1/ Why universal mask wearing in buildings is important

First, the persons infected are unknown. So, if everyone wears a mask the infectors are also wearing masks. Second, everyone else (receptors) get an extra layer of protection for also wearing a mask.
2/ The % reduction in volume (or mass) of aerosol particles inhaled by a receptor is given by the following equation: Z = A + 0.01 x B x (100 – A)
3/ Where Z = combined % reduction in inhaled particle volume by receptors, A = % reduction in emissions by virtue of infector(s) wearing masks, B = % reduction in inhaled volume by virtue of receptors wearing masks.
4/ So (as an example), if the infector’s mask is 50% efficient at reducing emissions (A = 50%) and the receptor’s mask is 40% efficient at removing particles during the inhalation process (B = 40%), then Z = 50 + 0.01 x 40 (100 – 50) = 50 + 0.4 x 50 = 70%.
5/ That is a GR8 first layer in a layered risk reduction strategy. After that the additional layers (ventilation, filtration, etc.) are all focused on that remaining 30%. With right combo of increased ventilation and filtration it is possible to reduce dose by 90% or more!
6/ This analysis is for aerosol particles. If infectious viruses are uniformly distributed across particle sizes (it is not clear that this is the case), then the reduction in inhaled dose by particle volume described above also holds for virions.
7/ And there's extra benefit of infector(s) wearing masks as they will also emit less large (ballistic) droplets, thus reducing droplet impact w/ receptor’s face (including mouth, eyes, nostrils), hair/clothing, as well as other surfaces that can lead to transmission by fomites.
8/ And masks on receptors also protect the nose and mouth from large droplets.
9/ Moral of story = wear a mask in any indoor environment other than your own home & do not go in unless everyone else is wearing a mask. Layered DOSE reduction leads to layered RISK reduction. Step 1 (a very inexpensive step) is everyone wearing a mask. Just do it!

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More from @CorsIAQ

24 Oct
1/ Not surprising & consistent w/ comments I made in a recent article by @RobertRoyBritt. Exposure time is NOT the only factor that determines risk, whether in close contact or indoor far field. We should focus on inhaled deposited dose (IDD) & not just on distance or time.
2/ IDD = Concentration (C) (#/L) x Respiratory minute volume (B) (L/min) x Time of exposure (t) (min) x Fraction deposited in respiratory system (f) (-). C & f are taken for specific particle sizes and the resultant size-specific IDD values summed (integrated) for total IDD.
3/ Concentration (C) depends on a number of factors, including distance, emission mode and strength, air flow conditions (e.g., cross-flow, mixing/TKE) & whether the receptor is wearing a mask (as concentration is in breathing zone inside mask), quality of mask, etc.
Read 11 tweets
22 Oct
There has been good work on dilution near source using thermal, breathing manikins. I will try to look this up & post a couple of good papers. more
Of importance is also if someone in close contact is breathing heavily, e.g., in an aerobic workout class, etc. In that case the respiratory minute volume can be 10 to 15 x what it is at rest AND the emitter might be emitting much more. Does that mean 1 minute or less contact?
I spoke about the new CDC guideline today on a radio show with listener questions. We need to be clear that there is nothing magical about 6 ft or 15 minutes. These are gross generalization that provide something "simple" for the public. more...
Read 4 tweets
22 Oct
1/ On new CDC guidelines that move from single-event exposure time for close contact to 24-hour integrated exposure time over 24 hours. My take ....
2/ First, while exposure time is an important factor, it is incomplete. At corsiaq.com, I describe inhalation dose. Time is an important factor, but so is concentration of aerosol particles in the breathing zone & respiratory minute volume.
3/ Concentration (C) decreases with distance from the source, and so 10 ft is better than 6 ft, etc. Take advantage of this, but also know that some dose occurs in the far field as well. Further, C is taken in breathing zone, e.g., air inside mask. Thus masks reduce C!
Read 9 tweets
13 Oct
1/ Important tweet by @linseymarr.

Aerobic activities in gyms are a bad idea during this pandemic, and it is difficult to get risk down to what I believe are acceptable levels. Results of a gym simulation I did several months ago are shown here w/ specs next .... Image
2/ In this simulation I used Infector X in a restaurant in China with a significant outbreak of COVID-19. By analyzing metadata for the restaurant it is possible to ask the following question ... more ...
3/ If infector X is placed in a gym as a staff member, how much volume of virus laden aerosols would someone in the gym inhale relative to those who were infected in Restaurant X? The value "omega" on the vertical axis shows this ratio for different conditions. more ..
Read 6 tweets
12 Oct
1/ Starting to post hand-written notes and (soon) slide presentations for undergraduate and graduate indoor air quality and exposure science courses that I taught at UT Austin for decades. At corsiaq.com under teaching. Will take awhile to populate. More ....
2/ I almost always provided handwritten notes to students well in advance of class meetings. The "blank" sections were for problems or derivations that I did on the board so that students could write in the notes I provided during lecture. More ...
3/ Some notes are admittedly "cleaner" than others. You can tell by the handwriting when I was up until 3 a.m.!

These notes are primarily intended for fellow academics who might be developing such courses, but might also be of interest to others who wish to know more about ...
Read 5 tweets
12 Oct
1/ The rebreathed fraction of air is the fraction of air that one inhales that came out of the collective respiratory systems of others in an indoor space. If a rebreathed fraction is 0.05 (a high value), every breath you take contains 5% of the collective breath of others.
2/ An important parameter is the average rebreathed fraction while you are in a space with others. The average rebreathed fraction (f) can be determined using CO2 measurements as f = (Cavg - Cout)/Cbreath.
3/ Cbreath is the CO2 concentration exhaled and is approximately 38,000 ppm (some variation with diet, etc.). Cavg is the average indoor CO2 concentration during the time you are in an indoor space in ppm, and Cout is the CO2 concentration outdoors in ppm.
Read 12 tweets

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