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It is important to control airborne particles in residential houses for protecting human health. Indoor particulate matter of <2.5 μm (PM_2.5) can be effectively monitored and managed using an air purifier. In this study, the actual clean air delivery rates in residential houses (CADR_Actual) were acquired by comparing decay rates of fine particles with and without operations of the air purifier under actual conditions, following the standard CADR of an air purifier obtained in a closed test chamber. The measurements of CADR_Actual at different outdoor PM_2.5 concentrations over a month in two residential houses revealed different airtightness levels, compared to the standardized clean air delivery rate of the air purifier (CADR_AP). Air changes per hour at 50 Pa (ACH50) was 4.8 h⁻¹ for “house A” (built in 2007) and 2.1 h⁻¹ for “house B” (built in 2018). The CADR of the air purifier used in this study was 10.6 m³/min, while the averaged CADR_Actual at the “house A” was 7.2 m³/min (approximately 66% of the CADR of the air purifier) and 9.5 m³/min at “house B” (approximately 90% of the CADR of the air purifier). Under the outdoor PM_2.5 concentrations of <35 μg/m³, the averaged CADR_Actual of house A and house B were 7.8 ± 0.3 and 9.7 ± 0.4 m³/min, respectively. However, under the outdoor PM_2.5 concentrations of >35 μg/m³, the analogous averaged concentrations were 6.8 ± 0.6 and 9.6 ± 0.3 m³/min for houses A and B, respectively. The measured CADR_Actual agreed well with the theoretical estimates of CADR_Actual acquired by the mass balance equation using the infiltration rate of ACH50/20. We also estimated CADR_Actual/CADR_AP for house C built in 2017, where the ACH50 was 1.8 h⁻¹. Overall, this study demonstrated how CADR_Actual/CADR_AP of an air purifier at residential houses can be predicted according to outdoor PM_2.5 concentration and airtightness of the house. As shown, it can be closer to 1 at lower ACH50 houses and at lower outdoor PM_2.5 concentrations.