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
- 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...
- 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
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View full threadDespite 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...
- 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.
- 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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View full threadAn 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...
- 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.
- Reposted by Chris IddonThe 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.
- 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 🧵
- 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
- 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
- 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...
- 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. 🧵
- 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.
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View full threadmarquees 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...
- Reposted by Chris IddonA 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...
