Astheworldisgettingmoreandmoredenselypopulated,airpollutionhasbecomeasevereproblem.Airpollutionmainlyderivesfromfourmajorhumanactivitysectors:industry,energy,transportationandagriculture.Energiesareneededtorunfactories,topowertrains,planesandbuses.Nearlyalloftheseenergiesareproducedbyburningfuels,whichwouldcauseairpollution.Scientistsarestudyingnewwaysofgeneratingelectricitythatcanbelessdamagingtotheenvironment.Theincreasedpublicenvironmentalawarenesshasledtotheadventofactivitiessuchasrecycling.