Efficacy of HEPA Air Cleaner on Improving Indoor Particulate Matter 2.5 Concentration

High-efficiency particulate air (HEPA) filters is a potential tool used to remove fine particles and improve indoor air quality. This study aims to analyze the real-world efficacy of portable HEPA air cleaners in a household environment. Laser light dispersion PM 2.5 sensors are used to continuously monitor the indoor and outdoor PM 2.5 level before and after HEPA air cleaner filtration. Overall, HEPA air cleaners significantly reduce the indoor PM 2.5 level (33.5 ± 10.3 vs. 17.2 ± 10.7 µg/m 3 , mean difference (MD) = −16.3 µg/m 3 , p < 0.001) and indoor/outdoor PM 2.5 % (76.3 ± 16.8 vs. 38.6 ± 19.8%, MD = −37.7%, p < 0.001). The efficacy to reduce PM 2.5 is strongest in three machines with medium-flow setting group (indoor PM 2.5 MD: −26.5 µg/m 3 , indoor/outdoor PM 2.5 percentage MD: −56.4%). Multiple linear regression demonstrates that outdoor PM 2.5 , machine number, airflow speed, and window ventilation are significant factors associated with indoor PM 2.5 concentrations (R = 0.879) and percentage of the indoor/outdoor PM 2.5 ratio (R = 0.808). HEPA air cleaners can effectively improve indoor PM 2.5 air pollution. Adequate air cleaner machine numbers, appropriate airflow, and window ventilation limitations are important to achieve the best efficacy of the HEPA air cleaner.