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Visualizing droplet dispersal for face shields and masks with exhalation valves - PubMed

Visualizing droplet dispersal for face shields and masks with exhalation valves

Siddhartha Verma et al. Phys Fluids (1994). .

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.

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Figures

FIG. 1.
FIG. 1.

(a) A face shield, which is similar in design to those used by healthcare workers in conjunction with masks and other protective equipment. The vertical laser sheet used for visualizing the expelled droplets is visible in this panel. (b) An N95 mask with an exhalation valve located at the front. Both cloth-based and N95 masks can be found equipped with such exhalation ports.

FIG. 2.
FIG. 2.

Near-field view of the droplet spread when a face shield is used to impede the emerging jet. (a) Prior to emulating a cough/sneeze, (b) 0.57 s after the initiation of the emulated cough, (c) after 3.83 s, and (d) after 16.57 s. The ejected plume is illuminated by both a vertical and a horizontal laser sheet. Droplets illuminated by the horizontal laser sheet can be observed in (c) and (d). Multimedia view:

https://doi.org/10.1063/5.0022968.1
FIG. 3.
FIG. 3.

Far-field view of droplet spread when a face shield is used to impede the jet. (a) 2.97 s after the initiation of the emulated cough, (b) after 6.98 s, and (c) 10.77 s. Multimedia view:

https://doi.org/10.1063/5.0022968.2
FIG. 4.
FIG. 4.

Visualization of the droplet spread when an N95 mask equipped with an exhalation port is used to impede the emerging jet. (a) Prior to emulating a cough/sneeze, (b) 0.2 s after the initiation of the emulated cough, (c) after 0.63 s, and (d) after 1.67 s. Multimedia view:

https://doi.org/10.1063/5.0022968.3
FIG. 5.
FIG. 5.

Visualization of droplet spread when a regular N95-rated mask is used to impede the jet. (a) Prior to emulating a cough/sneeze, (b) 0.13 s after the initiation of the emulated cough, (c) after 0.33 s, and (d) after 0.83 s. Multimedia view:

https://doi.org/10.1063/5.0022968.4
FIG. 6.
FIG. 6.

Visualization of the droplet spread when a surgical mask (brand “A”) is used to block the jet. (a) Prior to emulating a cough/sneeze, (b) 0.37 s after the initiation of the emulated cough, (c) after 0.62 s, and (d) after 2.33 s. Multimedia view:

https://doi.org/10.1063/5.0022968.5
FIG. 7.
FIG. 7.

Visualization of droplet spread when a surgical mask (brand “B”) is used to block the jet. (a) Prior to emulating a cough/sneeze, (b) 0.5 s after the initiation of the emulated cough, (c) after 0.83 s, and (d) after 3.13 s. Multimedia view:

https://doi.org/10.1063/5.0022968.6

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