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Making a Homemade Mask As Effective as a Medical or N-95 Mask
Question: How can I make a mask that is as good as a surgical mask or an N-95 mask?
Answer: If made with the right household materials, you can create a mask that may be as effective as a medical mask or even an N-95 respirator, according to several laboratory studies. Masks can be used alone or, for increased protection, particularly for the eyes, with a face shield.
How materials compare in blocking coronavirus
The first study, from the University of Illinois at Urbana-Champaign, found that many household fabrics can be as effective as the material in surgical masks for blocking droplets the size of the COVID-19 (SARS-CoV-2) coronavirus. The blocking efficiency of a commercial medical mask was found to be 96.3%, while the blocking efficiency of a used dish cloth (85% polyester and 15% nylon) was slightly better -- 97.9%. In addition, most household fabrics were more breathable than the material in a medical mask. The dish cloth, for example, was twice as breathable as the medical mask (Aydin, medRxiv 2020 --preprint). (See the CDC website to learn how to make a cloth face covering.)
A study at the University of Chicago and Argonne National Laboratory found that tightly woven, high-thread count cotton (600 thread-per-inch (TPI) sheet by Wamsutta) was more effective in filtering large droplets (similar to larger-sized SARS-CoV-2 droplets) than loosely woven cotton with a lower thread count (quilters cotton, 80 TPI), while fabrics with an electrostatic charge (such as silk and chiffon) were best for blocking aerosols -- the smaller sized droplets that remain suspended in air for extended amounts of time. Using layers of both fabrics, together, was most effective for blocking both large and small droplets. For example, two layers of 600 TPI cotton fabric had a large particle and small particle blocking efficacy of 99.5% and 82%, respectively, but one layer of 600 TPI cotton combined with two layers of chiffon (90% polyester, 10% spandex from Jo-Ann Stores) had a large particle and small particle blocking efficacy of 99.2% and 97% -- which is nearly as good as a properly-fitted N95 mask for blocking large particles and better than the N-95 with respect to small particles, of which only 85% are blocked by an N-95 mask). The researchers also found that small holes or leaks around the edges of the fabrics could decrease the blocking efficacy by 50% or more, and emphasized the importance of a good fit (snug and without gaps) (Konda, ACS Nano 2020). [Note: An illustration in the study shows the electrostatic layer of fabric as the inner layer when fabrics were combined. However, ConsumerLab contacted the author of the study who suggested that electrostatic fabric (such as chiffon) may be best used as the outer layer of the mask to avoid humidity from the nose or mouth, which could interfere with the electrostatic properties, but emphasized that was his suggestion, not something that was tested in the study.]
How to reduce air leakage around a mask
A way to reduce air leaks was suggested by a study, at Northeastern University in Boston, which showed that pulling an 8 to 10-inch tube of nylon (cut from a queen-sized nylon stocking) down over a regular mask and to the top of the neck. This significantly prevented air leakage around the mask and improved particle filtration efficiency, making the combined masking nearly as effective as an N-95 respirator which, unlike a medical mask, has an electrostatic charge and is specifically designed to prevent air leakage (Mueller, medRxiv 2020 --preprint; Godoy, NPR.org 4/22/20).
How to clean a cloth mask
Cloth masks can be washed in a washing machine. They can also be cleaned using heat, but a washing machine is preferred.
Any benefit to wearing masks at home?
Although CDC guidelines do not currently include the use of face masks at home, the rate of transmission from one household family member to another was 79% lower when members wore face masks prior to the first member developing COVID-19 symptoms, according to a study of 124 families in Beijing in which there was at least one infected person. Overall, there was a 23% rate of transmission of COVID-19 from an infected family member to another, but this was no lower when mask wearing began after the first member developed symptoms. These results are consistent with the fact that viral load is highest two days before symptoms and on the first day of symptoms. Daily use of disinfectants reduced transmission by 77%. Transmission rates were four times higher if the primary case had diarrhea and 18 times higher when there was frequent daily close contact (less than 3 feet apart). The researchers recommended use of face masks in families in which a member has been at risk of getting infected. In China, over 70% of transmission occurred within families (Wang, BMJ Global Health 2020).
Potential carbon dioxide build-up
If you make a mask that is as effective as an N-95 respirator, it may share the same potential that N95 masks have to reduce oxygen intake and, over time cause carbon dioxide build-up inside the mask (Sinkule, Ann Occup Hyg 2013). According to researchers at Stanford University, N95 masks are "are estimated to reduce oxygen intake by anywhere from 5 to 20 percent. That's significant, even for a healthy person. It can cause dizziness and lightheadedness." However, a small study in the U.S. found that wearing an N-95 mask for up to one hour did not cause any significant adverse effects in healthy healthcare workers performing moderate activities, despite significantly decreased inhaled oxygen and increased inhaled carbon dioxide levels (Roberge, Respir Care 2010). A representative from the CDC told Reuters.com that "...the level of CO2 likely to build up in the mask is mostly tolerable to people exposed to it. You might get a headache but you most likely [would] not suffer the symptoms observed at much higher levels of CO2. The mask can become uncomfortable for a variety of reasons including a sensitivity to CO2 and the person will be motivated to remove the mask. It is unlikely that wearing a mask will cause hypercapnia [elevated blood levels of carbon dioxide]."
The CDC advises that face masks should not be placed on children under the age of two, anyone who has trouble breathing, or anyone who would not be able to remove the mask without assistance.
N-95 vs. KN-95 respirators
Be aware that masks labeled as "KN-95" are not N-95 respirators and their filtration may not match that of N-95 masks. KN-95 masks may also not have a proper fit to prevent air leakage and typically have "ear loops" rather than the head bands used on N-95 respirators. The CDC has published its own tests of many of KN-95 and related masks, by brand, showing filtration levels as low as 13.6% to over 99%; however air leakage was not tested, so a KN-95 mask filtering 99% may be inferior to an N-95 respirator that provides a proper fit even though its filtration efficiency need not be greater than 95%).