4 ms·

This is pretty much meaningless. An N95, in order to be N95, must filter out 95% of 2.5um particles. For larger particles, this is almost certainly an lower bo

This is pretty much meaningless. An N95, in order to be N95, must filter out 95% of 2.5um particles. For larger particles, this is almost certainly an lower bound as well, but for smaller particles manufacturing techniques and tolerances can produce a wide variety of responses. Since COVID has been detected in aerosolized droplets well under the micron size, as well as much larger particles in the 5-10um range, we really don't have a ton of information on how this translates to infectiousness for particulate sizes. We don't even have a good understanding of the degree to which infections can spread through tear ducts, which none of these masks address in any significant way. And from an epidemiological perspective, I don't think we'll ever find a signal; behavioral differences between regions is a far larger factor than mask type composition in a population. That said, I think it is intuitively likely that higher filtration and better fitting masks will generally reduce transmission and infection, but I don't think we have an understanding of what those order of magnitudes are. Studies like this seem especially misleading. And some things are somewhat counter-intuitive; like the way that plastic valves actually seem to make things worse for liquid transmission because of their lack of absorbance even as they are more effective at filtering dry particles.

travisporter 5y ago
> larger particles, this is almost certainly an upper bound as well Shouldn't it be better at filtering larger particles, and therefore a lower bound? Or am I getting confused Agree that putting numbers like this are misleading.
- andrewla 5y ago
  Yes, I meant lower bound. Corrected.
aldebran 5y ago
If N95s weren’t effective wouldn’t more medical pros be catching Covid?
- xdennis 5y ago
  Compared to what? Haven't a lot of them already had covid?
fxleach 5y ago
I believe the study mentions many of the issues you just described. Are you saying the article about the study is meaningless or the study itself is meaningless?
vngzs 5y ago
I'm not aware of airborne particle size distribution data existing specifically for covid-19 (please share it if you have it), but the majority of particles that might transmit an airborne virus are usually 0.3 microns or greater in size. That is, while some virus particles may linger and those may be smaller than 0.3 microns, the quantity of particles is highly relevant in determining whether someone will get infected - if we can cut a patient's exposure to virus particles by a significant factor, this may indeed prevent infection. > The count median diameter (CMD) of the cough aerosol particles was fairly similar for all subjects and did not vary significantly during and after illness (p = 0.9340). When the subjects had influenza, the CMDs of all the cough aerosols were between 0.57 and 0.71 μm (average 0.63 μm, SD 0.05), and the geometric standard deviations (GSD) were between 1.54 and 1.83. After the subjects had recovered, the cough aerosol CMDs ranged from 0.57 to 0.89 μm (average 0.63 μm, SD 0.100), and the GSDs were between 1.53 and 2.28. Similar results were seen for the volume median diameters (VMD); the average cough aerosol VMD while ill was 2.44 μm (SD 1.17) and was 2.24 μm (SD 1.50) after recovery. The GSDs ranged from 1.66 to 2.31 while ill and 1.54 to 2.21 after recovery. The difference in VMD while ill and after recovery also was not statistically significant (p = 0.7780). It is notable that the particle sizes did not differ significantly when the patient was infected with a virus versus the control group. This makes me think that covid-19 is similar. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676262/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676262/
cameldrv 5y ago
This isn't quite correct. An N95 must filter 95% of all tested particle sizes (individually) to be certified. In general these types of filters do best with really small particles and really large particles. There is a size in the middle that in practice is between 0.1um and 0.3um that is called the Most Penetrating Particle Size, i.e. the worst case size, and the N95 must filter 95% of that size. For smaller and larger particles, an N95 will filter better than 95%. For Covid, my understanding is that the infectious particles you're worried about are generally larger than the MPPS and that generally an N95 will filter 99%+ of these. As was shown in the article, ultimately it becomes all about fit, because even if only 1% of the air is getting around the mask, improving the filter doesn't help that much in overall efficiency. If you want better than this, you need to go to a higher rated mask with a better face seal like an elastomeric mask with a rubber seal.
wrycoder 5y ago
Actually, 2.5 micron is the most difficult size to filter. For N95 with an electret layer, filtration is better above and below that size.
foo_barrio 5y ago
I thought N95 and other HEPA filters are not simple sieves? You can't just say anything smaller will be collected less as they rely on different effects of the materials to trap particles: * https://en.wikipedia.org/wiki/HEPA https://en.wikipedia.org/wiki/HEPA

travisporter 5y ago

> larger particles, this is almost certainly an upper bound as well Shouldn't it be better at filtering larger particles, and therefore a lower bound? Or am I getting confused Agree that putting numbers like this are misleading.

andrewla 5y ago
Yes, I meant lower bound. Corrected.

aldebran 5y ago

If N95s weren’t effective wouldn’t more medical pros be catching Covid?

xdennis 5y ago
Compared to what? Haven't a lot of them already had covid?

fxleach 5y ago

I believe the study mentions many of the issues you just described. Are you saying the article about the study is meaningless or the study itself is meaningless?

vngzs 5y ago

I'm not aware of airborne particle size distribution data existing specifically for covid-19 (please share it if you have it), but the majority of particles that might transmit an airborne virus are usually 0.3 microns or greater in size. That is, while some virus particles may linger and those may be smaller than 0.3 microns, the quantity of particles is highly relevant in determining whether someone will get infected - if we can cut a patient's exposure to virus particles by a significant factor, this may indeed prevent infection. > The count median diameter (CMD) of the cough aerosol particles was fairly similar for all subjects and did not vary significantly during and after illness (p = 0.9340). When the subjects had influenza, the CMDs of all the cough aerosols were between 0.57 and 0.71 μm (average 0.63 μm, SD 0.05), and the geometric standard deviations (GSD) were between 1.54 and 1.83. After the subjects had recovered, the cough aerosol CMDs ranged from 0.57 to 0.89 μm (average 0.63 μm, SD 0.100), and the GSDs were between 1.53 and 2.28. Similar results were seen for the volume median diameters (VMD); the average cough aerosol VMD while ill was 2.44 μm (SD 1.17) and was 2.24 μm (SD 1.50) after recovery. The GSDs ranged from 1.66 to 2.31 while ill and 1.54 to 2.21 after recovery. The difference in VMD while ill and after recovery also was not statistically significant (p = 0.7780). It is notable that the particle sizes did not differ significantly when the patient was infected with a virus versus the control group. This makes me think that covid-19 is similar. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676262/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4676262/

cameldrv 5y ago

This isn't quite correct. An N95 must filter 95% of all tested particle sizes (individually) to be certified. In general these types of filters do best with really small particles and really large particles. There is a size in the middle that in practice is between 0.1um and 0.3um that is called the Most Penetrating Particle Size, i.e. the worst case size, and the N95 must filter 95% of that size. For smaller and larger particles, an N95 will filter better than 95%. For Covid, my understanding is that the infectious particles you're worried about are generally larger than the MPPS and that generally an N95 will filter 99%+ of these. As was shown in the article, ultimately it becomes all about fit, because even if only 1% of the air is getting around the mask, improving the filter doesn't help that much in overall efficiency. If you want better than this, you need to go to a higher rated mask with a better face seal like an elastomeric mask with a rubber seal.

wrycoder 5y ago

Actually, 2.5 micron is the most difficult size to filter. For N95 with an electret layer, filtration is better above and below that size.

foo_barrio 5y ago

I thought N95 and other HEPA filters are not simple sieves? You can't just say anything smaller will be collected less as they rely on different effects of the materials to trap particles: * https://en.wikipedia.org/wiki/HEPA https://en.wikipedia.org/wiki/HEPA