Carbon　dioxide　concentration　(CO2)　is　an　important　index　for　estimating　the　air　exchange　rate　(AER).　To　clarify　the　actual　conditions　of　the　AERs　in　city-type　residences　in　China,　CO2　concentrations　were　measured　at　5-minute　intervals　during　a　period　of　one　to　two　weeks　in　six　cities　(Harbin,　Dalian,　Beijing,　Shanghai,　Wuhan,　and　Changsha)　in　summer　and　winter.　The　AERs　were　estimated　by　the　reciprocal　average　concentration　method　and　the　CO2　fitting　technique,　both　of　which　rely　on　measured　CO2　concentrations.　The　ranges　of　CO2　concentrations　in　winter　and　summer　were　400-9794　ppm　and　400-4519　ppm,　respectively.　The　difference　of　the　minimum　concentrations　was　not　remarkable　between　winter　and　summer　among　the　six　cities　because　indoor　CO2　concentrations　are　strongly　influenced　by　outdoor　CO2　concentrations.　On　the　other　hand,　for　the　maximum　concentrations,　winter　showed　CO2　concentrations　higher　than　those　in　summer.　This　finding　indicates　that　the　residents　do　not　frequently　open　their　windows　in　winter,　and　so　indoor　CO2　concentrations　trend　to　a　high　value.　As　estimated　by　the　CO2　fitting　technique,　it　became　clear　that　when　windows　were　closed,　AERs　were　0.5　h-1　or　less.　In　the　city-type　residences　in　China,　the　CO2　concentrations　at　the　time　of　sleeping　with　closed　windows　reached　a　maximum　of　10,000　ppm　around　daybreak,　since　airtightness　was　also　higher.　However,　since　windows　are　customarily　opened　in　the　daytime,　the　lowest　concentrations　in　the　daytime　were　comparable　with　the　outdoor　air　concentrations　in　both　summer　and　winter.