A group of scientists created a computer model of the airflow around someone wearing a face-shield — one of those see-through plastic visors you attach to your head.
They discovered that if you're wearing a face shield and someone with COVID-19 sneezes in front of you — and towards you — it creates "vortex rings" that carry virus particles up under the bottom of your mask.
Mind you, health-care professionals likely already know this. When they wear face-shields it is usually as only one in several layers of protection — they'll have an N95 mask covered by a surgical mask covered by a face-shield, if they're lucky enough to work for a health-care institution that has good gear.
But you do sometimes see civilians wearing face-shields while out shopping, with no mask on underneath, possibly under the apprehension that it'll stop COVID-1 — or maybe prevent their own exhalations from reaching the public? Either way, this research suggests what common sense might also: It won't work very well.
Their online paper is free in full here, and includes several cool animations worth checking out — here's a gif of the sneeze vortex hitting the face shield and wrapping underneath the bottom …
The conclusion, from their paper:
It was confirmed that the airflow in the space between the face shield and the face was observed to vary with human respiration. The high-velocity flow created by sneezing or coughing generates vortex ring structures, which gradually become unstable and deform in three dimensions. Vortex rings reach the top and bottom edges of the shield and form a high-velocity entrainment flow. It is suggested that vortex rings capture small-sized particles, i.e., sneezing droplets and aerosols, and transport them to the top and bottom edges of the face shield because vortex rings have the ability to transport microparticles. Droplets and aerosols from the patient's sneeze are transported mainly by the trailing vortex rings moving downward in front of the face shield, while the leading vortex ring moving upward transports a relatively small number of droplets and aerosols. It was also confirmed that some particles (in this simulation, 4.4% of the released droplets) entered the inside of the face shield and reached in the vicinity of the nose. This indicates that a medical worker wearing a face shield may inhale the transported droplets or aerosol if the time when the vortex rings reach the face shield is synchronized with the inhalation period of breathing.