Published Online: 01 September 2020
Accepted: July 2020

Visualizing droplet dispersal for face shields and masks with exhalation valve

Physics of Fluids 32, 091701 (2020); https://doi.org/10.1063/5.0022968
Siddhartha Vermaa), Manhar Dhanakb), and John Frankenfieldc)

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.

The COVID-19 pandemic has deeply affected every aspect of daily life worldwide. Several places across the world, including the United States, Brazil, and India, are experiencing a steep surge in infections. Healthcare systems in the most severely affected locations have been stretched to capacity, which also tends to impact urgent care for cases unrelated to COVID-19.1,2 Researchers have reported steady progress in the development of potential treatments and vaccines; however, it is estimated that widespread inoculation will not be available until sometime in the year 2021. It appears that the likelihood of vulnerable individuals struggling with severe health issues and the debilitating socio-economic ramifications of the pandemic will continue in the foreseeable future. Furthermore, widespread uncertainty regarding the re-opening of schools and universities for in-person instruction has created additional cause for concern, since these institutions have the potential to become focal points for unchecked community spread of the disease. In light of the acute circumstances, it has become crucial to establish clear and specific guidelines that can help mitigate the disease’s spread, especially given the high prevalence of asymptomatic and pre-symptomatic spread.3 A number of recent studies have contributed to this effort by significantly improving our understanding of various physical mechanisms involved in the disease’s transmission.4–7

Face masks have become increasingly accepted as one of the most effective means for source control (i.e., protecting others from a potentially infected wearer) and can help curb the community spread of the disease when used in combination with social-distancing and frequent hand-washing.8–12 Widespread mask-use by the general population has now been recommended or mandated in various places and communities around the world. Several private businesses have also adopted requirements for customers to use face coverings. Importantly, certain cloth-based masks have been shown to be effective in blocking the forward spread of aerosolized droplets13 (supplementary material, Movie 1). Although they are somewhat less capable than well-fitted medical grade masks, homemade masks constructed using certain materials can filter out a large proportion of respiratory droplets and particles.14–18 Moreover, cloth masks have the advantage of being readily available to the wider public in addition to being cost-effective, comfortable, and reusable when washed and disinfected properly. Additionally, they do not divert away from the supply of medical grade masks for healthcare workers.

While broad acceptance regarding the need for face coverings has risen steadily, 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 (Fig. 1). Face shields tend to have noticeable gaps along the bottom and the sides, and are used in the medical community primarily for protecting the wearer against incoming sprays and splashes while in close proximity to patients.19 Moreover, they tend to be used in conjunction with respirators, surgical masks, or other protective equipment. Masks with exhalation ports include a one-way valve, which restricts airflow when breathing in, but allows free outflow of air. The inhaled air gets filtered through the mask material; however, the exhaled breath passes through the valve unfiltered. There has been limited research on how effective face shields and masks with exhalation valves are as a means of source control.20,21

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