Thread by @DavidElfstrom

#COVIDCO2 readings in an Ottawa classroom. Never reached a steady state. At 3152 ppm every breath contains 7.2% of other people's breath‼️‼️ 🤮. #DontShareYourAir #Onted
Classroom has below 0.4 air changes per hour.

Target MIN: 3-4, Good: 4-5, Excel 5-6, Ideal 6+
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https://twitter.com/JenniferKShea/status/1362228242875355139

For COVID, air changes from HEPA filtration are equivalent to outdoor air. Typical classroom portable HEPA air cleaner:
400 CFM on max at 65 dB adds 2.7 ACH
200 CFM on med at 55 dB adds 1.3 ACH.
Two on med would be 58 dB.

BUT there's still the cognitive impact of 3000ppm!
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Here's the back-calculation assumptions used to reach 0.4 ACH in a classroom. I'm assuming 12 foot ceilings for an older naturally ventilated school. Other values are typical as well. The calculator throws an exception in red at 19 students, but not at 18.
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WITH one air cleaner on HIGH providing 2.7 ACH, the target CO2 level for 6 ACH total is 730 ppm.
On MEDIUM, target is 633 ppm.
NO air cleaner: 581ppm for this classroom.

Accuracy of the meter is +/- 50 ppm.
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CAUTION: Low humidity when opening windows. I haven't done the mixed air calculations or accounted for loads from breathing/perspiration. It's Ottawa in winter. Any outdoor air when heated is going to be well under 20% relative humidity (NOT good)
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⬆️Of interest to @YoniFreedhoff @ETFOeducators @covidschoolsCA @normsworld @CovidEcoles @RougeMatisse

⬆️of interest to @DFisman @CO2Guerrillas @covidisairborne
Plenty of thanks to @Don_Milton @j_g_allen @ShellyMBoulder @jljcolorado for the tools

In Ontario classrooms are empty due to virtual learner cohorts and govt refusal to fund smaller classes.

At a minimum the in-person learners should be assigned rooms with the best air quality. Rooms without mech ventilation, especially without exhaust, should be closed.

Several have asked about the rebreathed fraction of air. Here is a table. Turns out it's independent of activity level as noted by Prof Milton in reply

https://twitter.com/DavidElfstrom/status/1338327790874222592?s=19

Reminder that other infections can readily spread with low ventilation:

https://twitter.com/jeffgilchrist/status/1362366067650265088?s=19