Chris Iddon

The stuff you find when you actually read the RCTs in a systematic review...

This paper is one of the foundational studies on vitamin D to prevent respiratory infections in kids. Cited 1,400 times as per Google Scholar.

Associations of air cleaning interventions in primary school classrooms with indoor environmental quality and perceived symptoms Air cleaners are used to remove various pollutants from air in different types of indoor environments. Studies on the air cleaners’ efficiency to remo…

The authors conclude that the air quality in these classrooms is already good, the ventilation is good even without the air cleaners, so they suppose that the impact of air cleaners is limited in such environments.
sciencedirect.com/science/arti...

Kids also reported their perceived perception of the indoor air quality and their perception of the air quality improved with the presence of air cleaners, even when the air cleaners were off

A placebo effect - demonstrating the power of suggestion

6 schools and 27 classrooms under baseline, air cleaner off, HEPA on and SHAM (air cleaner on but no HEPA filter). Air cleaners also included activated carbon to remove TVOC.
If kids had symptoms they scored these, air cleaner reduces symptom severity with and without HEPA

New study on HEPA air cleaners in Finnish school study.
Kids fill in daily diary of respiratory symptoms and the prevalence of symptoms reduced when the air cleaner was on.
However the prevalence also decreased when the air cleaner was on but HEPA removed 🧵

marquees and other semi-outdoor venues can support large events with lower airborne transmission risk — but only if ventilation and occupancy are managed. This study provided key evidence for UK policymakers on reopening safely and responsibly. 🎪💨 doi.org/10.1016/j.bu...

There are currently no ventilation standards for semi-outdoor temporary structures — a major gap in regulation. The paper argues these spaces should be included in future building guidance and pandemic preparedness plans.

The research showed that semi-outdoor marquees aren’t automatically well-ventilated. When side panels were closed in wet or cold weather, or mechanical systems reduced, CO₂ levels rose sharply — highlighting how operation and crowding directly affect air quality.

The study found that most marquee spaces were well ventilated relative to occupancy (mean CO₂ <800 ppm). Short spikes to ~1200 ppm occurred during peaks in crowding — showing that occupancy and density drive air quality.

During the COVID-19 pandemic in 2021, researchers monitored air quality in marquees and semi-outdoor venues at UK mass gatherings to understand ventilation and airborne infection risk — work that directly informed how such events could safely reopen. 🧵

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

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

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

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

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.

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.

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...

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

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

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

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

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

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

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

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

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

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

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 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...

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

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

that's great news