Thread by @MarcelHarmon1

Really good overview of ventilation, filtration, & a few other air-cleaning strategies, via @B_resnick & @voxdotcom. Many of the right experts were consulted for this.

But many districts & universities have yet to fully address these issues... 1/

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vox.com/science-and-he…

due to a) decades of deferred maintenance: linkedin.com/pulse/2016-sta…, linkedin.com/pulse/addressi… & b) lethargic, inconsistent responses (from the local to national level, & by districts/universities themselves) to devote needed $, resources, expertise, driven by the lack... 2/

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of a national, coordinated effort to respond to the pandemic. And as a result many are simply not ready to reopen.

Here's another way to look at the importance of these variables. Let's take a 1,000 SF classroom, 9 ft ceiling, 25 students + 2 adults,... 3/

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20% RH (winter condition), MERV 7 filters, 455 CFM of outside air (OA or ventilation) required by code, which = 3.0 air changes per hour (ACH), 1.7 ACH of recirculated air.

Using @BranchPattern's Flu Infection Risk Estimator(TM): branchpattern.com/research/risk-… & assuming... 4/

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1 infected individual, a quanta value = 100, coughing, 6 hours of exposure, 0% vaccination, no masks, the total removal efficiency at 15 minutes (from 1 single cough) is 72.2%. The probability of infection (PI) for the kids after 6 hours is 33.3%. For adults it's 26.5%. 5/

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This is a simple model, & you can refer to the supporting documentation for the details & assumptions. But generally the removal efficiencies are more directly comparable to SARS-CoV-2 & the probabilities of infection only in a general sense.

So now lets up the... 6/

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classroom's ventilation to 900 cfm to achieve the 6.0 ACH recommended in the article, & keep everything else constant. Removal efficiency = 86.2%, Child PI = 22.8%, Adult PI = 17.9%.

Now let's also change the filters to MERV 13. Removal efficiency = 88.7%,... 7/

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Child PI = 21.1%, Adult PI = 16.5%.

Let's reduce the exposure time to 4.0 hours: Child PI = 14.6%, Adult PI = 11.3%.

Reduce it to 2.0 hours: Child PI = 7.6%, Adult PI = 5.8%. Again, let me emphasize this is a simple model & you should pay more attention to the... 8/

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deltas than the absolute values (also based on influenza). This points out the importance of ventilation, probably more important than building system filtration. But time of exposure is also critical, as is the volume of the space (not emphasized in this example). 9/

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Which is why moving learning activities to larger spaces (or outside) & reducing the time in the classroom / on campus are also strategies to consider, along w/ reducing density.

As many building systems won't be capable of achieving 6.0 ACH of ventilation in any,... 10/

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space, or consistently throughout the year during the more extreme external temperature conditions (particularly since we haven't devoted the needed time/resources - in the last six months or the last few decades), multiple strategies, that vary by,... 11/

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district, university, & building, will have to be considered. The article covered some of these additional strategies. But be wary of some as the article points out. Their use could lead to evolutionary mismatches, e.g. linkedin.com/pulse/evolutio….

And these are just... 12/

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some of the complex building/operational issues that must to be addressed. There are a whole host of behavioral issues we haven't fully wrapped our heads around either: linkedin.com/pulse/reopenin….

One of the most basic, mask wearing, is still controversial. Which is...13/

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unfortunate, considering the risk reduction benefits offered. The excel tool our online version is based on currently accounts for mask wearing (which will make it into v2 of the online tool). Using the same parameters as above (going back to 6 hours of exposure),... 14/

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if everyone is wearing a basic cloth mask, the Child PI = 15.8% and the Adult PI = 9.8%, vs 21.1% & 16.5% respectively for not wearing a mask. This conservatively assumes so-so fits of the mask to the face.

If we want to have even hybrid forms of school/university...15/

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during a pandemic, we must have more consistent efforts to understand and address all of the relevant variables. But short of a national, coordinated effort, I fear we may continue to limp along. 16/

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