Chris Iddon
@chrisiddon.bsky.social
Music, walking, ventilation.
Research fellow
Chair of the CIBSE natural ventilation group
Research fellow
Chair of the CIBSE natural ventilation group
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...
Redirecting
doi.org
November 6, 2025 at 1:44 PM
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.
November 6, 2025 at 1:44 PM
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.
November 6, 2025 at 1:44 PM
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.
November 6, 2025 at 1:44 PM
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.
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
October 16, 2025 at 1:14 PM
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
Not that that seems to matter these days. As long as what you have to say meets your priors the accuracy doesn't matter 🙄
October 15, 2025 at 8:28 PM
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
October 15, 2025 at 8:26 PM
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.
October 15, 2025 at 8:25 PM
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.
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...
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
October 15, 2025 at 7:33 PM
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...
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
x.com
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
October 15, 2025 at 7:33 PM
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
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
October 15, 2025 at 7:33 PM
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