Healthy　indoor　air　quality　is　a　basic　standard　for　good　living　environment,　and　air　cleaners　are　commonly　used　in　residential　applications,　especially　in　China,　to　control　indoor　PM2.5　pollutions　from　outdoors.　An　accurate　cleaner　selection　method　will　help　keep　satisfying　indoor　air　quality,　as　well　as　reducing　cost　and　materials　of　equipment.　Three　key　parameters,　namely,　air　exchange　rate　(a),　particle　penetration　factor　(P)　and　deposition　rate　(k),　have　been　suggested　as　direct　influence　on　air　cleaner　selection　when　pollutants　are　coming　from　building　infiltration.　In　relevant　standards,　however,　there　is　no　method　that　can　link　the　three　key　parameters　to　various　airtightness　levels　of　external　windows,　which　may　often　exist　in　real　applications.　This　study,　therefore,　has　proposed　a　calculation　method　for　deciding　a,　P　and　k,　based　on　monitored　indoor　and　outdoor　PM2.5　mass　concentrations,　with　given　recommended　design　values　under　different　external　window　airtightness　levels.　Results　showed　that　both　air　exchange　rates　and　penetration　factors　were　significantly　influenced　by　the　airtightness　level　of　windows,　and　both　their　values　were　found　to　be　different　from　the　recommended　values　in　the　current　Chinese　standard,　providing　evidence　to　support　future　standard　revision　and　update.　Meanwhile,　deposition　rates　showed　good　agreement　with　the　recommended　value　in　the　current　Chinese　standard　(i.e.　0.2h(-1)).　Additionally,　with　increased　window　airtightness　level,　the　required　clean　air　delivery　rate　(CADR)　of　air　cleaners　showed　a　downward　trend,　which　means　an　air　cleaner　with　smaller　size,　lower　energy　consumption　and　less　material.