Filtration efficacy was measured for masks made of the following materials: 100% cotton t-shirt, scarf, tea towel, pillowcase, surgical mask, vacuum cleaner bag, cotton mix, linen, and silk. Also noted was "pressure drop": a higher pressure drop makes it harder to breathe, which makes it less likely to be worn. A surgical mask as the filtration layer is the best option where available, but any densely woven filtration layer is superior to none at all: "All the materials tested showed some capability to block the microbial aerosol challenges.... The surgical mask had the highest filtration efficiency...followed by the vacuum cleaner bag, but the bag's stiffness and thickness created a high pressure drop across the material, rendering it unsuitable for a face mask. Similarly, the tea towel, which is a strong fabric with a thick weave, showed relatively high filtration efficiency...but a high pressure drop was also measured. The surgical mask (control) showed the highest filtration efficiency...[and], as expected, its measured low pressure drop showed it to be the most suitable material among those tested for use as a face mask."

For sewn masks to be optimally effective, they need an inserted filtration layer: "Various materials have been assessed for homemade masks with differing outcomes. A study by Davies et al. reported that homemade masks (made from pillowcase or 100% cotton shirt) were one-third as effective as medical masks; even so, homemade masks were significantly able to reduce the number of microorganisms expelled compared to no protection. On the other hand, a recent study comparing homemade masks made from four layers of kitchen paper and one layer of cloth with N95 masks and surgical masks reported comparable efficacy of 95.15% versus 99.98% and 97.14%, respectively, in blocking avian influenza aerosols made by a nebulizer."

This article validates the use of sewn masks but also makes clear that poor fit is the biggest reason why a sewn mask will under-perform compared to surgical masks (this is reported in many articles, but this one is the most thorough). This is the reason we were so meticulous about finding a mask that fits every face type with tight seals along all edges (including the jaw line / chin), and minimal leakage at the nose, and that will also accomodate a surgical mask as a filter layer: "Generally available household materials had between a 49% and 86% filtration rate for 0.02 µm exhaled particles whereas surgical masks filtered 89% of those particles.... When someone is breathing, speaking, or coughing, only a tiny amount of what is coming out of their mouths is already in aerosol form. Nearly all of what is being emitted is droplets. Many of these droplets will then evaporate and turn into aerosolized particles that are 3 to 5-fold smaller. The point of wearing a mask as source control is largely to stop this process from occurring, since big droplets dehydrate to smaller aerosol particles that can float for longer in air.... A comparison of homemade and surgical masks for bacterial and viral aerosols observed that "the median-fit factor of the homemade masks was one-half that of the surgical masks. Both masks significantly reduced the number of microorganisms expelled by volunteers, although the surgical mask was 3 times more effective in blocking transmission than the homemade mask." Research focused on aerosol exposure has found all types of masks are at least somewhat effective at protecting the wearer.... The preponderance of evidence indicates that mask wearing reduces the transmissibility per contact by reducing transmission of infected droplets in both laboratory and clinical contexts. Public mask wearing is most effective at stopping spread of the virus when compliance is high. The decreased transmissibility could substantially reduce the death toll and economic impact while the cost of the intervention is low. Thus we recommend the adoption of public cloth mask wearing, as an effective form of source control, in conjunction with existing hygiene, distancing, and contact tracing strategies. We recommend that public officials and governments strongly encourage the use of widespread face masks in public, including the use of appropriate regulation."

Mask comfort is so important, it is believed to be the reason that N95 and surgical masks often prove equal in preventing respiratory disease transmission in real-world usage: "There was an inverse relationship between the level of compliance with wearing an N95 respirator and the risk of clinical respiratory illness. It is difficult to ensure high compliance due to this discomfort of N95 respirators.... The reason for the similar effects on preventing influenza for the use of N95 respirators versus surgical masks may be related to low compliance to N95 respirators wear, which may lead to more frequent doffing compared with surgical masks. Although N95 respirators may confer superior protection in laboratory studies designing to achieve 100% intervention adherence, the routine use of N95 respirators seems to be less acceptable due to more significant discomfort in real-world practice."

"Symptomatic patients with cough or sneezing are generally advised to put on a face mask, and this applies equally to patients with pulmonary tuberculosis (airborne transmission) and influenza (predominantly droplet-transmitted). With the large number of asymptomatic [COVID-19] patients unware of their own infection, the comparable viral load in their upper respiratory tract [to symptomatic patients], droplet and aerosol dispersion even during talking and breathing, and prolonged viral viability outside our body, we strongly advocate universal use of face mask as a means of source control in public places during the COVID-19 pandemic."

On the importance of reduced air leakage at edges of masks: some studies indicate that a typical pleated surgical mask "does NOT provide the wearer with a reliable level of protection from inhaling smaller airborne particles and is not considered respiratory protection. Leakage occurs around the edge of the mask when user inhales."

"Surgical and N95 masks limit and redirect the projection of airborne droplets, and surgical mask wearing is estimated to be associated with a reduction in overall viral aerosol shedding and coronavirus. While diseases with a large proportion of symptomatic cases may result in most carriers reducing personal contacts (by choice or incapacity), Covid-19 is thought to have a high proportion of mildly symptomatic or asymptomatic cases, and therefore more infectious persons unaware of their status may continue to expose others. As such, even if masks offer limited personal protection, a general recommendation to wear masks in public may be particularly beneficial by containing transmission from unknowingly infectious persons."

In support of prioritizing distribution to elderly recipients: "We consistently observed that the random distribution of masks throughout the general population is a suboptimal strategy. In contrast, prioritizing the elderly population, and retaining a supply of masks for identified infectious cases generally leads to a larger reduction in total infections and deaths than a naïve allocation of resources."

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