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...
Masks for all (of course), less time in close contact, and greater distance are all things that WE can control when indoors. We need to articulate to business owners, building owners, building managers. school districts, etc. the importance of increased ventilation and controls.

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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
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
9 Oct
1/ At very beginning of this pandemic (late February/early March) we placed CO2 monitors w/ data loggers in several locations of the library @Portland_State, counted occupants on several floors, and flux in/out of the library to get an average time of approximately 2 hrs/visit.
2/ We used the data to determine the average rebreathed fraction of air on floors & occupancy numbers & average occupancy time to estimate reproductive numbers if infectors were present (w/ a range of quanta generation rates) using the Rudnick-Milton model. Upshot ...
3/ BIG differences in rebreathed fraction between floors, but small in general. However, w/ large # of occupants, even small rebreathed fractions can lead to predicted site-specific reproductive # between 1 and 5 for a single infector in teh space depending on quanta gen rate.
Read 4 tweets

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