Filtering out dumb mask ideas.

[CRGreathouse]

Originally Posted by a1call

For tier record N95 designation is not meant to filter out fumes:

https://www.massnurses.org/health-an.../openItem/1318

It is true that N-95s do not filter out gasses or most odors. (For that, you'd want a cartridge respirator, generally half-face or full-face, or an activated-carbon mask.) But the explanation in the article is actually incorrect! Let me quote it:

N95 respirators use a filter of densely woven fibers that can stop aerosol particles through impaction, interception and diffusion as the air being breathed in passes the mesh. They are 95 percent efficient in stopping particles down to about 0.1 micrometers (microns) in diameter. So they work well on tuberculosis, and other bacteria, that range in size from about 0.3 to 20 microns.

Gas molecules, however, range in size from only 0.0003 - 0.006 microns. As a result, gases like oxygen, chlorine, hydrogen sulfide and ammonia can all pass freely in the spaces between the fibers in an N95 mask.

The first sentence is correct. The key is that small particles are stopped by diffusion, medium-sized particles by diffusion and interception, and large particles by impaction. Impaction is highly efficient (big particles have trouble passing through fine mesh) and the respirator is more than 95% efficient here, say above 1 micron. Below 100 nm you're in the pure diffusion regime and the respirator is also more than 95% efficient (by 30 nm efficiency is well more than 99%). But in the murky middle ground diffusion doesn't work too well, but the particles are too small for impaction, so you can only rely on interception or the occasional diffusion. Efficiency reaches its nadir around 300 nm; to be certified as an N-95, test particles at this size must be filtered with 95% efficiency.

The size range of gas molecules, 0.3 to 6 nm, is perfect for diffusion. N-95s could filter particles of that size with essentially 100% efficiency. But gasses won't bind themselves with Van der Waals force to the fibers like particles will, so they just slip right past.

[CRGreathouse]

Originally Posted by kriesel

Possibly surgical over N95. Intake resistance is increased, as is exhale resistance, and mask consumption rate. Surgical under N95 would defeat the face seal of the N95. Does the surgical over N95 interfere with the check valve action? No samples here to examine.

This is hard. You want the N-95 to seal properly, like kriesel said, so putting it inside is the natural way. But putting a surgical mask outside wouldn't help and would definitely make breathing harder. It would probably make you less safe because you'd be more likely to defeat the seal drawing a harder vacuum.

But there could be value to putting something inside the N-95 to catch moisture, since the mask getting moist from exhalation can defeat its protection to some extent (after hours of use). It would have to be done very, very carefully.

[Uncwilly]

Those who regularly wear and are annually retrained and retested on halfmasks and SCBA know that you don't want chlorine based bleaches anywhere near the valves. It attacks the rubber that is used. When cleaning the masks the normal cleaning products don't have chlorine based bleach. When cleaning a halfmask for use a different user, the valves (and harness) are removed before sanitizing the face-piece in a very mild bleach solution.

I have been using halfmasks for decades. I have worn them for nearly a full 9 hour shift many times. Just last week I wore mine with HEPA prefilters and cartridges for acid gas/organic vapors/H₂S (that is a single cartridge like this https://www.grainger.com/product/HON...rtridge-16M236) while doing heavy physical labour.

Any alteration of respirators is forbidden by law. You may not add to them. Or modify them. Or use cartridges from a different manufacturer than the mask. All parts used to repair have to be OEM (like replacing bad valves or cartridge mounts, which I have done over the years) and designated by the manufacturer for that model and size.

When you use them to keep you from messing up your lungs (like I do at work) you pay close attention to things. I have had co-workers using SCBA go into IDLH environments routinely. And once several suffered severe injuries in what became (unexpectedly) an IDLH environment. One of them (as was told to me by those there at the time) had to have medical intervention to restart their heart and respiration. They were hospitalized for a number of days afterward. Their lungs were so damaged that they have had to be on oxygen as long as I have known them. They have been rehospitalized several times since then with lung issues. And their doctor warned them about the first SARS before it made the western news.


Oh, BTW, not all N95 masks have valves on them. I handled some at work last week. We are inventorying our stock of PPE to see that we have enough and if we have enough to share with healthcare users. I had to count boxes of gloves, N95's, tyvek suits, safety glasses, goggles, and face shields. Further I have to report how many nitrile gloves I am using each day at the end of shift.

And have you thought about what happens when an N95 with a central valve that has a downward exit (the valve is attached on the top) is used by a person who is also wearing a face shield? Where do that particles go? Is there a need to do anything else?

[kriesel]

Originally Posted by a1call

I might not have been clearly stating it but I was referring [to] a novel design, not an upgrade of existing N95 masks which seem to have turned the nursing homes into slaughter houses for the the elderly around the world.

What's your source for the implied claim that the infectious nursing home staff were wearing masks, much less properly wearing initially uncontaminated N95s? People working in low paid jobs like food service or as CNAs are notorious for working while ill due to economic necessity. And some in the nursing home industry are known to increase profits by cutting corners on quality of care. As a former employer put it, contemptuously, "he made a killing in the nursing home business." https://www.nytimes.com/2020/04/02/u...sing-home.html "Life Care did not have an adequate infection control system in place and failed to provide quality care, among other findings."
"Life Care" or its patients presented a high enough infection risk that numerous first responders became infected from going there. Note the only mention of PPE in this article is a change in policy for first responders. https://www.businessinsider.com/coro...ak-2020-3?op=1

[kriesel]

Given that estimated eventual production rate of N95 masks by the 3 leading manufacturers is 300 million / month, after ramp-up of production, not adequate for single use in peak pandemic demand by medical staff, and increasing mask usage by the general public, with consequent improvising of cloth masks by both health care workers and the general public, study of cloth mask effectiveness is needed. The initial results are not encouraging.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4420971

[kriesel]

Originally Posted by Uncwilly

BTW, not all N95 masks have valves on them.

No N95 mask I've ever owned or used had any check valve. That's from buying as PPE for painting, construction, etc, not medical use, though what's left in the large box made in India years ago may get repurposed.

[Till]

Damn, looks as if I somehow lost my ABC mask. The guys in my local bank would surely welcome me showing up with it to protect them, no?

[Uncwilly]

I am going to give blood (platelets actually) today. This will be the first time that I pull out an N95 that I have sitting in my drawer. I have surgical masks, the 3D style, and N95 all that I have bought 3 or more years ago to ~8 months ago.

[ewmayer]

Originally Posted by CRGreathouse

The key is that small particles are stopped by diffusion, medium-sized particles by diffusion and interception, and large particles by impaction. Impaction is highly efficient (big particles have trouble passing through fine mesh) and the respirator is more than 95% efficient here, say above 1 micron. Below 100 nm you're in the pure diffusion regime and the respirator is also more than 95% efficient (by 30 nm efficiency is well more than 99%). But in the murky middle ground diffusion doesn't work too well, but the particles are too small for impaction, so you can only rely on interception or the occasional diffusion. Efficiency reaches its nadir around 300 nm; to be certified as an N-95, test particles at this size must be filtered with 95% efficiency.

By way of reference, per Wikipedia, the diameter of Coronavirus particles is around 120 nm.

[kriesel]

Originally Posted by Till

The guys in my local bank would surely welcome me showing up with it to protect them, no?

Northern Wisconsin retail locations get their share of hunters, loggers, snowmobilers and skiers in ski masks / balaclavas. (worn as in upper left in photo, or with less exposure, and with UV blocking aviator or wraparound sunglasses) https://en.wikipedia.org/wiki/Balaclava_(clothing)

[Till]

Originally Posted by kriesel

Northern Wisconsin retail locations get their share of hunters, loggers, snowmobilers and skiers in ski masks / balaclavas. (worn as in upper left in photo, or with less exposure, and with UV blocking aviator or wraparound sunglasses) https://en.wikipedia.org/wiki/Balaclava_(clothing)

Is this guy from Northern Wisconsin? (Alert: Video probably not apt for people below 18)

https://www.youtube.com/watch?v=hTbPmkSAk70


By the way I think that ski masks don't offer too much protection against Covid-19. An ABC mask is much safer!