Aims:  To investigate the performance of an iodine‐releasing filter medium for use as a protective device against airborne pathogens. Methods and Results:  The filter’s physical and viable removal efficiencies (VRE) were investigated with challenges of MS2 bacteriophage aerosols, and the infectivity of MS2 collected on the filter was analysed. To test a proposed inactivation mechanism, media containing thiosulfate or bovine serum albumin (BSA) were put in impingers to quench and consume I2 released from the filter. In direct plating experiments, treated filters presented significantly higher VREs than did untreated filters; however, collection in excess BSA decreased VRE by half and in thiosulfate the apparent VRE decreased drastically. No significant difference in infectivity of retained viruses on treated and untreated filters was observed at the same environmental condition. Conclusions:  Evidence presented herein for competition by dissolved I2 in infectivity assays supports a mechanism of induced displacement and capture of I2. It also requires that dissociation of iodine from the filter and capture of iodine by MS2 aerosols as they pass through the filter be factored in the design of the assessment methodology. The filter’s strong retention capability minimizes reaerosolization but also makes it difficult to discriminate the antimicrobial effect at the surface. Significance and Impact of the Study:  This study shows the direct plating assay method to be sensitive to interference by iodine‐releasing materials. This requires reevaluation of earlier reports of VRE measurements.

Aims:  To assess the effectiveness of iodine‐treated biocidal filter media against bacterial spore aerosols. Methods and Results:  Bacillus subtilis spores were aerosolized and introduced into a filtration system. Both treated and untreated filters exhibited high viable removal efficiency (>99·996%) with negligible variation in pressure drop during the entire experiment. The viability of collected spores on the filter was investigated by enumeration of spores extracted from the filter by vortexing. At room temperature and low relative humidity (RH), the survival fraction of the treated filter was significantly lower than that of the untreated filter (P‐value < 0·05). Meanwhile, at room temperature and high RH and at high temperature and high RH, the survival fractions on the treated medium were statistically the same as the untreated control at room temperature and low RH. Conclusions:  Both treated and untreated filters achieved excellent viable removal efficiency for spores. The pressure drop of the treated filter was not affected by the iodine treatment. The viability of collected bacterial spores was decreased because of the exertion of iodine disinfectant. Significance and Impact of the Study:  The evaluation demonstrates that the iodine‐treated filter is a viable medium for respiratory protection against infectious spore aerosols. The results warrant further evaluation of smaller biological agents, which exhibit higher penetration.