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Chris Iddon @chrisiddon.bsky.social · 8h

That said, it seems the study thought they were adding an extra 6ach of clean air, but I get around 3ach and I think the discrepancy lies in two different classroom sizes 250m3 or 4000cf used by the authors 13/n

Chris Iddon @chrisiddon.bsky.social · 8h

And they have 4 HEPA units giving the equivalent of 200L/s, so that is a total equivalent clean air of 408L/s. The average class occupancy is 19, therefore 21.5L/s/person which exceeds ASHRAE 241 20L/s/p requirement 12/n

Chris Iddon @chrisiddon.bsky.social · 8h

They don't meet ASHRAE 241! Well this study pre-dates 241. But 241 looks for the total equivalent ventilation ie all removal mechanisms are additive, ventilation and filtration and bio decay (eg by UV). The average ventilation is 3ach and the average class 250m3 = 208L/s 11/n

Chris Iddon @chrisiddon.bsky.social · 8h

That's not to say they may have been off some of the time. The researchers did go to lengths to reduce this happening eg over-riding the on/off switch and checking regularly that they were still plugged in. Anyway, i think we can say the HEPA was working to an extent 10/n

Chris Iddon @chrisiddon.bsky.social · 8h

and so to the critiques - maybe they were turned off? Perhaps, but the data from the asthma study (ie the same data set) showed a 20-40% reduction in particulate material with the HEPA present. You wouldn't expect to see filters filtering if they were off! 9/n

Chris Iddon @chrisiddon.bsky.social · 8h

The study provides no information on whether or not the presence of the HEPA had any impact on transmission of any of these respiratory illnesses or impacted absences. 8/n

Chris Iddon @chrisiddon.bsky.social · 8h

This shows that infector viral emission rate matters — if it’s high, air concentration stays high even with more ventilation or cleaning. You can’t always clean your way out of a heavy emitter. (We suggested this earlier 👇) x.com/moog77/statu... 7/n

x.com

Chris Iddon @chrisiddon.bsky.social · 8h

In other words, HEPA use didn’t cut the odds of being in the high–viral load group (so, no clear exposure reduction). But the study’s too small to rule out a modest effect. 6/n

Chris Iddon @chrisiddon.bsky.social · 8h

The study then uses a statistical model to group the airborne pathogen data into two categories: high and low viral load. About 22% of samples were in the high-load group — and HEPA use didn’t reduce the odds of being in it. 5/n

Chris Iddon @chrisiddon.bsky.social · 8h

The results show an interesting spread of viruses that change in prevalence over the year (note there is no SARS-CoV-2 bcos this predates the pandemic, plenty of other coronaviruses tho) 4/n

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Chris Iddon @chrisiddon.bsky.social · 8h

at the same time they collected air samples from these classrooms and have analysed these samples recently to assess how much and how many pathogen genomic material are present in the air with and without HEPA 3/n

Effect of School Integrated Pest Management or Classroom Air Filter Purifiers on Asthma Symptoms in Students

Chris Iddon @chrisiddon.bsky.social · 8h

First of all this was an initial study into the impact of HEPA purifiers with and without filters on childhood asthma undertaken between 2015-2020. The primary results of that are elsewhere (tldr HEPA had no impact on asthma incidence) jamanetwork.com/journals/jam... 2/n

This randomized clinical trial compares use of a school-wide integrated pest management (IPM) program vs use of high-efficiency particulate air (HEPA) filter purifiers in the classrooms for students w...

jamanetwork.com

Pushing the cooling limit: CIBSE Natural Ventilation Group - CIBSE Journal

Chris Iddon @chrisiddon.bsky.social · 8h

A lot of the analysis on this misses the mark:
• “They weren’t on” — maybe, but filtration data says otherwise
• “Too small / not ASHRAE 241” — data disagrees.

And none of it actually says anything about transmission of respiratory illnesses.
🧵1/n

CIDRAP @cidrap.bsky.social · 2d

HEPA purifiers not tied to less #viral exposure in elementary classrooms, analysis finds

#HEPA purifiers were associated with a 33% decrease in viral diversity, but the reduction wasn't linked to fewer school absences.

www.cidrap.umn.edu/i...

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Chris Iddon @chrisiddon.bsky.social · 13d

that's great news

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Reposted by Chris Iddon

Chris Iddon @chrisiddon.bsky.social · Sep 1

With UK heatwaves intensifying, are passive cooling strategies enough? At CIBSE’s seminar, experts explored natural ventilation limits, AirCon efficiency, façade carbon trade-offs & why overheating in schools is a learning crisis. cibsejournal.com/technical/pu...

The limits of passive cooling, healthy learning environments and mitigating overheating were the core subjects of a topical summer event hosted by the CIBSE Natural Ventilation Group in July. Chair Ch...

cibsejournal.com

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Chris Iddon @chrisiddon.bsky.social · Sep 1

With UK heatwaves intensifying, are passive cooling strategies enough? At CIBSE’s seminar, experts explored natural ventilation limits, AirCon efficiency, façade carbon trade-offs & why overheating in schools is a learning crisis. cibsejournal.com/technical/pu...

Pushing the cooling limit: CIBSE Natural Ventilation Group - CIBSE Journal

The limits of passive cooling, healthy learning environments and mitigating overheating were the core subjects of a topical summer event hosted by the CIBSE Natural Ventilation Group in July. Chair Ch...

cibsejournal.com

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Chris Iddon @chrisiddon.bsky.social · Jul 26

You're welcome

a man wearing a top hat and a vest is standing in front of a pile of rocks ..

ALT: a man wearing a top hat and a vest is standing in front of a pile of rocks ..

media.tenor.com

Chris Iddon @chrisiddon.bsky.social · Jul 14

Want to know more about how airflow and risk modeling work? The full paper dives deep into the math, assumptions, and trade-offs 👉 doi.org/10.1016/j.bu...

#ventilation #indoorairquality #publichealth #ASHRAE241
/end

Redirecting

doi.org

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Chris Iddon @chrisiddon.bsky.social · Jul 14

A great team effort from @benjonesieq.bsky.social, Marwa Zaatari, Pawel Wargocki and Richard Burns in providing this technical overview of the ANSI consensus process 10/n

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Chris Iddon @chrisiddon.bsky.social · Jul 14

Bottom line: ASHRAE 241 offers a science-informed way to reduce risk—but it’s not a silver bullet. Clean air helps, but vaccines, and reducing exposure time still matter too. 🏥🏢💨 9/n

Chris Iddon @chrisiddon.bsky.social · Jul 14

Other expert groups might’ve made different modeling choices—but this is the consensus approach the ASHRAE 241 committee agreed on, based on current science and engineering judgment. 8/n