Face masks release water vapour, but where does it go? An early observational study

ObjectivesThe aim of this observational study was to demonstrate the behaviour and trajectory of exhaled material from an individual wearing an FFP3 mask. Valves allow material release, but we theorised that valve design may direct material downwards towards patient and surrounding environment. Limiting transmission of diseases with aerosolised spread is a current and serious concern within healthcare worldwide. Filtering face piece masks (FFP) are an essential piece of protective equipment when treating patients with ongoing infection. However, valved masks in other settings such as elective theatre and by the general public may have unforeseen negative effects. DesignA heating coil-based vaporiser was used to produce visible water vapour. A healthy test subject was filmed wearing a variety of different masks and exhaling the water vapour. ResultsFlexible pleated and solid-shell FFP masks direct exhaled material downwards in plumes exceeding 25 cm. Duckbill-shaped masks appear to direct exhaled vapour laterally, with a smaller plume. The effect is influenced by mask design and type of valve. Fluid repellent surgical masks reduce material directed downwards, and when used in conjunction with an FFP3 mask, appear to reduce the size and density of the exhaled vapour plume. The use of a visor was ineffective in reducing plume expulsion. InterpretationA properly fit-tested FFP3-rated protective mask may only moderately limit expulsion of aerosolised particles from asymptomatic healthcare workers to patients, particularly in cases where procedures are being performed in close proximity to patients or in cases where mucosal surfaces are exposed. Further research in this area is needed.