Masks Reduce the Risk of COVID-19 Infection by Suppressing the Cough-Cloud

by | Nov 17, 2020 | 2020, COVID-19

Written by Joyce Smith, BS. Researchers use a mathematical model to explore the mechanism by which a pandemic virus is spread when coughing occurs in confined spaces.

Covid-19 coughingIt is well-documented that the cough of a COVID-19 infected person can contaminate a large volume of surrounding air with the coronavirus 1-3. It is also well-documented that the coronavirus is airborne 4. The importance of viral transmission through airborne respiratory droplets is particularly relevant in confined spaces that lack adequate ventilation. It is unknown when a person coughs what volume of air becomes contaminated by the cough because the cough-cloud coming out from the person’s mouth is dispersed into the surrounding air to eventually become part of it. Therefore, a much larger volume of air is affected by coughing than is initially ejected.

To better understand the mechanisms of pandemic spread, the team of Agrawal and Bhardwaj sought to determine the amount of air infected by coughing in a room with negligible air ventilation. Knowing the maximum number of people that can be safely accommodated in a hospital ward and the minimum rate at which air in confined spaces must be circulated to reduce infection risk would be very beneficial in reducing viral transmission. The team also calculated thermodynamics parameters, such as temperature and relative humidity, since they can also affect the droplet size and distribution in the cough cloud 5. To date, a number of studies experimental 3,6 and numerical 1,2,7 studies have been done to better understand the safe distance between persons and the benefits of face masks during our current pandemic. One such study suggested that at large wind speeds varying from 4 km/hour to 15 km/hour, a cough-cloud could travel up to 6 meters 1. Verme et al. compared different types of masks and reported that well-fitted homemade masks could reduce the speed and range of the emulated cough jets significantly 3: however, loosely folded face masks and bandana-style coverings provide only minimal stopping-capability for the smallest aerosolized respiratory droplets 6.

Testing the effectiveness of face shields indicated that the expelled droplets from cough-clouds can move around the visor, while an exhalation port in a mask allows a large number of droplets to pass through unfiltered, thereby reducing their effectiveness 6.

Utilizing the measurements of the cloud-cough model Agrawal and Bhardwaj 8 found that the evolving volume of the cough-cloud was independent of its initial velocity but rather dependent on the distance it travelled. Based on their analysis, they suggest that the first 5 –8 seconds after coughing are important and necessary for suspending the exhaled droplets in air, after which the cloud than begins to disperse. They found that at the point of dispersion, their measurements revealed an infected air volume around 23 times more than the volume ejected by coughing. However, when a mask is worn, the volume of ejected air is dramatically reduced, thus significantly reducing the risk of infection to others who are present. The volume of the cough-cloud without a mask is about 7 and 23 times larger than in the presence of a surgical mask and an N95 mask, respectively. The presence of a mask drastically reduces this volume and significantly cuts down the risk of the infection to the other persons present in the room. Similarly, coughing into one’s elbow and the use of a handkerchief are actions that can significantly cut the distance traveled by the cloud, and reduce its volume, thus reducing the chances of viral dispersion. It should be noted that coughing into the elbow or using a handkerchief is better than no mask, it should not be a substitute for the continuous use of a face mask. With respect to the effect of temperature and relative humidity, data show that the cough-cloud takes on the room temperature and retains slightly more moisture relative to the humidity of the room. This model, while based on measurements of coughing, could also apply to sneezing. The authors cite, as further validation of their work, a study by Yin et al that demonstrated how, in a hospital patient’s room, ceiling ventilation rates of 0.057 m3/seconds and 0.085 m3/seconds could remove the volume of air equal to that of a cough-cloud in about 3 seconds in a worst case scenario 9.

Source: Agrawal, Amit, and Rajneesh Bhardwaj. “Reducing chances of COVID-19 infection by a cough cloud in a closed space.” Physics of Fluids 32, no. 10 (2020): 101704.

© Published under license by AIP Publishing

Posted November 17, 2020.

Joyce Smith, BS, is a degreed laboratory technologist. She received her bachelor of arts with a major in Chemistry and a minor in Biology from  the University of Saskatchewan and her internship through the University of Saskatchewan College of Medicine and the Royal University Hospital in Saskatoon, Saskatchewan. She currently resides in Bloomingdale, IL.

References:

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