With the global spread of COVID-19 (coronavirus disease 2019), each country has been making efforts to ensure that human interactions are kept at a minimum to prevent further virus transmission. Prepandemic, people preferred spending time indoors to prevent health complications from air pollution due to increasing air particulate matter (PM), especially as these are classified as carcinogens (Group 1) by the International Agency for Research on Cancer (IARC). 1 With the COVID-19 outbreak, human mobility has been further restricted, resulting in increased occupancy ratio indoors. 2,3 Therefore, improving indoor air quality is imperative to protect human health.Air purifiers are generally used to regulate indoor air quality.Their performance is evaluated by their "clean air delivery rate (CADR)," which is expressed as the product of the air flowrate and the particle removal efficiency. 4,5 CADR indicates how quickly the PM is removed in an enclosed space. [6][7][8] As air flowrate and particle removal efficiency are negatively correlated, in which high air flowrate reduces the time for ultrafine particles to diffuse, 9-13 it is necessary to find the compromise between air flowrate and particle removal efficiency to obtain the highest CADR. The flow rate of commercial air purifier can be increased using two methods: selecting filter media with low pressure drops or using a pleated filter. For the former, the most effective approach to increase particle removal