NEW CLOTH FACE MASK RESEARCH
Last Updated by Cory Calendine, MD, Orthopedic Surgeon, 11/25/20
Cloth masks are highly effective in protecting the people who wear them as well as those around them, according to a new study from leading aerosol scientist at Virginia Tech University, Linsey Marr.
“Filtration works both ways,” Marr said. “If it works for source control,” she said during a media presentation on Monday — that is, if the mask filters out particles coming from the wearer’s mouth — “it’s going to work pretty well for exposure reduction to protect the wearer also.”
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Virginia Tech Research Summary
The newest Virginia Tech research involved testing different types of masks by putting them over the faces of mannequins. Those mannequins were then placed inside a plastic bin and sprayed with a saline solution. Marr and her team recorded how many particles made it through the mask and into the mannequin’s mouth (which was fitted with a sensor). The experiment setup was designed to replicate 'real-life' conditions of a mask wearer. Results focused on the ability of various mask materials and layers ability to filter 1-3 micron-sized particles. Marr explained that, although the actual SARS-CoV-2 virus is much smaller, it does "not travel naked". Scientists have learned over the past 12 months that the SARS-CoV-2 virus is expelled from the nose and mouth and carried "piggy-back" in the air on moisture particles that average 1-3 microns in size.
As an aerosol research specialist, Marr is one of a handful of researchers in the world with unique expertise in how airborne pathogens interact with and travel through the air. The team’s study, published last week on the preprint platform medRxiv, has not yet been peer reviewed. They are awaiting publication. Below is a summary of the core study findings and link to the original article and study abstract:
- Cloth masks help to reduce the amount of virus in the air and reduce the amount that the mask wearer breathes-in.
- Testing 11 different types of face coverings (9 cloth masks made from coffee filters, cotton and other materials, surgical masks and face shields), Marr and co-author Jin Pan found that many of the materials tested were able to filter out up to 75% of the particles representing covid-laden aerosol droplets
- A high-quality cloth mask consisting of three layers could allow filtration up to 90% - of both incoming and outgoing particles. So cloth masks can be equally as good at protecting you from the virus as reducing the likelihood of spread to others.
- A mask that contains an interior filter made from a common vacuum bag “achieved outstanding performance”
- Masks are not 100% effective and should be used in conjunction with other interventions including social distancing and handwashing.
- Recommendations, based on this study, are that people use a three-layer mask. The outer two layers should consist of a tightly woven but flexible material that allows the mask to conform to your face. The middle layer should be a filter material. A two-layer mask could potentially be equally effective, provided the cloth had a “good, tight weave”
See Original Article: Study shows cloth face coverings help protect both wearers and those nearby, Virginia Tech University
Study Abstract: Inward and outward effectiveness of cloth masks, a surgical mask, and a face shieldJin Pan, Charbel Harb, Weinan Leng, Linsey C. Marr
What Face Mask Material Works Best?
Original Blog by Cory Calendine, MD
Research continues to support that COVID-19 spreads by the transmission of respiratory droplets, and the Centers for Disease Control and Prevention recommends homemade cloth face coverings for use by everyone over the age of 2 years old in public spaces. The effectiveness of face masks and revised recommendations have driven researchers to evaluate what materials provide the best filtering of airborne particles. The different materials tested are ranked based on how well they filter small airborne particles and how easy it is to breathe through the material.
How Does the COVID-19 Virus Spread?
The COVID-19 virus is very small (0.06 to 0.14 microns). The virus is spread by passage of respiratory secretions from an infected patient directly to others and indirectly via shared contact with a contaminated surface. The virus particle is most commonly passed to others by piggy-backing on respiratory secretions (of an infected person) emitted from the mouth and nose.
We now know that when an infected person coughs, sneezes,speaks or even breathes, virus-laden respiratory secretions are emitted in all sizes. Some of the respiratory particles are larger, while others are very small. The larger droplets (>5 microns) may travel only a few feet - or up to 26 feet if propelled by a sneeze - before falling to the ground or onto another person. Smaller particles can behave like an aerosol and remain suspended in the air for a much longer period of time and be carried by air movement for much farther distance. Traditional respiratory disease control measures are designed to reduce transmission by droplets produced in the sneezes and coughs of infected individuals. New research suggests a large proportion of the spread of COVID-19 appears to be occurring through airborne transmission of aerosols during breathing and speaking, prompting the CDC to broaden face mask recommendations.
Masks can provide a critical barrier by reducing the amount of virus emitted in exhaled breath, especially in asymptomatic people and those with mild symptoms. Although not 100 percent protective, cloth face masks can decrease the amount of virus laden respiratory droplets and aerosol emitted from the nose and mouth. Researchers have now confirmed that some materials provide more efficient filtering with some (properly fitted) homemade masks recently found to be similar to that of some medical masks that were tested.
RELATED ARTICLE: Cloth Face Mask Tips and Recommendations
Household Material Particle Filtering Research
Reseachers at the University of Illinois at Urbana-Champaign tested the breathability and droplet blocking ability of 10 common household fabrics. The fabrics had different combinations of cotton, polyester and silk and included two- and three-layered T-shirt fabric. In two layers, T-shirt fabric had a 98% droplet blocking efficiency and had better breathability than the medical masks (even when used in 3-layers).
The Smart Air company, led by Paddy Robertson, CEO expanded testing to include 30 additional household materials including paper towels, bra pads and scarfs. Smart Air used a laser particle counter to measure the penetration of mask material for larger (1.0 micron) particles and smaller (0.3 micron) particles. The COVID-19 coronavirus measures 0.06-0.14 microns by itself and 5-10 microns when piggy-backing on respiratory droplets.
For larger particles (> 1.0 micron) the N95 mask, surgical mask, and HEPA filter performed best, capturing over 99% of particles. Surprisingly, the HERO coffee filter captured 98% while paper towels, canvas, denim, and the cotton bed sheet also captured more than 90%. All materials were far better than nothing, blocking over 50% of particles. The four worst-scoring materials were the wool scarf, polyester neck warmer, cotton bandana, and light scarf.
To evaluate the filtering effect of smaller particles, the Smart Air group measured 0.3 micron particles (the most difficult particles to capture). The N95 mask, HEPA filter, and surgical mask still did best, but there was a much wider range in effectiveness (all captured >75%). The HERO coffee filter again came in next, capturing 62% while only four other materials filtered more than 48% of the smaller particles: 40D nylon, CHEMEX coffee filter, dish towel, and canvas.
How Easy Is It to Breathe Through These Materials?
In general, natural fibers (like cotton and paper) filter better than synthetic fibers (like polyester and polypropylene). Based on the above filtering data, it would seem reasonable to recommend the HERO coffee filter or blue nylon as the best materials for homemade masks. However, some materials that are excellent at filtering particles are also very difficult to breathe through. For practical purposes, researchers identified the materials with breathability similar to medical face masks that filtered most efficiently. When tested for breathability, double layer 100% cotton T-shirt, bra pad, 70D nylon, paper hand drying towel, cotton bed sheet (120 thread) and denim (10 oz) all fell within the breathability range of a surgical and N95 mask.
Smart Air recommended a Top 5 list of household materials based on a balance between filtering efficiency and breathability. Materials like coffee filters perform exceptionally well at filtering tiny particles, but they are very difficult to breathe through. Based on a combination of breathability and filtration effectiveness, Smart Air researchers recommend the use of:
- Denim (10 oz)
- Bed sheets (80-120 thread)
- Paper towel
- Canvas (0.4-0.5 mm thick)
- Shop towels
Further, researchers from the Argonne National Laboratory and the University of Chicago found that multiple layers and mixing up fabrics worked best to filter particles. Although the filtration efficiencies for single layer fabrics ranged from 5 to 95% (for particle sizes of <300 nm ), the filtering improved when multiple layers were used. Further filtering effectiveness was improved by combining different fabric types.
"Filtration efficiencies of the hybrids (such as cotton–silk, cotton–chiffon, cotton–flannel) was >80 percent (for particles <0.3 microns) and >90 percent (for particles >0.3 microns). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration."
Mechanical filtration is just the fabric physically catching the particles – like previous research teams, they found that fabrics such as cotton were effective (high thread count work the best). Electrostatic-based filtration is a little different. Materials with 'static-cling' type properties such as polyester seem able to capture the smaller aerosol particles inside the static environment.
One layer of a tightly woven cotton sheet combined with two layers of polyester-spandex chiffon (a sheer fabric often used in evening gowns) filtered out the most aerosol particles (80-99%, depending on particle size) --- performance close to that of an N95 mask material. Substituting the chiffon with natural silk or flannel, or simply using a cotton quilt with cotton-polyester batting, produced similar results.
Best Filtering Face Mask Materials
Tightly woven cotton (80-120 thread count) is recommended in most studies based on a balance of filtering efficiency and breathability. You can enhance the filtering of smaller airborne particles by adding a second layer of cotton material or (even better) a layer of an electrostatic material like chiffon or natural silk. The real take-home message is that you can easily make a homemade face mask that can filter out a majority of airborne droplets - YOU CAN SAVE LIVES, MAYBE YOUR OWN.
Regardless of which face mask material you choose, remember that the correct fit is extremely important. Your face mask should fit snugly over your nose and mouth and remain snugly against your face while breathing. Improper fit and gaps around your mask can easily decrease the filtering ability by more than 60%.
Wear your Face Covering Correctly
- Wash your hands before putting on your face covering
- Put it over your nose and mouth and secure it under your chin
- Try to fit it snugly against the sides of your face
- Make sure you can breathe easily
- Wear a face covering to help protect others in case you’re infected but don’t have symptoms
- Keep the covering on your face the entire time you’re in public
- Don’t put the covering around your neck or up on your forehead
- Don’t touch the face covering, and, if you do, wash your hands
- No face coverings should be used with children < 2 years old or with anyone having breathing difficulty
New COVID-19 research seems to be coming daily. We have a lot more to learn about this virus, and it's not hard to find varying information and recommendations. As we work to end this pandemic, we have to follow the science, focus on the facts, and do everything we can to protective ourselves and others. These are some of those facts:
- We can now confidently say that COVID-19 laden respiratory secretions are emitted as small particles (aerosolized, <5 microns) even with routine breathing and talking.
- Aerosolized viral secretions from an infected person can travel with air currents for distances much farther than 6 feet, and it can remain detectable in the air longer than 3 hours.
- Masks can provide a critical barrier by reducing the amount of virus emitted with breathing, talking, coughing, sneezing, etc. (especially important in asymptomatic people or those with only mild symptoms).
- Tightly woven cotton is an effective barrier face mask material. Filtering efficiency can be improved by layering with additional materials including chiffon, natural silk or even a coffee filter. Adequate breathability and proper fit are important.
- Face mask use is not a substitute for social distancing, hand washing and staying at home when possible.
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