Editor’s note: Researchers at Florida Atlantic University tested different kinds of face masks to see how well they worked to stem the spread of the coronavirus. They published their findings in a scientific journal. Here, for Tampa Bay Times readers, they kindly explain their findings for those of us who aren’t scientists.
There has been a lot of controversy and confusion surrounding the use of masks by regular, healthy people. Some of the confusion can be traced back to changes in guidelines put forward by the public health agencies. Early on in the COVID-19 pandemic, public health officials recommended mask-use only for people who had symptoms, and for those caring for the ill. However, recent recommendations call for universal mask usage in a public- or crowded setting. Most of this back and forth can be attributed to our continually improving understanding of the disease and the ways in which it is transmitted from one person to another.
Over a third of us who might get infected with COVID-19 at some point will remain asymptomatic, that is, we will not know that we have the infection, unless we get tested. But even for those who never exhibit symptoms, there is a high chance of inadvertently infecting others who might be much more susceptible to the disease. Moreover, current estimates suggest that about 50 percent of the time infection transmission happens at the pre-symptomatic stage. This means that before an infected person displays overt symptoms such as fever, coughing, shortness of breath, etc., it is still highly likely that they might transmit the virus to healthy individuals.
All of these factors, which are based on continually emerging evidence from a wide range of scientific and clinical studies, have led to several health agencies around the world to recommend the use of masks as a protective measure, in addition to social-distancing.
In order to consider what masks actually do, we must first understand how exactly COVID-19 is transmitted from one person to another. Anytime we cough or sneeze, we expel an invisible cloud of tiny droplets from our mouth and nose, made of saliva, mucus, and other material found in our respiratory tracts. In fact, we also generate such droplets while talking, singing, and even just breathing. All of these activities produce a wide range of droplet sizes, with the smallest being on the order of a few microns, to the largest being well over 500 microns (approximately 1/50th of an inch). Each of these respiratory droplets can contain hundreds to thousands of virus particles capable of causing an infection. When these virus-carrying droplets enter the body of a healthy human being, either by landing in/being inhaled through the nose or mouth, or by landing on the permeable surfaces of the eyes, they are likely to lead to an infection.
The ways in which these droplets can reach us differ markedly, depending mainly on the size of the droplets. Once respiratory droplets leave our mouth and nose, the larger, heavier ones tend to fall to the ground within a few feet. They may also land on inanimate objects, from where they might be picked up by unsuspecting individuals who touch the object afterwards.
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The smaller, much lighter droplets behave very differently. These aerosolized droplets can remain suspended in the air for long periods of time, and are easily carried over from one place to another by very light breezes; for a familiar analogue, think of tiny dust particles floating in a beam of sunlight. Although these smaller droplets carry fewer virus particles, they can penetrate deep into our lungs when inhaled and can result in an infection even for relatively low virus concentrations. They present an especially worrying concern in poorly ventilated indoor settings with large crowds, where virus-laden aerosolized droplets can accumulate over time, which significantly increases the possibility of cross-infection.
All of this is where masks come into the picture. When we cover our nose and our mouth, we are attempting to restrict the spread of the respiratory droplets, both large and small, as much as we possibly can. Using simplified representations of coughs and sneezes, and employing visualization techniques that have long been used in studies of flow physics, we have been able to observe how these expelled droplets spread in a calm indoor setting.
We observed that when a cough was simulated with no covering obstructing the mouth/nose, the smaller aerosolized droplets could travel up to 8 feet on average. While this is more than the 6-foot recommendation that we have all become so familiar with at this point, it is important to understand that 6 feet is the minimum recommended distance; being farther apart is preferable whenever possible, since both the number and concentration of droplets keeps decreasing with increasing distance.
The main purpose of masks is to cut down on both the travel range of respiratory droplets, as well as the number released into the ambient environment. We observed in our tests that different types of homemade masks were able to restrict the extent of droplet spread to varying degrees. Using a single-layer bandanna style covering made of thin fabric reduced the range of droplets to slightly below 4 feet, or approximately half that of an uncovered cough. A quickly folded cotton handkerchief shaped into a mask using rubber bands performed slightly better, reducing droplet spread to just over a foot.
The best-performing homemade mask was made of two layers of quilting cotton, which were stitched together and shaped for a snug comfortable fit on the face. This mask cut down the range of droplet spread to merely two and a half inches, which is comparable to some high-quality commercial non-medical masks that we have also tested. Overall, the results demonstrate that while any sort of covering is effective in reducing droplet spread (especially for the larger droplets), it is better to opt for good quality fabrics with a tight weave, and a snugly fitting design that is comfortable to wear over long periods of time.
While the results described here show that masks can help mitigate the airborne transmission of infectious pathogens, they do not eliminate the risk of transmission completely. Although masks restrict the forward spread of droplets, there always tends to be some leakage from the gaps along the top of the mask and the bridge of the nose. Thus, it is essential to use masks in combination with social-distancing measures. Moreover, there has been a rising trend of people using masks equipped with exhalation valves. Although these tend to be more comfortable compared to plain regular masks, they allow respiratory droplets to pass through the valve unfiltered, which defeats the primary purpose of restricting droplet spread.
On a personal level, masks are our effort to protect the most vulnerable members of our society from severe harm that we might unintentionally bring upon them. This is no different from the strong impulse that we feel to assist others whenever we see them in need of help. On a societal level, masks are our best tools to bring some sense of normalcy back to our lives. If everyone wore a mask, it would help bring down infection rates for COVID-19 to a point where we can return to our daily lives sooner rather than later.
Siddhartha Verma, an assistant professor in the Department of Ocean and Mechanical Engineering at Florida Atlantic University, was lead author of the paper “Visualizing the Effectiveness of Face Masks in Obstructing Respiratory Jets” in the Physics of Fluids. Other co-authors were Manhar R. Dhanak, chair of that department at FAU, professor and director of SeaTech; and John Frankenfield, a technical paraprofessional with the department.