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Air pollution  occurs when harmful or excessive quantities of substances including gases (such as  carbon dioxide ,  carbon monoxide ,  sulfur dioxide ,  nitrous oxides ,  methane  and  chlorofluorocarbons ),  particulates  (both organic and inorganic), and  biological molecules  are introduced into  Earth's atmosphere . It may cause diseases, allergies and even death to humans; it may also cause harm to other living organisms such as animals and food crops, and may damage the  natural  or  built environment . Both human activity and natural r .