The use of an exhalation valve means that the filter fails to capture some of the outgoing virus particles from the wearer’s breath. Some types of N95 also have valves that cause the same problem. Depending on the positioning of the valve, maybe it’s not a big problem? Or it might be easy to mitigate by covering it with cloth or something.
Yes, this is a drawback of any mask with exhalation valve. It is, as you suggest, easy to mitigate by covering the exhalation valve with cloth or a surgical mask. This is however imho not really necessary under most conditions, since the idea is to not get infected in the first place, so you do not shed any virus particles anyway.
“The idea is to not get infected in the first place” is not good thinking.
Given the long asymptomatic infectious period, both the “protect myself” and “protect others” effectiveness matter in a proposal for universal public wearing, although of course compliance is more incentivized by “protect myself”.
That said, I don’t question that these are good overall.
Well, if your chances of getting infected are drastically reduced, then so is the use of the “protect others” effect of wearing the mask, so overall these masks are likely to be very useful.
That said, a slightly modified design that filters air both on the in- and the out- breath might be a good idea. This way, you keep your in-breath filters dry and have some “protect others” effect.
Everybody wearing a respirator could be one of the equilibrium point of the social evolution under the COVID-19, though may be not the only one. Unfortunately, I did not figure other equilibrium point yet. To my best knowledge, nobody gives other end point of the social evolution in a rigors way.
Agree. My comments can certainly be seen as suggesting a starting approach and then refining that approach. However, I would actually expect to see a lot of zigs and zags, and possible multiple types of solutions that will work in different settings.
About the only thing I would say I have any high confidence about is that we need to start doing something. Masks, just like telecommuting for some, is one starting point. It might just end up being the crutch used to get over some hurdle. So I see things more as a stage where we will likely do some things that could be called trial and error approaches (but the error needs to be more on the “we cannot do this because I cannot stand doing X” side and not “we cannot do it this way because we just tripled R0!” side)
For sure people having a hard time breathing already will skimp on optional exit valve filtering, but you can sell it as helping-others to comply, and people will at least brag about how they’re doing it.
The N95 mask does a poor job of keeping virus particles from being inhaled from an aerosol; the water droplets are largely caught by the filter media, and then evaporate, leaving the virus to be sucked through.
The virus isn’t expected to spread by straight airborne transmission between people, but that’s poor evidence that it won’t be airborne from the distance from your mask to your lungs.
As EGI says that doesn’t seem to be correct but he is talking about N99 not N95 so I’m wondering about this.
My understanding is that the N95 standard is about filtering 95% of the particles and the test level is for particle distributions with a mean of 75nm and standard deviation of less than 2nm. SARS-C0V-2 is thought to be 80 − 120 nm in size (and generally spherical). While the smallest would be close to the mean for N95 seems to suggest 95% should still be filtered (assuming the filter is not damaged).
Adsorption air filters are not sieves or membrane filters, particles are captured by adsorption to the filter medium, not by size exclusion. The pessimum of filtration efficiency is afaik around 1 µm with higher capture efficieny below that due to higher collision probability due to more brownian motion. Not completely sure of the numbers though.
I’d go with P 99 or 100 since they are not that much more expensive / unpleasant to wear and we want to have as little particle leakage as possible since we do not know how much dose reduction is needed to reduce infection probability by one to two orders of magnitude. A hundredfold seems plenty though.
Also note that virus particles do not fly alone since they are allway bound in liquid or whatever remains after the droplet dries. CoV-2 seems to be nonviable when dried though so you need not worry about dry stuff.
Interesting. While I knew some of the filtering was stuff just sticking together I had thought some level of particle size filtering was also present—and not on a gross size scale.
See my reply to you above. That said, even if the virus was dry airborne as you assume, P3/N99 filters would still capture way above 99% of these particles as explained in my original post.
I don’t think that it’s safe to generalize “99% of particles” to “99% of virus particles”. It’s likely better than nothing and might even be adequate, especially since N95 seems to be the gold standard.
Yeah, it will be way more than 99% of virus particles since most virus particles are bound in larger droplets where filtration efficiency is much higher than 99 %.
The use of an exhalation valve means that the filter fails to capture some of the outgoing virus particles from the wearer’s breath. Some types of N95 also have valves that cause the same problem. Depending on the positioning of the valve, maybe it’s not a big problem? Or it might be easy to mitigate by covering it with cloth or something.
Yes, this is a drawback of any mask with exhalation valve. It is, as you suggest, easy to mitigate by covering the exhalation valve with cloth or a surgical mask. This is however imho not really necessary under most conditions, since the idea is to not get infected in the first place, so you do not shed any virus particles anyway.
“The idea is to not get infected in the first place” is not good thinking.
Given the long asymptomatic infectious period, both the “protect myself” and “protect others” effectiveness matter in a proposal for universal public wearing, although of course compliance is more incentivized by “protect myself”.
That said, I don’t question that these are good overall.
Well, if your chances of getting infected are drastically reduced, then so is the use of the “protect others” effect of wearing the mask, so overall these masks are likely to be very useful.
That said, a slightly modified design that filters air both on the in- and the out- breath might be a good idea. This way, you keep your in-breath filters dry and have some “protect others” effect.
Everybody wearing a respirator could be one of the equilibrium point of the social evolution under the COVID-19, though may be not the only one. Unfortunately, I did not figure other equilibrium point yet. To my best knowledge, nobody gives other end point of the social evolution in a rigors way.
Agree. My comments can certainly be seen as suggesting a starting approach and then refining that approach. However, I would actually expect to see a lot of zigs and zags, and possible multiple types of solutions that will work in different settings.
About the only thing I would say I have any high confidence about is that we need to start doing something. Masks, just like telecommuting for some, is one starting point. It might just end up being the crutch used to get over some hurdle. So I see things more as a stage where we will likely do some things that could be called trial and error approaches (but the error needs to be more on the “we cannot do this because I cannot stand doing X” side and not “we cannot do it this way because we just tripled R0!” side)
For sure people having a hard time breathing already will skimp on optional exit valve filtering, but you can sell it as helping-others to comply, and people will at least brag about how they’re doing it.
The N95 mask does a poor job of keeping virus particles from being inhaled from an aerosol; the water droplets are largely caught by the filter media, and then evaporate, leaving the virus to be sucked through.
The virus isn’t expected to spread by straight airborne transmission between people, but that’s poor evidence that it won’t be airborne from the distance from your mask to your lungs.
As EGI says that doesn’t seem to be correct but he is talking about N99 not N95 so I’m wondering about this.
My understanding is that the N95 standard is about filtering 95% of the particles and the test level is for particle distributions with a mean of 75nm and standard deviation of less than 2nm. SARS-C0V-2 is thought to be 80 − 120 nm in size (and generally spherical). While the smallest would be close to the mean for N95 seems to suggest 95% should still be filtered (assuming the filter is not damaged).
Adsorption air filters are not sieves or membrane filters, particles are captured by adsorption to the filter medium, not by size exclusion. The pessimum of filtration efficiency is afaik around 1 µm with higher capture efficieny below that due to higher collision probability due to more brownian motion. Not completely sure of the numbers though.
I’d go with P 99 or 100 since they are not that much more expensive / unpleasant to wear and we want to have as little particle leakage as possible since we do not know how much dose reduction is needed to reduce infection probability by one to two orders of magnitude. A hundredfold seems plenty though.
Also note that virus particles do not fly alone since they are allway bound in liquid or whatever remains after the droplet dries. CoV-2 seems to be nonviable when dried though so you need not worry about dry stuff.
Interesting. While I knew some of the filtering was stuff just sticking together I had thought some level of particle size filtering was also present—and not on a gross size scale.
See my reply to you above. That said, even if the virus was dry airborne as you assume, P3/N99 filters would still capture way above 99% of these particles as explained in my original post.
I don’t think that it’s safe to generalize “99% of particles” to “99% of virus particles”. It’s likely better than nothing and might even be adequate, especially since N95 seems to be the gold standard.
Yeah, it will be way more than 99% of virus particles since most virus particles are bound in larger droplets where filtration efficiency is much higher than 99 %.
And, as I’ve said above, I think that it’s not sufficiently safe to assume that they inactivate within seconds of drying out.