5 ms·

Iono, from page 6: "However, taking into account the available studies evaluating pre- and asymptomatic transmission, a growing compendium of observational evi

Iono, from page 6: "However, taking into account the available studies evaluating pre- and asymptomatic transmission, a growing compendium of observational evidence on the use of masks by the general public in several countries, individual values and preferences, as well as the difficulty of physical distancing in many contexts, WHO has updated its guidance to advise that to prevent COVID-19 transmission effectively in areas of community transmission, governments should encourage the general public to wear masks in specific situations and settings as part of a comprehensive approach to suppress SARS-CoV-2 transmission (Table 2)." The later stuff is on filtration by cloth masks. I don't think that's why cloth masks are effective. They're terrible filters. Don't think of cloth masks as filters. N95 does that, but cloth masks are useless for that. What they do is alter the trajectory of respiratory droplets coming out of your mouth. When you talk, your vocal cords vibrate, and that aerosolizes tiny liquid droplets from your respiratory tract. These droplets may contain viral particles. When you wear a cloth mask, you're not filtering out those particles, you're altering their trajectory. With the cloth barrier, the droplets catch on the mask, or are deflected at an angle that is not straight-ahead at the level of other people's mouth. So if everyone is wearing masks, then it disrupts the vectors of virus-containing droplets being directed from one person's mouth directly into another person's airway, especially during vocalization. The redirected particles spent more time in the air, and so have more of a chance to dry out before being inhaled. It's one-big harm-reduction strategy, where the goal is not to completely filter out viral particles, but rather to make sure that you're not directly inhaling too many on a given exposure (sub-infectious dose). That's how I think of it. There's no RCT or anything that proves it, but I haven't thought of any good argument for why this isn't a good model for what's going on. Although hand-washing and physical distance are the gold-standard, part of the problem is that it doesn't seem to be feasible as an option (at least in the US), so we need to rely more heavily on the single best harm-reduction strategy, which seems to be masks at the moment. It's the difference between the best option in a lab, and the best option in a crowded grocery store or subway train where people aren't necessarily going to follow the rules.

parliament32 6y ago
It's a fun theory but "filtration efficiency" applies in both directions, including droplets you're spewing out. Trajectory would matter if the droplets were falling to the ground, but considering 1) the aerosolized droplets you're talking about linger in the air for 5-8 minutes, and 2) when talking, they don't travel further than 0.3m in >90% of cases[1], it really matters little. To clarify, you said "With the cloth barrier, the droplets catch on the mask, or are deflected at an angle" -- I'm saying that "catch on the mask" is precisely the filtration efficiency noted above, and "deflected" doesn't matter if they're just hanging in the air for several minutes, <30cm from head level (even in the pre-covid days, I don't think it'd be socially acceptable to talk to someone within a foot of their head). I'm not saying cloth masks don't help at all, just that they help very, very little. Consider that the filtration efficiency drops from the numbers in that table immediately after you start wearing it, and that it gets reduced a further 1-4x after you've washed it the first time[2], it's not really worth the effort, hence WHO discourages it. [1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843952/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843952/ [2]https://www.koreascience.or.kr/article/JAKO201521839156288.page https://www.koreascience.or.kr/article/JAKO201521839156288.p...
- phobosanomaly 6y ago
  I dunno, I've posted two quotes from your WHO sources that are both instructions by the WHO to governments to advocate cloth mask usage to prevent transmission of SARS-CoV-2 in community-based environments where ideal social distancing isn't possible. They do seem to be advocating it, not discouraging it. The first paper you linked to does not look at respiratory droplet transmission with a mask on. The second paper only has the abstract in English. I can't read Korean, but based on the abstract they're looking at pressure drops across a cloth filter. Again, it's not about direct filtration. It's about disrupting the stream of viral particles from one person's airway directly into another person's airway. I'm 100% open to criticism on why my model doesn't make sense, but my foundational premise is that masks function primarily by disrupting direct stream of viral particles from one person's airway to another primarily when facing each other and vocalizing. This paper shows the change in dispersion of respiratory droplets when placing a surgical mask on a coughing patient (surgical mask being very similar to the cloth masks people are using). Exhaled Air Dispersion during Coughing with and without Wearing a Surgical or N95 Mask https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516468/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516468/ "During normal cough, exhaled air dispersion distances along the median sagittal plane was significantly reduced to ≤ 30 cm when the HPS wore either a surgical mask or a N95 mask." Figure 3. really shows this in a graphical form. Here's another cool paper. Textile Masks and Surface Covers—A Spray Simulation Method and a “Universal Droplet Reduction Model” Against Respiratory Pandemics https://www.frontiersin.org/articles/10.3389/fmed.2020.00260/full https://www.frontiersin.org/articles/10.3389/fmed.2020.00260... "Using a bacterial-suspension spray simulation model of droplet ejection (mimicking a sneeze), we quantified the extent by which widely available clothing fabrics reduce the dispersion of droplets onto surfaces within 1.8 m, the minimum distance recommended for COVID-19 “social distancing.” All textiles reduced the number of droplets reaching surfaces, restricting their dispersion to <30 cm, when used as single layers."

parliament32 6y ago

It's a fun theory but "filtration efficiency" applies in both directions, including droplets you're spewing out. Trajectory would matter if the droplets were falling to the ground, but considering 1) the aerosolized droplets you're talking about linger in the air for 5-8 minutes, and 2) when talking, they don't travel further than 0.3m in >90% of cases[1], it really matters little. To clarify, you said "With the cloth barrier, the droplets catch on the mask, or are deflected at an angle" -- I'm saying that "catch on the mask" is precisely the filtration efficiency noted above, and "deflected" doesn't matter if they're just hanging in the air for several minutes, <30cm from head level (even in the pre-covid days, I don't think it'd be socially acceptable to talk to someone within a foot of their head). I'm not saying cloth masks don't help at all, just that they help very, very little. Consider that the filtration efficiency drops from the numbers in that table immediately after you start wearing it, and that it gets reduced a further 1-4x after you've washed it the first time[2], it's not really worth the effort, hence WHO discourages it. [1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843952/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2843952/ [2]https://www.koreascience.or.kr/article/JAKO201521839156288.page https://www.koreascience.or.kr/article/JAKO201521839156288.p...

phobosanomaly 6y ago
I dunno, I've posted two quotes from your WHO sources that are both instructions by the WHO to governments to advocate cloth mask usage to prevent transmission of SARS-CoV-2 in community-based environments where ideal social distancing isn't possible. They do seem to be advocating it, not discouraging it. The first paper you linked to does not look at respiratory droplet transmission with a mask on. The second paper only has the abstract in English. I can't read Korean, but based on the abstract they're looking at pressure drops across a cloth filter. Again, it's not about direct filtration. It's about disrupting the stream of viral particles from one person's airway directly into another person's airway. I'm 100% open to criticism on why my model doesn't make sense, but my foundational premise is that masks function primarily by disrupting direct stream of viral particles from one person's airway to another primarily when facing each other and vocalizing. This paper shows the change in dispersion of respiratory droplets when placing a surgical mask on a coughing patient (surgical mask being very similar to the cloth masks people are using). Exhaled Air Dispersion during Coughing with and without Wearing a Surgical or N95 Mask https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516468/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3516468/ "During normal cough, exhaled air dispersion distances along the median sagittal plane was significantly reduced to ≤ 30 cm when the HPS wore either a surgical mask or a N95 mask." Figure 3. really shows this in a graphical form. Here's another cool paper. Textile Masks and Surface Covers—A Spray Simulation Method and a “Universal Droplet Reduction Model” Against Respiratory Pandemics https://www.frontiersin.org/articles/10.3389/fmed.2020.00260/full https://www.frontiersin.org/articles/10.3389/fmed.2020.00260... "Using a bacterial-suspension spray simulation model of droplet ejection (mimicking a sneeze), we quantified the extent by which widely available clothing fabrics reduce the dispersion of droplets onto surfaces within 1.8 m, the minimum distance recommended for COVID-19 “social distancing.” All textiles reduced the number of droplets reaching surfaces, restricting their dispersion to <30 cm, when used as single layers."