1/ I am seeing many interpreting this as "it's safe to send kids back to school." That's a misinterpretation. First, safe implies certainty, which is not true here. What can be said is that it is much safer to re-open schools if specific risk reduction measures are taken.
2/ Densely-populated, poorly-ventilated (the majority of schools I have studied in Texas) schools, especially during any periods when masks are not worn are a recipe for infection. Ask Israel.
3/ Proper layered risk reduction works, is not rocket science, and does not have to break the bank. More on LRRS in my webinar here:
4/ I have done simulations that show >92% risk reduction by lower inhalation dose of virus-laden particles at a cost of roughly $400/classroom/year. We can send children, teachers, and staff into much safer schools if we are smart about it ... and it should not break the bank!
5/ Of course, there are other spaces in schools that also need attention, outdoors is BETTER than indoors, and lowering community spread is also important.
6/ School health, community health, local business health, healthcare health, are an interconnected system. If one part of the system fails the others suffer. We must realize this to starve this virus of its hosts. Just do it!
• • •
Missing some Tweet in this thread? You can try to
force a refresh
We are a couple of weeks away from the anniversary of the first known COVID-19 death in the US. In February 2020 the US had approximately 20 total deaths. As we move into February 2021 we will be pushing 500,000 deaths w/ a trajectory aimed much higher.
We failed a year ago. Infections got too far ahead of us. Our response should now be an all-out war to starve this virus & its new variants of hosts, to do what we should have done 10-11 mos ago. We can lower the future death total before vaccines get us to where we need to be.
The ABSOLUTE MINIMUM that must be done to reduce inhalation dose in both the near- and far-fields is (1) UNIVERSAL mask wearing in ALL indoor environments other than one's own home in the presence of immediate family, and (2) physical distancing of AT LEAST six feet.
I agree w/ @linseymarr that lower than 800 ppm is often needed, but we also need recognition that what is a "safe" CO2 concentration in on environment may not be safe at all in another, and vice-versa.
The actual max acceptable value depends on # of people present, typical time spent in environment, mask requirements, respiratory minute volume, and effectiveness of controls.
For 25 people spending 2 hrs in the same indoor space, a rough estimate based on a quanta generation rate similar to that in Restaurant X is CO2 < 695 ppm for an infection probability of < 5% w/o masks or controls. Assuming worst (no masks or controls) provides a safety factor.
1/ Automobiles and Intake Fraction. Since cars are back in the news I thought I would retweet this model result I offered in early April 2020. I focused only on 1 micron particles & accounted for windows completely closed & cracked slightly open.
2/ Related air exchange rates were based on experimental results in literature for mid-sized sedans. Particle deposition to indoor surfaces were accounted for, as the surface to volume ratio in a 3 m3 cab is large. An important outcome was the intake fraction (IF)
3/ Here, IF is the number of particles (or virions in collective particles) inhaled by a receptor DIVIDED BY the number or particles (or virions in collective particles) emitted by an infector.
1/ Thank you, Dr. @lisa_iannattone, and right on! And as an environmental engineer I never learned about physiology, internal organ function, etc. The best scenario is when we all work and learn form one another. That's the way we take on big challenges effectively. 👇👇👇👇
2/ Many years ago I was a PI on an NSF IGERT grant that involved students across 6 colleges at UT Austin. They wrote a wonderful editorial about the importance of interdisciplinary collaboration. onlinelibrary.wiley.com/doi/full/10.11…
3/ The last paragraph in their editorial is particularly relevant.
Total US COVID-19 deaths = 385K (400K by end of weekend)
1 of every 855 Americans dead by COVID-19
1 of every 43,600 S. Koreans dead by COVID-19
End of Feb 2020
US deaths = 20
South Korean deaths = 17
By end of Feb 2021 US likely exceeds 500K
S. Korea may exceed 1,400
2/ Yes, the vaccine will ultimately help. But unless we work relentlessly to starve this virus of its hosts the case and death count will remain out of control until then. And the new variant means that we need to quadruple down on reducing inhalation dose.
3/ Inhaled dose (D) is important in BOTH the near (close) and far fields.
D = C x B x t x f
D = # virus-laden particles deposited (ultimately convert to vol), C = concentration in breathing zone (#/L), B = resp minute volume (L/min), t = time (min), f = fraction deposited.
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 ....