[FRIAM] Masks and Face Shields

Wed Aug 26 18:02:02 EDT 2020

Owen, the masks from https://rendallco.com/ seem roomier than others I've
tried.   The "Sentry" feels a bit bigger than the "Ace", though I like
both.  A bit on the pricey side.

Carl

On Mon, Aug 24, 2020 at 9:36 AM Owen Densmore <owen at backspaces.net> wrote:

> I probably missed it, but: Where can you find reasonably effective
> masks of the sort described?
>
> Size appears to be an issue for me as well. I bought a good multilayer,
> filter pocket mask. It fits tightly and has a nose wire. But it is too
> small from nose to chin. One size does not fit all. I got a large mask and
> it does help but it is not as well built as the first mask.
>
> I haven't had much luck finding a website that has multiple sizes, filter
> pockets, good descriptions etc. Any suggestions?
>
>    -- Owen
>
> On Wed, Aug 19, 2020 at 4:43 PM uǝlƃ ↙↙↙ <gepropella at gmail.com> wrote:
>
>> This excellent description landed in my spam folder. So it's likely it
>> did for others as well. Hopefully, the filters that reroute Frank's emails
>> won't be the ones that reroute mine.
>>
>> The "80% of viral particles from entering your nose and mouth" link in
>> the Conversation article was from 2010, but it talks about the 20-1000 nm
>> range: https://academic.oup.com/annweh/article/54/7/789/202744
>>
>> Your link to the built environment website is much more generally
>> informative. Thanks.
>>
>> On 8/17/20 7:43 PM, Frank Chambers wrote:
>> > I have been following reports on masks, face shields, and social
>> distancing with interest. The analysis is mostly fluid mechanics and
>> filtration. My specialty is fluid mechanics and I have conducted research
>> on automotive air filtration. I have served on the SAE Air Filter Test Code
>> Committee and been an expert witness on air filter testing. To start with,
>> the symptomatic who are coughing and sneezing are producing droplets that
>> are about 5 microns. The asymptomatic who are expelling droplets while
>> breathing, speaking, and singing are expelling droplets which are around 1
>> micron. The 6 foot social distancing rule is based on very old research
>> about how far 5 micron droplets travel before falling to the floor.
>> >
>> > I got concerned when I learned about the 1 micron droplets because of a
>> rule of thumb used for measurements using optics. Laser Doppler Anemometry
>> and Particle Imaging Velocimetry measure turbulent air flow using what are
>> called ''seed particles" to reflect laser light. One really measures
>> particle velocity, but makes sure that the particles are small enough to
>> move with the air. The rule of thumb is that 1 micron particles follow
>> laboratory air flows very well. Thus for a face shield, the gross 5 micron
>> particles from those with symptoms sneezing impact on the shield, but the 1
>> micron particles of the asymptomatic move with the air that is sucked in or
>> out by breathing behind the shield. The small particles just flow around
>> the corners of the shield. These aerosol particles can remain airborne and
>> travel through buildings. Shields only are effective for the larger
>> droplets. When Sen. Daschle received his envelope of anthrax powder, the
>> particles were about 1 micron. That
>> > indicated that it was sent by someone who knew what they were doing.
>> >
>> > These droplets of mucus surrounding virus particles change size as a
>> function of humidity as they evaporate, etc. The importance of asymptomatic
>> transmission has been becoming more recognized, but there still are
>> questions about how long the aerosols remain viable.
>> >
>> > On masks, there are different types of N95 masks. The basic standard is
>> that they filter particles which are 0.3 microns and larger at 95%
>> efficiency. They capture both the 5 micron and 1 micron droplets well The
>> N95 masks work very well for medical purposes except for the ones which
>> have a bypass valve making it easier to breathe out. These let out the
>> virus you are expelling. Surgical masks and homemade masks also work, but
>> not as well. They do a good job on larger particles, but are not as good on
>> the small ones, though they still are useful, even with filtration
>> efficiencies of 40 and 50%. There is a pretty good, very comprehensive
>> report on masks. It does, however, give more credence than deserved to the
>> study done at Duke which indicated gaiters were worse than nothing. A story
>> about this in the Washington Post generated lots of publicity this past
>> week. I read the report carefully and they were not even doing standard
>> efficiency measurements, ratioing downstream to
>> > upstream measurements. They just measured downstream and compared to
>> measurements without a mask. One has to be careful, because there are a lot
>> of non-peer-reviewed reports coming out from those who are novices at
>> filtration. It is easy to mess up, for the filtration efficiency can be a
>> strong function of the velocity through the filtration media. If one can’t
>> measure flow rate well, one can’t measure filter performance accurately.
>> >
>> >
>> http://built-envi.com/what-kind-of-mask-should-i-be-wearing-to-protect-against-covid-19/
>> >
>> >
>> > Prof. Linsey Marr of Virginia Tech has been writing and been
>> interviewed a lot on these topics. I have talked with her and she is pretty
>> good.
>> >
>> > Frank
>> >
>> > Frank W. Chambers
>> > 2 Sabroso Pl.
>> > Santa Fe, NM 87508
>> > Home:  (505) 466-1942
>> > Cell:  (405) 614-4353
>>
>>
>> --
>> ↙↙↙ uǝlƃ
>>
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