Factors influencing bacterial viability on face masks and bactericidal effect of disinfection methods

Face masks are contaminated with bacteria during daily use and storage. Several studies have already been conducted on face mask decontamination and bacterial survival during the pandemic. However, there are limited reports on methods of decontamination that can be implemented in daily life. In this study, we inoculated commercially available nonwoven face masks with Staphylococcus aureus and/or Staphylococcus epidermidis which are the main indigenous bacteria of human skin to obtain knowledge on the prevention of bacterial infection when wearing face masks in daily life, and investigated factors affecting bacterial viability on face masks and the bactericidal effect of different disinfection methods. Commercially available face masks were inoculated with two bacterial species and the effects of temperature, humidity, washing, contamination (sweat, sebum, saliva, protein, and cosmetics), and initial bacterial count and density on the viability of adherent bacteria were examined. After each bacterium was collected from the face mask, colonies were measured by the agar plate dilution method and the number of viable colonies. Various experiments were also performed to determine how to disinfect bacteria on face masks. Bacterial survival of face masks increased in the presence of artificial saliva and 1% peptone. By sealing the face mask after 70% ethanol spraying, the number of bacteria was reduced by 6 log order of magnitude and no colonies were detected. However, the presence of contaminants on the face mask weakened the bactericidal effect. Furthermore, direct exposure of the face masks to sunlight was suggested as a useful method for bacterial disinfection. Our findings revealed factors affecting bacterial viability on face masks, providing data that can be applied for the effective household disinfection of face masks.