Size-dependent efficiencies of ultrafine particle removal of various filter media

Ultrafine particles (UFPs) are hazardous air pollutants produced by human activities in outdoor and indoor environments. Filtration in mechanical ventilation systems or air purifiers is an effective technology for removal of outdoor-or indoor-originated particles, to reduce human exposure to UFPs in indoor environments, where people spend most of their time. The filtration efficiency of filter media is critical to performance, and the efficiency significantly depends on particle diameter. Thus, the size-dependent efficiencies of 47 commercial filter media with 17 filter classes were measured at typical air flow velocities. The pressure drops at different velocities were also measured to obtain the ratios of pressure drops to media velocities. Total efficiencies of UFP removal were estimated based on the size-dependent efficiencies and size distributions of UFPs in three typical cases. The average efficiency of the UFP removal of coarse filters, fine filters, high-efficiency particulate air filters, and ultra-low particulate air filters in typical cases were 10.85%, 76.87%, 99.37%, and 99.92%, respectively. The maximum range of the total efficiency was 18.17%, which was significantly smaller than that of size-dependent efficiency (45.15%). The efficiencies for particles with the mean diameter and mean value of the total efficiency of the UFPs were similar, with absolute differences of less than 5%. These results can provide guidance for selection of filter media under the comprehensive consideration of air quality and energy consumption and offer reference values for the evaluation of new filter media as well as basic data for the assessment of human exposure to UFPs.