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Chris Iddon @chrisiddon.bsky.social · 1h

and on a tangent on this, if there is some discrepancy and the 4000cf is correct, then they have sized to provide an additional 6.4ach which exceeds the CDC guidelines at the time of 5ach. So I'd still argue that these filters have been well sized for the application

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Reposted by Chris Iddon

Chris Iddon @chrisiddon.bsky.social · 19h

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 · 18h

Not that that seems to matter these days. As long as what you have to say meets your priors the accuracy doesn't matter 🙄

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Chris Iddon @chrisiddon.bsky.social · 18h

I should clarify that the study did find that absences increased in those classrooms designated high viral load in the air, compared to low viral load. And that HEPA did not reduce your chance of being a high viral load classroom

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Chris Iddon @chrisiddon.bsky.social · 18h

Air sampled for a week, frozen and analysed several years later. Some kind of preamplification then quantitative PCR. I imagine the most cost will be setting up air sample and collection.

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Chris Iddon @chrisiddon.bsky.social · 19h

n this study we can conclude that increasing air cleaning with HEPA did not reduce the chance of being exposed to a class with a high viral load. We need more similar studies to see if there's an impact on incidence of these respiratory illnesses \end
jamanetwork.com/journals/jam...

Air Purifier Intervention for Respiratory Viral Exposure in Elementary Schools

This secondary analysis of a randomized clinical trial examines the association of HEPA (high-efficiency particulate air) filters with respiratory viral exposure in elementary school classrooms.

jamanetwork.com

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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 · 19h

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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

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

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

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Chris Iddon @chrisiddon.bsky.social · 19h

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...

1 4 8

Chris Iddon @chrisiddon.bsky.social · 14d

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...

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 · 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

2 2

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

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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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