Theconcentration(集中)ofpeople,economicactivityandenergydemandintheworld’s
growingcitiesmeansthatpoorairqualityisoftenregardedasanurbanproblem.Yetpoorair
qualityalsoaffectsmanyrural(乡村)communities,particularlywherehouseholds(家庭)
continuetorelyonsolidbiomassforcookingandkeroseneforlighting.Moreover,themajor
pollutants–includingsecondary(二次)pollutantsformedbychemicalreactionsinthe
atmosphere–canbetransportedlargedistancesfromtheirsources.