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 I-2 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 I-2 in infectivity assays supports a mechanism of induced displacement and capture of I-2. 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.