By Michelle Koh
Since the beginning of April, the CDC has recommended the general public to wear cloth face coverings in public to protect themselves and others around them from infection with SARS-CoV-2 (see our posts on the nomenclature for COVID-19 and face coverings for more information). While surgical masks and N95 masks are some of the most effective methods for preventing transmission of the virus, the CDC has strongly urged the public to use cloth face coverings instead so that these critical supplies can be reserved for those who need them most, such as healthcare workers.
After the announcement by the CDC, there’s been plenty of various tutorials for making DIY face coverings on Facebook and Youtube, in addition to the official instructions released by the CDC. So while there’s no lack of information on how to make masks, you may be wondering exactly which materials you should use to make masks that offer sufficient protection. Here, we’ll go over some common fabrics or materials you could use to make your own masks and how effective they may be in preventing transmission. But first, what should a face cloth covering do?
What should a cloth face covering do?
As research suggests that the COVID-19 virus spreads through respiratory droplets that can also aerosolize (form tiny particles), a face cloth covering should be able to trap and filter these aerosol particles. In addition, the CDC recommends that a face cloth covering cover both your nose and mouth and fit “snugly” and securely over your face. If air and respiratory droplets can flow in through gaps between your face and the covering, the covering - no matter what material you use - will not be able to as effectively protect you from infection.
In fact, research shows that even a 1% gap can reduce the effectiveness of the mask by half or more. Recent research (that has not yet been peer-reviewed) suggests that adding an outer layer of a nylon stocking over a homemade face covering can significantly increase filtration efficiency and decrease airflow around the edges of the covering.
Nevertheless, while a face covering should filter tiny particles and fit snugly, you should also be able to breathe through your face covering. And finally, you should be able to clean your face covering (which you should do regularly, depending on how frequently you use the covering) by machine-washing and drying it.
How effective are surgical masks and N95 masks?
Medical grade N95 masks are designed to prevent the wearer from inhaling particles by capturing >95% of particles that are over 0.3 microns in size (the gold standard, since particles smaller than 0.3 microns are actually less difficult to capture). N95 masks are particularly effective since they’re designed to be worn with a tight fit and made with a non-woven electrostatic material that is able to trap particles with electrostatic attraction.
On the other hand, surgical masks are generally used to prevent the wearer from spreading particles and only capture around 65% of 0.3-micron particles. These masks are designed to be worn more loosely and made of a non-woven material that is more permeable and not as effective at blocking transmission.
Both of these are essential supplies that must continue to be reserved for healthcare workers and other medical first responders, according to current CDC guidance. Though you probably won’t be able to make an N95-grade mask at home, by using combinations of certain materials, you may be able to make face coverings that are as effective as surgical masks (or even better) at filtration. However, if you don’t have these materials at hand, remember that any face covering is better than no face covering when protecting yourself and those around you.
What are some common materials I can use to make face coverings and how effective are they?
Recent peer-reviewed research by Konda et al. investigated the filtration efficiencies of various materials you might find at home. When looking at these numbers, it’s important to remember a key caveat: these results represent the filtration efficiencies of materials when there are no gaps. For instance, the study found that an N95 mask without a gap could filter 99.9% (+/-0.1% margin of error) of particles and that a surgical mask without a gap could filter 99.6% (+/-0.1). However, filtration efficiencies dropped drastically when even a 1% gap was present, decreasing to 12% (+/-3%) for N95 masks and 44% (+/-3%) for surgical masks.
Cotton fabrics can be effective for filtering particles mechanically, or physically trapping and blocking particles. Cotton fabrics with high thread counts, or threads per inch (TPI), are significantly more effective than those with lower thread counts. A single layer of high thread-count quilter’s cotton (600 TPI) filtered around 98.4% (+/-0.2%) of particles over 0.3 microns in size. Two layers of 600 TPI quilter’s cotton was even more effective, filtering around 99.5% (+/-0.1%) of >0.3-micron particles.
In comparison, a single layer of lower thread-count quilter’s cotton (80 TPI) (for reference, most cotton t-shirts have a thread count of 40-50) filtered around 14% (+/-1%) of particles; two layers filtered around 49% (+/-3%).
Even a traditional cotton quilt (Konda et al. measured a sample that consisted of two 120 TPI quilter’s cotton sheets with 0.5cm batting (90% cotton−5% polyester−5% other fibers) in between) was still inferior to high thread-count quilter’s cotton, filtering around 96.1% (+/-0.3%) of >0.3-micron particles. However, the cotton quilt was better at filtering particles smaller than 0.3 microns (96% (+/-2%) for the cotton quilt vs. 82% (+/-19%) for a double-layer of 600 TPI cotton).
Flannel, Natural Silk, or Chiffon
Ever get frustrated when some of your clothes get staticky? According to Konda et al.’s research, some of these fabrics, like natural silk, flannel, and chiffon, may be effective for filtering particles electrostatically (like most N95 masks). Flannel (cotton-polyester) filtered around 44% (+/-2%) of >0.3-micron particles. A single layer of natural silk filtered around 56% (+/-2%) of particles, while a double-layer filtered around 65% (+/-2%). Four layers of natural silk managed to filter 88% (+/-1%) of particles. Of the three fabrics, chiffon (polyester-Spandex) could filter the highest percentage of >0.3-micron particles: 73% as a single layer and even 90% as a double-layer.
Combine cotton with flannel, natural silk, or chiffon
Combinations of cotton and “electrostatic” materials may also be highly effective (see Figure 1). The combination of one layer of high thread-count cotton (600 TPI) + one layer of flannel filtered around 96% (+/-1%) of particles, while one layer of 600 TPI cotton + two layers of silk filtered 98.5% (+/-0.2%) of particles. The combination of one layer of 600 TPI cotton + two layers of chiffon had the highest filtration efficiency at 99.2% (+/-0.2%). Though this is still lower than the filtration efficiency of two layers of 600 TPI cotton for particles larger than 0.3 microns, these combinations were overall more effective at filtering particles smaller than 0.3 microns.
Furnace Filters or HVAC Filters
Other preliminary research (that has not yet been peer-reviewed) has shown that furnace filters or HVAC filters may have high filtration efficiencies. In his preliminary data, Dr. Yang Wang from Missouri University of Science and Technology found that four layers of furnace filter had the highest filtration frequency, compared to four layers of a 600 thread-count pillowcase, scarf, 400 thread-count pillowcase, or a bandana (see Figure 2).
However, since air filters can release fibers that are dangerous to inhale after being cut, take caution when using this material. If you do choose to use air filter material to make your face covering, make sure to sandwich the filter material between layers of thick cotton fabric to prevent inhalation of these fibers. In a collaboration with TSI, a particle-testing equipment company, fashion designer Chloe Schempf found that two layers of Kona 100% Cotton + 1 layer 1500 Filtrete Furnace Filter was able to filter around 80% of particles (you can visit her site for more details on the material combinations she tested).
Similarly, though HEPA vacuum bags have relatively high filtration efficiencies, some bags may contain fiberglass that is also dangerous to inhale. In addition, vacuum bag material may not be very breathable, which is also not optimal for a face covering.
Filti Face Covering Material
Filti has recently started being marketed as a commercially available face mask material. While Schempf found that a combination of one layer of Filti + one layer of 6.5g Cotton Rip Stop could also filter over 80% of particles, this material has not yet been formally approved and confirmed to be safe for masks. Filti is also not washable.
The U.S. Army’s Chemical Biological Center also tested various common materials to assess their effectiveness at filtration. Their research showed that four layers of microfiber fabric had one of the best filtration efficiencies at over 75%.
Coffee Filters & Paper Towels
The CDC mentions coffee filters as potential materials for making face coverings in its official instructions. However, coffee filters may not be very breathable when layered and even then, provide relatively low filtration. Paper towels may be more breathable when layered, but tests show that they also have low filtration efficiency. Schempf also found that coffee filters and paper towels may both be less effective at filtration than cotton t-shirts.
If you don’t have these materials at hand, the light test is an easy way to check if you’re using a material that is non-porous enough. If you can see light through the fabric when you hold it up to a light, it probably won’t be very effective for filtration.
In addition, research by Dr. Loretta Fernandez at Northeastern University found that nylon stockings may also be useful in up-grading the filtration efficiencies of home-made face coverings. Although her research is not yet peer-reviewed, adding a layer of nylon stocking over a home-made face covering (see Figure 3) could reduce gaps around the covering and increase filtration efficiency by 15-30%.
Finally, remember that you should use cloth face coverings in addition to abiding by social-distancing measures (at least 6 feet from other people) and not interchangeably to most effectively protect yourself and others from infection.