Article by Ellis Kim
Contributors: Sophia Gonzales, Lauren Huang, Jessica Ly, and Kenneth Thai
To Wear a Mask is Your Task
Despite the abundance of research articles, scholarly journals, and CDC explanations on safety precautions during this pandemic, mask efficacy has been a hot topic leading to protests, arguments, and misinformation. This article compiles a gradation of masks from the most to least effective. At the beginning of the pandemic, face masks were not required, but new scientific evidence revealed that they effectively limit the spread of viral infections. Therefore, the World Health Organization and U.S. Centers for Disease Control and Prevention (CDC) began to recommend cloth, surgical, and N95 masks for health care professionals.1 The coronavirus is spread through respiratory droplets and is the most contagious within the 48 hour range before symptoms begin to appear.2 Therefore, in order to contain these droplets, face coverings became a mandate for public spaces. Between the end of March and late May of 2020, the mask mandate in 15 states led to a large slowdown of the daily growth rate in coronavirus cases, preventing up to 450,000 COVID-19 cases.3 Furthermore, researchers are finding that humidity in masks can lessen the severity of Covid-19 particles. The study found that a thick cotton mask had the largest increase in humidity, compared to a surgical mask that had the lowest increase in humidity. With high humidity levels, the spread of the Covid-19 particles to the lungs can decrease since humidity would hydrate the respiratory tract, a mechanism that has been found to benefit the immune system. This study displays another mechanism that demonstrates how masks can help provide protection against the coronavirus.4 While participating in economic, medical, and international activities, many COVID-19 patients were able to prevent the spread of the virus just by wearing a mask and following social distancing guidelines.5
Surgical masks are masks that are used specifically for healthcare workers. They contain respiratory droplets, filter out large particles in the air, and reduce the exposure of saliva to the public. These surgical masks are loose-fitting, and, therefore, do not create a tight seal for the face or completely protect against germs and other large contaminants.6 There are two types of these surgical masks: medical-grade and non-medical grade. Medical-grade surgical masks are regulated by government agencies including the FDA, NIOSH, and OSHA. They are further performance-tested by third-party facilities in order to meet certain fluid barrier protection standards and flammability tests for certification. There are four levels with five basic criteria of ASTM (American Society for Testing and Materials) certification that surgical masks can be classified as depending on the level of protection they provide to the wearer.7
These are the five basic criteria:
BFE (Bacterial Filtration Efficiency): ability to filter bacteria.
PFE (Particulate Filtration Efficiency): ability to filter sub-micron particles
Fluid Resistance: ability to prevent fluid penetration through the mask to the wearer
Delta P (Pressure Differential): measures how light and breathable a mask feels
Flame Spread: ability to withstand exposure from flame.
The four levels of ASTM certification are the following:
Level 1: Low Rated Medical Masks
Consists of ear loops and used for low-risk settings where there will be no fluid, spray, or aerosol. Used during general procedures, minimally invasive surgery, ophthalmology, and respiratory etiquette.
BFE at 3.0 micron: ≥ 95%
PFE at 0.1 micron: ≥ 95%
Fluid resistance to synthetic blood: 80 mmHg
Delta P: < 5.0 mm H2O/cm2
Flame Spread Class: 1 per 16 CFR part 1610
Level 2: Medium Rated Medical Masks
Used for light to moderate fluid, spray, and aerosol. Used in general surgery and designed for resistance to splash or spray.
- BFE at 3.0 micron: ≥ 98%
PFE at 0.1 micron: ≥ 98%
Fluid resistance to synthetic blood : at 120 mmHg
Delta P: < 6.0 mm H2O/cm2
Flame Spread Class: 1 per 16 CFR part 1610
Level 3: High Rated Medical Masks
Used for heavy possible exposure to fluid, spray, and aerosol. Contains the highest fluid resistance.
BFE at 3.0 micron: ≥ 98%
PFE at 0.1 micron: ≥ 98%
Fluid resistance to synthetic blood: at 160 mmHg
Delta P: < 6.0 mm H2O/cm2
Flame Spread Class: 1 per 16 CFR part 1610
Overall, the fitted filtration efficiency, or FFE, is 71.5% for medical-grade surgical masks with ties or 38.1% for those with elastic ear loops.
These medical grade surgical masks have three layers of nonwoven fabric. Nonwoven fabrics are able to filter out microparticles of bacteria more effectively than woven fabrics. These nonwoven fabrics are most often made of polypropylene derived from plastics, or polystyrene, polycarbonate, polyethylene, and polyester. The outer layer is a hydrophobic, nonwoven layer that is designed to repel water, blood, and bodily fluids. The middle layer, the most crucial layer, is the melt-blown layer. The purpose of this layer is to prevent very small particles of bacteria from entering or leaving the mask. This layer is created by spinning polymer fibers into webs, drawing the fibers into filaments in extruders by high-speed hot air, and thus forming ultra fine melt-blown non woven fabrics. These nonwoven fabric fibers have an average diameter of 1 to 2 micrometers. Surgical masks can filter out droplets larger than 5 micrometers, meaning they cannot filter out droplet nuclei that are smaller than 5 micrometers. Although coronavirus particles are individually 0.1um, they cannot survive well on their own and the coronavirus is primarily transmitted through respiratory droplets larger than 5 micrometers. It is still unclear on how contagious these droplet nuclei may be and their transmission rates, though research finds that they linger in the air for longer periods. The last layer is a soft, absorbent non-woven layer that absorbs spit, water, and sweat.
Even stronger than a surgical mask is a N95 mask that can filter out both large non-oil aerosol droplets and small particle aerosols. Unlike surgical masks, N95 masks are very tight-fitting and, if properly worn, there is minimal leakage around the mask when the user inhales.8 The filtration efficiency of these masks are 95% or more effective.
The KF94 masks are another option that has been getting much attention with one media source claiming it to be “highly protective”9 and much more readily available and inexpensive than that of the N95 mask. These masks originate from South Korea and are comparable to the N95. The “KF” stands for Korean filter, and the 94 refers to its filtration efficiency. These KF94 masks are more accessible for the general public, so are gaining much popularity.10 They can be used to protect against covid-19 particles, but some users have claimed it to be uncomfortable in fitting.11 The masks are very lightweight and have two folding sections that make a boat-like shape. They include a wired band on the nose area for adjustability. With the different structure of the mask, it is important to note that fit is one of the most important factors in masks. In fact, a study suggests that N95, KN95 or FFP2 masks do not perform their full effectiveness without a proper fit. They are reduced to become comparable to cloth or surgical masks.12
Since the COVID-19 pandemic, many non-medical grade surgical masks have also been mass-produced to accommodate consumer demand. These masks cover the user’s nose and mouth and may or may not meet fluid barrier or filtration efficiency levels. Due to the lack of strict regulation, there is no set filtration efficiency of these masks. These masks are typically two or three ply made from non-woven material. However, what distinguishes these masks from medical-grade surgical masks is that they lack the critical melt-blown filter layer. This makes these masks much less effective against microparticles of bacteria and viruses.
Moving forward in our gradation of effective masks, cloth masks are not able to protect against infection as effectively as medical-grade masks. Cloth masks should not be used for “high-risk situations,” for example, where healthcare professionals are working in the same setting as infectious patients. However, for the general public, cloth masks are more than effective in reducing the spread of respiratory droplets.13 A study by the New England Journal of Medicine displays a laboratory experiment that used laser-light-scattering methodology in order to demonstrate a visual representation of respiratory droplets as subjects repeatedly spoke the words “stay healthy”. The study found that covering the speaker’s mouth with a damp washcloth blocked nearly all of the droplets that ranged from 20 to 500 micrometers in size.14
CDC specifically recommends the use of “non-valved multi-layer cloth masks” because they block the release of exhaled respiratory particles and microorganisms. They can effectively block up to 50-70% of droplets and particles as small as 10 microns, and they limit the continued spread of particles that have not been captured. However, it is important to note that multiple layers of cloth with a higher thread count (600TPI (threads per inch), compared to 80TPI for moderate-thread-counts) can filter nearly 50% of fine particles less than 1 micron. These cloth masks usually have a special material like polypropylene that enhances the filtration by creating a form of static electricity (triboelectric charge) which boosts the ability of charged particles to be captured in the mask.15
Cloth masks should have a minimum of 2 layers and are the most effective at 3 layers. If surgical masks aren’t available, the CDC recommends home-made face coverings as an alternative for the general public. It is important to note that single layer cloth masks may reduce the amount of droplets spread when speaking, but are not effective when the user is coughing or sneezing.16 In fact, researchers have confirmed that single-ply cotton cloth face masks are comparable to toilet paper in their ability to contain the spread of respiratory droplets.17 In order to prevent the spread of droplets when a user coughs or sneezes, it is important to have a double-layer cloth face covering that does a significantly better job at spreading such particles. In fact, the CDC states that face coverings should have at least 2 layers of fabric in order to be effectively used in general public settings.18 The most effective is the three-ply surgical mask that is able to help prevent the spread of any time of respiratory emission. Findings from a research study with WHO found that fabric masks should contain an inner layer that absorbs, a middle layer that filters, and an outer layer that has non-absorbent material like polyester.19 In fact, Dr. Jing Wang, a clinical instructor at the University of British Columbia, states that although a two-layer cotton mask is about 60% effective in containing the virus particles from the user’s mouth, adding an extra layer of filter can increase the effectiveness to about 80-90%.20 As the new coronavirus variant sparks fear in the public, researchers recommend adding an extra filtration layer made of tight-weave fabric. A PM2.5 Carbon filter or spunbond material made of polypropylene are able to use the power of static electricity to trap the virus particles. In fact, a Stanford University Researcher, Yi Cui, stated that adding a filter with two layers of polypropylene can increase a cotton mask’s filtration efficiency as much as 35%.21
Yet, even before analyzing the number of layers that a cloth mask contains, it is important to make sure the mask is able to fit tightly around the sides and front of the face, completely covers the nose and mouth, is secured with ties or ear loops to prevent the mask from falling, and does not block or make it difficult to breathe.22
Weakest/Unique face coverings
In addition to the surgical, medical-grade, non-medical grade, and cloth masks are weaker mask alternatives such as gaiters, bandanas, silk masks, plastic masks/face shields, clothing/T-shirts, and gas masks/paint respirators. There are not nearly as many studies and research projects that tested the effectiveness of these weaker face coverings, so it is important to note the lack of a large amount of sources about the following face coverings.
Gaiters are single layer masks that may contain 92% polyester and 8% spandex type. In one study, they found that gaiters have high porosity and are very thin in layer but still have better effectiveness than a cotton 3 layer mask. In fact, the 3-layer cotton mask and 2-layer nylon mask have been found to have the lowest fitted filtration efficiency.23 A research group at MIT confirmed that a neck gaiter is better than no mask at all, despite previous news reports that claimed otherwise. Even though there was a study that suggested that respiratory droplets could be split into smaller droplets in the fabric, making the gaiter worse than no mask, it is unlikely for the droplets to split up or even release more droplets than with no mask since any face covering will block a percentage of droplets according to aerosol scientists. The researchers conclude that there could have been other factors in this original study that could have led to such conclusions.24
Bandanas are usually very loose fitting and have an average jet distance of droplets of about 3 ft 7 in. They contain a fabric that can get saturated after prolonged use, which will affect the filtration potential. Therefore, bandana-style coverings and loosely folded face masks provide very minimal protection from the smallest aerosol particles. In a study done by research group Physics of Fluids, researchers found that a stitched mask made of quilting cotton was the most effective, then a commercial mask, a folded handkerchief, and coming in last was a bandana. Even though the bandana had the highest thread count, it was found to have the least effectiveness.25 Folding or stacking of bandanas can increase the filtration efficiency due to the increase in the number of layers, but can also lead to a large pressure drop which leads to discomfort, difficulty breathing, and leakage of droplets. Most importantly, this can lead to inefficiency at collecting aerosols. Therefore, using these fabrics with fibrous filters in between layers and fitted properly like homemade cloth masks can be much more effective in blocking the spread of aerosol particles.26
In a laboratory-based study, researchers examined the hydrophobicity of fabrics like cotton, polyester, and silk. They measured the resistance to penetration of droplets as transmission and found silk to be the most effective at blocking the penetration and absorption due to its hydrophobicity. It repelled droplets and was found to be more breathable because it didn’t trap humidity.27
Clear face masks which are not tight around the nose and mouth have become popular amongst a few celebrities and the general public. These clear face masks are rigid due to the plastic material and do not form a tight seal around the face. Since the bottom edge is entirely open, it cannot block the aerosol particles from touching the face.28 Similarly, face shields should not be used as solitary face/eye protection due to their lack of seal around the face. An article written prior to the pandemic states that while there were 92-96% reductions in risk of inhalational exposure immediately after a cough, smaller aerosols created more inhalational exposure. Face shields are far less effective than face masks because of particle penetration and should be used with face masks but not alone.29
Multi-layered cloth masks were found to be 95.15% effective, while T-shirt mask efficacy is dependent on the material of the T-shirt and its thickness. Despite the layer and fabric, it is important to ensure that there are no open gaps at the bottom because aerosol particles are able to enter through these gaps.30
Paint respirators and dust masks have no medical relevance at all and do not protect from the coronavirus aerosol particles. 31 Gas masks are known as air purifying respirators, which can clean chemical gases and, possibly, particles out of the air as you breathe. They are only effective if used with the correct cartridge or filter for particular biological or chemical substances.32
As explored, there are various types of face masks with differing levels of filtration efficiencies in containing respiratory droplets. The most effective masks are N95 and medical-grade surgical masks. Non-medical grade surgical masks are not under strict regulation, so it is important to check the scientific evidence behind these masks. Medical-grade surgical and N95 masks are also the most inaccessible to the general public and are limited for healthcare workers. Therefore, lower-grade masks such as homemade cloth masks of a minimum of 2 layers, and most effectively at a layer of 3, may be sufficient for the general public. These masks may not be as effective as filtering out respiratory particles especially in high-risk zones. However, they can be modified by adding additional layers and adjusted to better fit one’s face to become highly effective. Weaker and unique masks, such as gaiters, bandanas, and silk masks, can have minimal protection in containing respiratory droplets, while clear face masks, T-shirt masks, paint respirators, dust masks, and gas masks are largely ineffective and near to no medical relevance in protecting against the coronavirus. There is not a grand source of information about these unique and weaker masks, so it is important to use a 2- or 3- layer mask in public spaces. It is everyone’s task to wear a mask.
5 https://www.cmaj.ca/content/192/15/E410, https://www.washingtonpost.com/business/2020/06/17/masks-salons-missouri/, https://jamanetwork.com/journals/jama/fullarticle/2768533
10 https://www.npr.org/sections/goatsandsoda/2021/01/22/959683338/coronavirus-faq-why-am-i-suddenly-hearing-so-much- about-kf94-masks
13 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4420971/?fbclid=IwAR0VXF8NdjsNnNzBAmYVjPN_Pt8xROn1wN89ILPYq HPu0Txr6hqzZ631yXs
15 https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html https://medical.mit.edu/covid-19-updates/2020/08/how-do-i-choose-cloth-face-mask
20 https://www.thestar.com/life/health_wellness/2020/11/04/trudeau-questioned-on-new-3-layer-mask-guidelines-in-house-of- commons.html
24 https://medical.mit.edu/covid-19-updates/2020/08/neck-gaiters 25 https://aip.scitation.org/doi/10.1063/5.0016018