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Since we're talking about counterintuitive things related to the coronavirus crisis, I was surprised to learn that N95 masks, that remove 95% of particles of si
by alister 6y ago
Since we're talking about counterintuitive things related to the coronavirus crisis, I was surprised to learn that N95 masks, that remove 95% of particles of size 0.3 μm, are even more efficient with smaller particle sizes. In turns out that 0.3 μm is the hardest particle size to filter--they call it the most penetrating particle size--hence filter testing is done at this worst-case size. The reason is that filters work due to several mechanisms which dominate at different particle sizes according to a curve:
"For very small particles, less than 0.1 μm in diameter, the primary filtration mechanism is diffusion and the filter is very efficient. For particles between approximately 0.1 and 0.4 μm the filter is less efficient as the particles are too large for a great diffusion effect and too small for a large interception effect. Particles above approximately 0.4 μm enter the region where interception along with inertial impaction are predominant and the filter is very efficient again."[1]
So if your filter is 95% efficient at 0.3 μm, it's even better at bigger and smaller sizes. The SARS-CoV-2 virus is 0.05 to 0.2 μm in diameter, so an N95 mask will be substantially better than 95% for coronavirus.
[1] https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24aedba14/ITI-041-A4?ext=.pdf https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24ae...
- whatever1 6y agoSame reason that we care about the concentration of PM2.5 particles in the atmosphere. Once we inhale them they manage to reach the very end of our lungs.
- EForEndeavour 6y agoIs it? That's different from the counterintuitive N95 filtration efficacy. PM2.5 designates particles of diameter 2.5 microns and smaller, and I've read nothing that implies particles smaller than 2.5 microns are somehow easier for the respiratory tract to keep out of the lungs. The graph of filtration efficacy as a function of particle size is U-shaped for N95 masks, but not for the human respiratory tract.
- henvic 6y agoYou might find it interesting to know too that HEPA filters get more effective as they approach end-of-life.
- davidwihl 6y agoSo is the only reason to change filters to avoid blockage and reduced airflow but not filtering capacity?
- deleted 6y ago[deleted]
- Gibbon1 6y agoI think they can also collect enough organic matter for mold to start growing on the downstream side. I base that on pictures of automotive cabin filters that are revolting.
- sizzle 6y agoWould some generous Lysol spray get rid of the mold?
- Gibbon1 6y agoJust flip it around
- AstralStorm 6y agoYes, but it will ruin dust filtration, so the proper choice is hot water vapor, dry heat, or hydrogen peroxide. Perhaps silver ions too. HEPA filters use exactly the same meltblown fabrics as respirators, except thicker and in accordion form.
- weaksauce 6y ago> So if your filter is 95% efficient at 0.3 μm, it's even better at bigger and smaller sizes. The SARS-CoV-2 virus is 0.05 to 0.2 μm in diameter, so an N95 mask will be substantially better than 95% for coronavirus. small nit. the corona virus gloms onto other particles of differing sizes in an aerosol though it's not super clear what size they are. > The size of infective airborne viral particles is not well known and the aerosol transmission of viral diseases is currently debated in the literature (Roy and Milton, 2004; Tellier, 2006). https://academic.oup.com/annweh/article/52/3/177/312528 https://academic.oup.com/annweh/article/52/3/177/312528
- caseysoftware 6y agoFor a more observable example, think about sorting American coins. A dime is the easiest because it is the smallest. Quarters are much larger so are also easy. In the middle are pennies and nickels which are the hardest to separate from one another because they're so close in size. Here's a video that demonstrates it: https://www.teachengineering.org/activities/view/cub_coinsorter_activity1 https://www.teachengineering.org/activities/view/cub_coinsor...
- jbay808 6y agoSince a filter sorts particles from air molecules, this would imply that the hardest particles to filter are just a couple atoms large.
- mokus 6y agoTo be fair, that’s probably true. It’s just that the particles at that scale are other molecules and they are indeed very hard to filter out mechanically.