Wednesday, October 27, 2021

Mask media – research is mixed but provides valuable lessons

They may seem like simple pieces of material, but masks are actually heavily researched and often sophisticated articles of personal protective equipment. In our previous examination of the science of masks, we looked at the clear hierarchy of protection from standard masks: N95 respirators, surgical masks, and last, cloth masks. We covered a lot of ground, but there is much more data to bring into your next debate over the effectiveness of face coverings.

First, masks are mainly used for source control of a virus. Source control generally refers to sick people wearing masks with intent to prevent spread of disease to healthy people (MacIntyre & Chughtai, 2020). One 2016 in vitro study confirmed that masks are more effective as a source control than as a preventive measure for the recipient (Patel et al, 2016). The Patel study compared a range of variables to come to this conclusion, including simulating a normal breathing pattern, coughing, and using various forms of facial protection in the forms of a “natural fit” surgical mask, a fitted surgical mask, and two types of N95 respirators (one with a Vaseline seal, and one without). Patel et al found that with coughing, source control was the superior method unless the recipient used a Vaseline sealed N95 respirator. With normal breathing, source control was similar or slightly better than having a mask on the receiver.

In the case of surgical masks, not all are equal. Surgical masks are ranked according to bacterial filtration efficiency (BFE) (Wibisono et al, 2020). Filtration efficiency is determined by numerous factors, including fibre selection and fabrication method (Wibisono et al, 2020). Surgical masks fall under three general categories of protection: low barrier (BFE of 95% or higher), moderate barrier, (BFE of 98% or higher) and high barrier (Wibisono et al, 2020). It should be noted that surgical masks are classified by protection against bacteria, not viruses. They are designed for single use, as opposed to the multiple-use scenario that has been typical in the general population during the COVID-19 crisis.

Cloth masks can be washed, but what about surgical masks? A dry sterilization method, and a 15-minute steam process at 121 degrees Celsius (with the mask in a sealed bag) were found effective, and did not affect the permeability or filtration capacity of the surgical mask (Wibisono et al, 2020). Unfortunately, most people do not have access to these processes, so we are stuck with single-use surgical masks.

With masks in general, it was found that masks with ear loops performed worse for protecting the wearer than masks with head ties (Rowan & Moral, 2021). This is likely because with head ties it is easier to achieve a mask shape more fitted to one’s face. The mask has been the predominant face cover throughout the COVID-19 pandemic, but we occasionally see a person wearing a face shield. The World Health Organization (WHO) says that face shields are meant to protect the face from “splashes of biological fluid,” and therefore are insufficient from protecting from any form of aerosolized virus.[1]The face shield can be important for protecting your eyes (because they are mucous membrane and, therefore, a place at which a virus can enter your body), but it needs to be complemented by a face mask for optimal viral protection.

Mask breathability is another important consideration. Breathability is a very difficult variable to quantify as it varies between individuals (Guha et al, 2021). To measure breathability, researchers examined the pressure drop across a mask, where the larger the pressure drop, the more difficult it is to breathe (Rowan & Moral, 2021). When observing the pressure drop across different masks, Guha et al found that, based on thresholds from the FDA, even a single-layer, tightly woven mask was higher than the maximum threshold permissible for children. As a result, Guha et al recommended that if children wear masks, they should wear three to four layers of loosely knit or woven cotton or polyester. Looking at permissible pressure drop thresholds from the CDC, Guha et al found that adults should use caution when choosing tightly woven fabrics, and therefore recommend using a combination of one tightly woven material with other “easier to breathe” layers such as a cotton, polyester, or nylon. Research by Wibisono et al supports these findings. They found that, based on the best filtration efficiency they found with 100% cotton and pillowcase-based masks, if layers are doubled, the filtration efficiency is not affected, but the pressure drop doubles. From these studies, we see that pressure drop increases with particular fabrics added to the mask, making it more difficult to breathe. Therefore, it is important to choose fabrics that achieve good filtration, and pair them with fabrics that have good breathability.

Unfortunately, there is a lot of contradictory evidence and theorizing in the science of masks. Many studies have been conducted, but it is evident that the body of research is still growing and evolving. One study theorized that mask use may cause immune priming through “variolation”. This occurs when a small dose of the virus makes it through the layers of a mask, thereby stimulating an immune response and possibly helping a population move toward herd immunity (Rowan & Moral, 2021). Other research shows that cloth masks may increase risk of infection, suggesting that medical workers should not wear cloth masks (Wibisono et al, 2020). Relatively porous, easily penetrated cloth masks may allow virus particles to build up inside the mask in larger doses, and in so doing increase risk of infection. With the potential build-up of fluid on the inside of a mask, assuming a cough frequency of 0.5 per hour, and assuming people produce 0.1 mL per hour of sputum, even having a mask using two 99% efficient layers can theoretically allow around 2.7 x 10^3 copies of SARS-CoV-2 (COVID-19) through the mask, which is within the limits of an infectious dose of a respiratory virus (Guha et al, 2021).

Are we using masks in vain? Not necessarily. Research on the application of membrane technology in mask use is encouraging. Membrane technology regarding virus research involves using composite-membrane filters that aid in virus retention. This technology has succeeded in filtering out viruses significantly smaller than coronavirus such as retroviruses (> 50 nanometres) and parvoviruses (18-24 nanometers) (Wibisono et al, 2020).

As discussed in both of our articles on the science of masks, none of the standard options provides 100% viral protection. For now, your best bet is to keep these findings in mind as you make your mask choices. Filtration efficiency and breathability should trump fashion, and remember – a mask is more likely to make you a super spreader than a superhero if you are not using it properly and, most important, maintaining sufficient distance from others.

[1] WHO pamphlet: mask use in the context of covid 19

Guha S, Herman A, Carr IA, Porter D, Natu R, Berman S, et al. (2021) Comprehensive characterization of protective face coverings made from household fabrics. PLoS ONE 16(1): e0244626.

Patel, R. B., Skaria, S. D., Mansour, M. M., & Smaldone, G. C. (2016). Journal of Occupational and Environmental Hygiene, 13(7), 569-576.

MacIntyre, C. R. & Chughtai, A. A. (2020). A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients. International Journal of Nursing Studies, 108.

Rowan, N. J. & Moral, R. A. (2021) Disposable face masks and reusable face coverings as non-pharmaceutical interventions (NPIs) to prevent transmission of SARS-CoV-2 variants that cause coronavirus disease (COVID-19): Role of new sustainable NPI design innovations and predictive mathematical modelling. Science of the Total Environment 772.

Wibisono, Y., Fadila, C. R., Saiful, S. & Bilad, M. R. (2020) Facile Approaches of Polymeric Face Masks Reuse and Reinforcements for Micro-Aerosol Droplets and Viruses Filtration: A Review. Polymers 23(2516).

Skye Thompson
Skye Thompson is about to enter veterinary school in the U.K., and is someone who loves adventuring, jamming to music in her car, and learning about anything she can. In her spare time writing, Skye aims to offer strong analysis and unique perspective to current issues.

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