Chris Iddon
Music, walking, ventilation.
Research fellow
Chair of the CIBSE natural ventilation group
- New study looking at influenza transmission 1 infected person shared small low humidity and poorly ventilated room with 8 uninfected subjects (plus 2 observers). Chatting, playing uno, bit of exercise 5000+ppm CO2 no one got infected 🧵 1/n
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- This is the second similar study that the authors have conducted. The ethics are recognised
- Although there was low ventilation rates in these rooms, there was high mixing rates and the authors postulate that this could have rapidly mixed exhaled plumes that ordinarily would hold more concentrated viral material and support close range transmission 5/n
- Despite creating an environment that would be hypothesised to support flu transmission, other factors matter: immunity of recipients viral emission rates close range transmission number of coughs tbh i think this shows that "it's complicated" /end journals.plos.org/plospathogen...
- This study included multiple interactions between donors and recipients, some the CO2 only got to 2400ppm, but the authors note that despite the donors having high viral load from swabs they emitted low levels of virus 3/n
- air sample in the room only detected low levels of viral genomic material, and exhaled breath samples were also low in viral material. Items were passed around, tablet, microphone and marker pen. Some viral material found on one sample from the pen 4/n
- There's plenty that would baulk at CO2 = 5000ppm, raw dogging the air etc but as we've been pointing out for years, if the infector isn't emitting much virus then your inhaled dose will be low, even if ventilation is poor 2/n a link to the other place x.com/moog77/statu...
- Just announced - The Race For Space at Alexandra Palace, a 10(+1) year celebration. Join us if you can, please (and bring a friend). www.youtube.com/watch?v=udeP... Pre-sale starts 10am on Monday 26 Jan; general sale starts 10am on Thursday 29 Jan. FAQ here: www.patreon.com/posts/148198...
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View full threadIt's in the FAQ sir.. www.patreon.com/posts/148198...
- thanks for the comprehensive explanations :)
- 16+ venue, my kids will be disappointed. Any other gigs in the pipeline?
- New thesis on HEPA air cleaners in school classrooms: 🧵 TL;DR: In this school study, HEPA air cleaners showed little reliable impact on absenteeism, and air-quality improvements were limited during actual classroom use.
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View full threadThe study does control for important confounders (school effects, classroom effects, seasonality). But it can’t control for individual health, household exposure, immunity, or behaviour—so residual confounding remains likely.
- An interesting study which provides some more evidence of the impact of HEPA filter treatments on school absenteeism. Overall, the study offers little reliable evidence that HEPA air cleaners meaningfully reduce student absenteeism. proquest.com/docview/3281...
- Why the uncertainty? The study ran many overlapping models (different seasons, moving averages, interactions). Results change depending on the model → classic risk of model-dependence (and over-interpretation).
- This doesn’t mean the researchers did anything wrong—sensitivity analyses are useful. But without a single pre-specified primary model, statistically significant results should be treated cautiously.
- Other models show no statistically significant effect at all once you adjust for school, classroom, season, and time trends. In other words: the “benefit” disappears depending on how you model the data.
- And importantly: ⚠️ In Winter, some models show higher absenteeism in HEPA treated classrooms compared to controls—especially at higher equivalent ventilation rates.
- But the big question was absenteeism. Do cleaner classrooms = fewer sick days? Short answer: the evidence is mixed and weak.
- Some statistical models show small reductions in illness-related absenteeism in HEPA classrooms—mostly in Fall and Spring. But the effect sizes are tiny (fractions of a day per student).
- The study installed portable air purifiers (HEPA-based, some with extra filtration layers) in many classrooms and compared illness-related absenteeism to control classrooms with no purifiers.
- They didn't see much difference in particle counts between control and treatment (T) classrooms when occupied, but they only took a couple of 24hr measurements. Other studies with continual measurement have shown reductions in PM with filters present. Unoccupied = PM reduction
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- yeah the deposition stuff is interesting (tho predictable). But their conclusions on ventilation and PAC are dependent on the size of the PAC and ventilation provision, of which there are myriad combinations (here i think they use 2), and obvious from their relative ach
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- my point is that it's comparing a larger equivalent removal with a smaller removal and going oh look the larger removal removes more. Completely pointless to me. You can see that 3.15ach is greater than 0.5ach without having to run a model.
- what's the point of comparing ventilation at 0.5ach with a PAC with equivalent ventilation of 3.15ach and going, oh the PAC is better at removing particles? They model a 40m3 room which is tiny. If PAC_1 was in a 300m3 room with 0.5ach ventilation, the PAC_1 will now be running at 0.42ach 🤷♀️
- New study protocol for a pragmatic, cluster-randomised, parallel, two-arm, group sequential superiority trial of HEPA filters in school classrooms to commence later this year in Norway pmc.ncbi.nlm.nih.gov/articles/PMC... -Using school absence as outcome -be interesting to see the results
- They will provide air purifiers with HEPA filters running at 486m3/h (approx 3ach) or at 48m3/h.
- 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 🧵
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View full threadKids 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
- 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...
- 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
- 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. 🧵
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View full threadThere 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.
- 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...
- 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.
- 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.
- 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
- 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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View full threadThat 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 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
- Expertise has entered the chat
- Not that that seems to matter these days. As long as what you have to say meets your priors the accuracy doesn't matter 🙄
- 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...
- 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
- This is super interesting - do you know how much it costs to get a dataset like this? I'm wondering if it would be viable for community surveillance.
- 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.
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
