Signs of Fall 10: Face Masks!

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Cloth face masks have become a symbol of the COVID-19 pandemic. Looking at the evolving opinions about cloth face masks and their role in protecting us from viral transmission and infection is, in many ways, an intellectual trip that reflects the progress we have made from our initial assumptions and dearth of knowledge about the SARS-CoV2 virus up to our current much more nuanced understanding of this extremely dangerous pathogen.

Cloth face masks, of course, have been worn regularly in many countries, particularly countries in Asia, to prevent viral transmission. Most western health experts, however, dismissed the potential effectiveness of these types of masks fundamentally because of the obvious size discrepancy between the sizes of the viruses in question and the sizes of the spaces in between the threads of the woven cloth. For example, the average diameter of the SARS-CoV2 virus is 0.065 to 0.140 micrometers (a micrometer is one millionth of a meter) while the average spaces in between the threads in woven cloth can vary from 5 to 15 micrometers (in high thread count cloth) to 50 to 200 micrometers (in low thread count cloth).  How could, the logical/common sense argument went, a cloth filter remove particles that are anywhere from three hundredths to three ten thousandths the size of the holes in the filtering mesh?

As with many aspects of the initial approach to SARS-CoV2, this attitude was based on incomplete information and erroneous assumptions. This prevailing idea, though, that cloth face masks were ineffective in preventing the transmission of viruses was a major influence that initially led public health officials when they referred to “face masks” to only mean “medical-grade face masks” (including the very recognizable “N95” masks). At the beginning of the pandemic there were fears that these medical-grade masks, which were needed for the protection of front line medical and emergency response personnel, would be bought up and hoarded by a frightened public and, thus, be, unavailable for critical health workers.

According to Dr. Anthony Fauci, the Director of the National Institute of Allergy and Infectious Diseases, the administration was initially hesitant to recommend the widespread use of face masks by the general public because of this fear of critical, medical grade face mask shortages. The Center for Disease Control (“CDC”) repeatedly (in briefings from January 30 to March 10) stated that it did not “recommend the use of face masks for the general public.” It was not until April 3, 2020 that the CDC finally recommended that people wear cloth face masks in public in order to prevent SARS-CoV2 transmission. This change in position was based on new information about SARS-CoV2 transmission (especially on the apparent importance of pre-symptomatic and asymptomatic transmission) and also on new observations of the virus itself.

One of these new observations concerned the reality of how the virus is actually expelled from an infected person. SARS-CoV2 viruses are not shed as individual, viral particles but instead as clusters of viruses encased in tiny drops of fluid. Every respiratory act from breathing, to talking, to singing, to coughing and sneezing expels these viral-laden, fluid, respiratory particles from the mouth and nose.

Some terminology: the term “droplet” refers to fluid, respiratory particles that are greater than 5 micrometers in diameter. Droplets are theoretically so large and heavy that they can only be suspended in the air for a very short period of time. Droplets, then, quickly settle out of the air and land on surrounding surfaces. Fluid, respiratory particles smaller than 5 micrometers are called “aerosols.” Aerosols are small enough to remain suspended in air for many minutes or even many hours. Initial research and evaluation of subsequent SARS-CoV2 transmission data suggest that most of the SARS-CoV2 viruses expelled by an infected person’s respiratory system are in the form of droplets, although very recent experiments looking at the exchange of SARS-CoV2 between laboratory ferrets clearly shows that aerosol transmission is possible.

Since the primary pathway for SARS-CoV2 infection is the direct introduction of air-borne viruses into an individual’s respiratory tract, the likelihood of successful transmission of these large droplets greatly decreases over distance. This is the basis for Social Distancing and the recommendation of keeping at least six feet away from  the other people around you.

Aerosol dispersal of SARS-CoV2 may occur, but it is not thought to be a typical mode of transmission. If it were a more common dispersal mode, epidemiologists contend, the transmission rates of SARS-CoV2 would be much, much higher. For reference, the virus that causes measles is dispersed in respiratory aerosols, and this is one of the reasons that that the measles virus is so explosively contagious, and why a space contaminated by a measles carrier can remain dangerously infectious for many hours.

So, SARS-CoV2 is primarily expelled from the body of an infected individual in relatively large, fluid, respiratory droplets. It turns out that when these droplets pass through a cloth mesh, a significant number of them encounter the mesh and stick to it. The droplets also stick to previously captured droplets. The key idea to making a cloth face mask that is an efficient respiratory droplet filter is to construct a cloth system that makes droplet encounters with the cloth threads very likely to occur.

So, the nature of the cloth used in cloth face masks is extremely important. The types of cloth used in masks are also a source of some of the great variability in the measurement of the effectiveness of cloth masks to block both the outward transmission (“source control”) and the inward uptake of the SARS-CoV2 virus.

Woven cloth can be described by its “thread count” (the number of threads per inch of fabric (“TPI”)). A fabric with a high thread count has a large TPI, while a fabric with a low thread count has a low TPI. In general, the higher the TPI, the better the cloth functions as a filter.

A cloth can also have a range of geometries of thread weaves. A “plain weave” cloth has interwoven threads that cross at right angles, while a “twilled weave” cloth has threads that cross at twisted angles. Cloth can also be made of fibers that are compressed together (“felted cloth”). Felted cloth has very tortuous air passages and a very complex overall internal structure. In general, the more complex the weave or fiber pattern, the better the cloth acts as a filter.

The “heaviness” of a cloth is simply its weight per surface area. Again, in general, the heavier the cloth the better it functions as a filter. Cloth can also have surface features that arise from the weave pattern. A “napped” cloth has raised ends of fibers on one of its surfaces, and a “terry cloth” has loops of the weaving fibers on one of its surface. These naps and loops may also increase the droplet absorption properties of the cloth.

The Mayo Clinic has published a review on the effectiveness of cloth masks to accomplish filtration of air-bourn particles. Many of the cited research articles utilize bacteria (which are about 5 micrometers in diameter (the same size as the fluid respiratory droplets)) as biological markers to determine filtration efficiencies. The results below are summaries of the studies cited in this review article:

1. Flat cloth, T-shirt material: (t-shirt material is woven cloth with one napped edge):

One layer of T-shirt cloth as a face mask: 69% bacterial filtration efficiency

Two layers of T-shirt cloth as a face mask: 71% bacterial filtration efficiency

In an another study: oral bacterial were measured as the experimental subject coughed through two layers of T-shirt material. The result: 79% filtration efficiency

NOTE: medical masks in this study had an 85% filtration efficiency (statistically the same as the T-shirt efficiency)

In a third study: saline particles (0.02 to 1.0 micrometers in diameter) were inhaled through mask. The result: 50% filtration efficiency

 

2. Linen tea towels (“dish cloths”) (plain, woven cloth):

One layer of tea towel as face mask: 83% bacterial filtration efficiency

Two layers of tea towels as face mask: 97% bacterial filtration efficiency

Inhalation study (saline particles 0.02 to 1.0 micrometers in diameter through one layer of linen tea towel): 55 to 77% filtration efficiency

 

3. Cotton cloth with 600 TPI (a high thread count): transmitting particle size 0.075 to 0.1 micrometers:

One layer of high thread count cloth: 76% filtration efficiency

Two layers of high thread count cloth: 85% filtration efficiency

 

4. Muslin (plain weave cotton) with an inner flannel layer (flannel is a plain weave, napped cloth): measuring oral bacteria transmission: 88 to 99% filtration efficiency.

These experiments clearly show that cloth face masks can be very effective in filtering out small droplets and particles in both expired air and inhaled air. In many cases, the handmade, cloth face masks equaled the filtration efficiency of commercial, medical grade face masks. Wearing cloth face masks is an effective way to both reduce the spread of SARS-CoV2 from an infected person and also to reduce the probability of inhaling the virus.  Cloth face masks can also, very significantly, reduce the number of inhaled viral particles ( the “inhaled viral load”) taken in by the mask wearer. This reduced viral load has profound impacts on the nature and severity of a developing COVID-19 infection.

Some observations:

USS Theodore Roosevelt: This U. S. Navy aircraft carrier made the news last March when crew members tested positive for SARS-CoV2. Over the course of two months 1,156 crew members (out of 4500) tested positive for SARS-CoV2, and one sailor died. A variety of epidemic control methods were established on the Roosevelt to try to contain the infection. About half of the crew was evacuated from the ship and extensive cleaning and sterilization procedures were conducted. The use of cloth face masks and social distancing practices were established on the ship. The San Diego Union Tribune reported (June 9, 2020) that the Roosevelt sailors who wore face masks had a 55.8% infection rate while those sailors who did not wear face masks had an 80.8% infection rate.

In Switzerland, a small army base that housed 508 soldiers experienced a COVID-19 outbreak. A large number of the base’s soldiers (354) were initially infected with SARS-CoV2 and 30% of these individuals developed COVID-19 symptoms. Measures including wearing cloth masks, social distancing and strict handwashing and surface cleaning were implemented. These steps greatly slowed the spread of the virus through the base and also, significantly, reduced the size of the viral load impacting on new patients. The results were that none of the remaining 154 soldiers on the base developed COVID-19 symptoms although 13 of them were infected by SARS-CoV2 (as evidenced by antibody tests).

A number of cruise ship and poultry and meat packing plant COVID-19 outbreaks have shown the same trends as the U. S. S. Roosevelt and the Swiss army base: wearing masks reduces (but does not entirely prevent) the transmission and uptake of SARS-CoV2, but the reduction in the initial viral load that cloth masks accomplish reduces the overall seriousness of the resulting viral infection and lessens the chance that a full blown COVID-19 infection will result.

Face masks work! They keep an infected person from passing along SARS-CoV2, and they reduce the chances that the mask wearer will take in a large enough viral load to cause the most serious types of COVID-19 responses.