In general, room air cleaners are rated according to clean air delivery rate (CADR). However, ventilation systems have not yet been assessed using a metric similar to CADR. This study establishes a new mass balance equation that compares the CADRs of a ventilation system and a room air cleaner. Experiments and CFD simulations were conducted to evaluate and compare the CADRs of room air cleaners and ventilation system in the view of removing particles from indoor air. In addition, the operating cost effectiveness of ventilation system and room air cleaner was investigated. The results showed that the room air cleaners showed their performance independently, even when two or more room air cleaners were operated simultaneously. In the ventilation system, an air filter with MERV11 or higher rating was recommended to reduce the indoor particle concentration when 100% outdoor air was supplied. It was possible to select an air filter with MERV11 or lower rating when the recirculation airflow rate was increasing up to 70%. The CADR of the room air cleaner was higher than that of the ventilation system regardless of the particle size and the filter performance when the airflow rate was same. The operating cost effectiveness (CADR/kW) of the room air cleaner was higher than that of the ventilation system at the same airflow rate. Therefore, the room air cleaner must be more cost-effective than the ventilation system for reducing particle concentration indoors. (C) 2016 Elsevier B.V. All rights reserved.