined in many ways, one of the most used definitions of air pollution is the occurrence of chemical compounds in the atmospheric air that are toxic and present at concentration that may be injurious to animals, vegetation and humans. Air pollution decrease the quality of air due to presence of chemicals which are not present originally but resulting of human activity and it also change the lives on earth because of global warming and depletion of ozone layer [57].
