Visualizing droplet dispersal for face shields and masks with exhalation valves

TL;DR

This study visualizes how face shields and masks with exhalation valves perform in blocking aerosol droplets, revealing limitations that could impact COVID-19 mitigation efforts and suggesting the use of traditional masks instead.

Contribution

It provides qualitative visualizations demonstrating the aerosol dispersal around face shields and exhalation valves, highlighting their reduced effectiveness compared to regular masks.

Findings

Face shields allow droplets to spread around the visor.

Exhalation valves permit unfiltered droplets to pass through.

Traditional masks are more effective for source control.

Abstract

Several places across the world are experiencing a steep surge in COVID-19 infections. Face masks have become increasingly accepted as one of the most effective means for combating the spread of the disease, when used in combination with social-distancing and frequent hand-washing. However, there is an increasing trend of people substituting regular cloth or surgical masks with clear plastic face shields, and with masks equipped with exhalation valves. One of the factors driving this increased adoption is improved comfort compared to regular masks. However, there is a possibility that widespread public use of these alternatives to regular masks could have an adverse effect on mitigation efforts. To help increase public awareness regarding the effectiveness of these alternative options, we use qualitative visualizations to examine the performance of face shields and exhalation valves in impeding the spread of aerosol-sized droplets. The visualizations indicate that although face shields block the initial forward motion of the jet, the expelled droplets can move around the visor with relative ease and spread out over a large area depending on light ambient disturbances. Visualizations for a mask equipped with an exhalation port indicate that a large number of droplets pass through the exhale valve unfiltered, which significantly reduces its effectiveness as a means of source control. Our observations suggest that to minimize the community spread of COVID-19, it may be preferable to use high quality cloth or surgical masks that are of a plain design, instead of face shields and masks equipped with exhale valves.