Characterization of silver biocides and composites in commercial face masks with implications for risk assessment

Silver-based biocides applied in fabric-based mouth- and nose-covering face masks require characterization due to the potential toxic effects of silver to which users may be exposed. In the absence of reliable silver release data in realistic usage conditions, current safety assessment of face masks relies on a safe-by-design principle. To contribute to the refinement of specifications of safe face masks, types of particulate silver biocides actively applied in face masks on the market were identified and characterized. Ultramicrotomy followed by scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) analysis showed that a wide variety of silver-based biocides were applied. This includes metallic silver nanoparticles (NPs), silver salts in NP form, silver ion exchangers, and notably, various silver nanocomposites with other particulate materials such as synthetic amorphous silica (SAS), TiO₂, and ZnO. These composites are added to face masks to combine different modes of biocidal action or to facilitate gradual silver release, thereby extending biocidal activity. In five out of seven silver-containing masks, silver-containing NPs were identified on the surface of fibers. Additionally, significant numbers of other NPs (SAS, TiO₂) were found coating the fiber surfaces. Sizes of silver-containing NPs ranged from 3 to 200 nm across all masks, with the large majority of particles below 50 nm. These findings imply that for safety assessment, no-adverse effect levels of all incorporated compounds should be taken into account and that the effects of co-exposure to multiple compounds should be considered. The completion of particle release studies, exposure assessments, and regulatory oversight of face masks is recommended.