Influence of wearing masks on exhaled air aerodynamics.

Since aerosol inhalation is the most common mechanism for COVID-19 infection, the respiratory protective devices (RPDs) have the highest importance in personal protection. The aim of this study was to assess the efficiency of 10 different RPDs in shortening the travelling distance of exhaled air by range measurement using the schlieren imaging technique. When a RPD is worn by a person resting in a seated position, the expired air does not exceed the human convective boundary layer (CBL). Instead, the CBL lifts the expired aerosols vertically up. Thus, they have a prolonged travelling time in the surrounding air and become less harmful by several mechanisms of virus content decay. Coughing as well as expiration valves can cause far reaching expiration air clouds that cross horizontally the human CBL by opening leakage airway corridors into different directions. Measured by the range of expired air an FFP2 mask provided high security under all conditions tested. A non-vented full-face mask with two viral filters performed even better because of its airtight fit and the excellent filtering capacity of the viral filters during inspiration and expiration, even during cough manoeuvres.