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Dr. Richard Corsi Profile picture
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4 Jan, 5 tweets, 2 min read
Particle settling: In case anyone wants to see a derivation of Stokes equation for particle settling - here is one of my old course notes. Note that aerosol particles achieve terminal settling velocity almost instantaneously, unlike someone jumping out of an airplane. more ...
This derivation assumes spherical particles & does not adjust for different shapes that affect drag force. It also does not adjust for particle "slip" as diameter approaches the mean free path of air molecules & drag is reduced as the no slip condition is violated. more ...
A 0.1 micron particle settles three times faster for this reason than is shown at the bottom off the page. A 1 micron particle settles about 17% faster. Slip correction becomes smaller and smaller for larger particles. More ....
Note that in air (not water) settling velocity is effectively proportional to particle density. The absolute viscosity of air does not vary much across most indoor environments. The dominating factor for settling is particle diameter. 2 x diameter & settling velocity quadruples.
I'll be putting more old course notes up on my website at corsiaq.com this week (under teaching and course pull down). This is for anyone who is curious about this stuff or who wants to use the notes in your courses. Please let me know if you do. It will make my day!

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Dr. Richard Corsi Profile picture
31 Dec 20
1/ Dealing with an inferno of infectiousness.
We failed to do what was needed to keep this mess from getting out of control. Our situation is horrific and will soon get much worse due to end-of-year holidays + more infectious variant. Vaccine distribution also F grade to date.
2/ All of this means even greater mass casualties (for both those who live and die) in the coming months.
3/ Expect one-half million dead by end of February, a lot more of the following: refrigerated trucks outside of hospitals, hospitals faced with triage, schools that will not re-open, business closures, lost jobs, homeless, and (unfortunately) more.
Read 25 tweets
Dr. Richard Corsi Profile picture
27 Dec 20
1/ Teaching Indoor Air Quality

I taught both undergraduate and graduate indoor air quality classes for many yrs at UT Austin. Proud of the fact that many current professors (and some department chairs) took my courses as students & are now teaching a new generation.
2/ I loved infusing my research into lectures, having students bring different scented products to the classroom, exposing them to a small amount of ozone and measuring ultrafine particle formation (image). We also measured rebreathed fraction in the classroom, etc.
3/ Every class would have an assignment with CO2 analyzers checked out to groups w/ 1 wk to measure avg CO2 concentrations and rebreathed fraction & then use the Rudnick-Milton model to estimate probability of common cold & flu transmissions across campus (50+ locations/class)
Read 12 tweets
Dr. Richard Corsi Profile picture
25 Dec 20
1/ "To prevent the virus from spreading, companies that needed employees to return to in-person work put up plexiglass barriers, implemented daily temperature checks and required workers to wear masks while in proximity to others." What's wrong with this picture?
2/ Plexiglas barriers are not very effective at reducing transmission by aerosol particles. Temperature checks do not catch pre-symptomatic, asymptomatic, or symptomatic infectors w/o fever. Masks "while in proximity of others" = unacceptable. Masks at all times indoors.
3/ And we wonder why there are outbreaks in the workplace. LRRS = Layered Risk Reduction Strategy, not LRR Shortcut. LRRS must be done based on layered interventions that make scientific sense. Performance = sad. Almost a year in, folks. What will it take to get this right?
Read 5 tweets
Dr. Richard Corsi Profile picture
23 Dec 20
1/ More results (year 1) of our Healthy High School PRIDE study. Here we show a cumulative distribution plot of average rebreathed fraction (RF) of air in classrooms during the occupied day. RF is actually converted to a percentage on the vertical axis.
2/ Note that a rebreathed fraction of 0.03 (3%), for example, means that 3% of every inhaled breath originated from the collective respiratory systems of others in the indoor space (including anyone infected with COVID-19).
3/ In my recent USEPA webinar on layered risk reduction strategies for schools I argued for a maximum RF < 0.008 (0.8%) for classrooms during this pandemic. That translates to an average CO2 concentration of less than 700 ppm.

Read 7 tweets
Dr. Richard Corsi Profile picture
23 Dec 20
1/ Plots from our Healthy High School PRIDE (Partnership in Research on Indoor Environments) project, a 4-yr USEPA funded study and intense analysis of 46 high school classrooms in Central Texas.
2/ Plot A shows air exchange rate in permanent and portable classrooms in the unoccupied setting (end of school day to start of next school day). The x is mean value and central line on bars is median. Variation shown via percentiles and min/max.
3/ The mechanical systems were off in plot A. All ventilation was via infiltration. Note much lower values and spread for permanent classrooms. Portable classrooms are connected directly to outdoors, leakier, and prone to greater ventilation by infiltration.
Read 6 tweets
Dr. Richard Corsi Profile picture
20 Dec 20
1/ @CDCgov has bought into layered inhalation dose, and therefore risk, reduction strategies. Pleased by this recognition, but not ready to shower the agency with adulation. The agency's credibility has taken a hit. I want to see a sustained effort of focus & doing right.
2/ My life has been about looking forward. But it is difficult not to reflect on where we would have been today had recognition of inhalation of virus-laden aerosol particles in both the near and far fields been recognized by @CDC 10 months ago.
3/ Future planning for the next pandemic or continued battle with SARS-CoV-2 must rely on continued innovation, but also on a deep forensics analysis of all of the failures (on so many fronts - not just @CDC) that fueled an inferno of infection in the US & elsewhere.
Read 4 tweets

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