Two natural products; i.e., grapefruit seed extract (GSE) and propolis, were investigated for use in antimicrobial air filters. Staphylococcus aureus was investigated as a test bioaerosol, and was deposited on the antimicrobial filters, which were treated by spraying with various areal densities of GSE and propolis. The pressure drop and particle penetration were investigated to assess the filtration performance of the bioaerosol, and the bacterialinactivation performance of the filters was evaluated by quantifying S. aureus. There was little change in the pressure drop as a function of the areal density of GSE up to 185.9 mg/ cm 2 ; however, a significant change in the pressure drop was found for the air filter coated with propolis at an areal density of 98.4 mg/cm 2 . The penetration levels of bioaerosols in both filters were uniform and in the range 1.4-2.0% (based on particle number), regardless of the areal density of the deposited GSE or propolis. The inactivation rates of the filters with identical deposition masses of GSE and propolis were similar in the ranges of 92. 1-100%, 75.2-89.1%, and 54.4-75.5% at the control filters with colony numbers of 10 3 , 10 4 , and 10 5 CFU/mL, respectively. The bacterial inactivation rate could be described by an exponential function of the areal density of GSE/propolis per number of colonies.

Antimicrobial air filters using a new natural product agent, grapefruit seed extract (GSE) have been investigated for application in air purifiers or heating, ventilation, and air-conditioning (HVAC) systems. The disk diffusion method was used to evaluate the antimicrobial activity of GSE, propolis, and shiitake against four bacteria: Staphylococcus aureus, Micrococcus luteus, Psedomonas aeruginosa, and Escherichia coli. The inactivation of S. aureus was then investigated on air filters treated with two natural products, GSE and propolis (selected for comparison to GSE) using two test methods based on different deposition weights.GSE displayed the highest antimicrobial activity against S. aureus, P. aeruginosa, and E. coli of the three natural products tested by the disk diffusion method. Similar inactivation rates of 58-67% for GSE and propolis air filters were observed at a relatively low deposition weight of 94-98 μg/cm 2 using the aerosol deposition method. However, the inactivation rate was considerably superior on the GSE air filter (~98%) compared to the propolis air filter (~75%) at deposition weights of 5000-8000 μg/cm 2 using the film attachment method. The inactivation rate for both GSE and propolis air filters could be expressed as an exponential function [in the form of 1 -exp(-ax b )] of the deposition weight per unit area of the natural product. The superior inactivation performance of the GSE air filter using the film attachment method was probably due to the high wettability of GSE for bacteria cultures, with a contact angle less than 20°. Therefore, GSE air filters will be more effective and economical than propolis air filters due to their high microbial activity and relatively low price.