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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

The trial suggest that implementation of HEPA filters won't make a meaningful impact on resident respiratory episodes and therefore investment in HEPA filters is not warranted, which is predicted to save as much as £154M due to care homes not implementing 12/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

Nor on staff absenteeism 11/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

After the trial no effect is found on the primary outcome of resident respiratory infection episodes 10/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

Interestingly at the start of the trial there is a strong belief that HEPA filters can reduce infections 9/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

These care homes are probably already well ventilated as the perception at the start of the trial is that occupants are satisfied or very satisfied with care home air quality and odors 8/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

Making simple assumptions about this space & 6 hour exposure of residents in small communal room I predict a 53% reduction in dose, but magnitude of the dose matters when it comes to infection. Low virus emitters mean dose is low with and without HEPA. High emitters are rare 6/n

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Chris Iddon @chrisiddon.bsky.social · 28/07/2026

Results of a cluster RCT on HEPA filters in care homes tldr; finds no effect on winter respiratory infections respiratory infection rate per bedroom resident ❌ Staff absenteeism ❌ the additional CADR is pretty meagre a 🧵 1/n

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Chris Iddon @chrisiddon.bsky.social · 24/07/2026

No significant impact on frequency of symptoms either, in fact in some weeks there were far more symptoms in HEPA classrooms 5/n

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Chris Iddon @chrisiddon.bsky.social · 24/07/2026

Because they have previously reported good reductions in particulate matter we can assume that the air cleaners were regularly used in these classrooms during the trial, but little impact on classroom attendance 4/n

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Chris Iddon @chrisiddon.bsky.social · 24/07/2026

These schools have HVAC with MERV13, but it is not clear if any recirculation of air through the HVAC is part of the ventilation strategy. Classrooms either have air cleaner with HEPA filter or HDPE cloth with minimal filtration 2/n

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Chris Iddon @chrisiddon.bsky.social · 24/07/2026

New preprint on randomized crossover trial of HEPA air cleaners on school classroom attendance tldr; no statistical significant impact on absenteeism or symptom scoring deeper dive, read on... 1/n

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Chris Iddon @chrisiddon.bsky.social · 23/01/2026

This is the second similar study that the authors have conducted. The ethics are recognised

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Chris Iddon @chrisiddon.bsky.social · 23/01/2026

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

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Chris Iddon @chrisiddon.bsky.social · 23/01/2026

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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Chris Iddon @chrisiddon.bsky.social · 23/01/2026

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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Chris Iddon @chrisiddon.bsky.social · 13/01/2026

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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Chris Iddon @chrisiddon.bsky.social · 13/01/2026

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.

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Chris Iddon @chrisiddon.bsky.social · 13/01/2026

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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Chris Iddon @chrisiddon.bsky.social · 21/11/2025

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

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Chris Iddon @chrisiddon.bsky.social · 21/11/2025

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

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Chris Iddon @chrisiddon.bsky.social · 21/11/2025

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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Chris Iddon @chrisiddon.bsky.social · 06/11/2025

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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Chris Iddon @chrisiddon.bsky.social · 16/10/2025

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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Chris Iddon @chrisiddon.bsky.social · 15/10/2025

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 · 15/10/2025

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 · 15/10/2025

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 · 15/10/2025

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 · 14/07/2025

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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Chris Iddon @chrisiddon.bsky.social · 14/07/2025

ASHRAE Standard 241 sets guidelines for how much “clean air” (via ventilation, filtration, etc.) is needed in buildings to reduce the airborne spread of infectious diseases like COVID-19. But how were these numbers chosen? 1/n

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Chris Iddon @chrisiddon.bsky.social · 25/06/2025

🔍 Conclusion: Better trials, not bigger fans. Until we have high-quality evidence, stick to standards, target known harms, and prioritize interventions with proven impact. #evidencebased #ventilation rehva.eu/rehva-journa... 5/5

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Chris Iddon @chrisiddon.bsky.social · 25/06/2025

📉 Advocates often ignore the law of diminishing returns: more airflow ≠ proportionally more benefit. Beyond current standards, major upgrades need strong evidence—which we don’t yet have. #IndoorAirQuality 2/5

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

Whilst this paper shows promising improvements in air quality by replacing kerosene lamps with solar lamps in reducing respiratory symptoms, they suggest improving gut microbiome communities could be an answer thanks to p-hacking loads of gut flora! /end ehp.niehs.nih.gov/doi/10.1289/...

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

They suggest maybe it is the gut microbial communities that impacts the respiratory health, and poor IAQ changes these communities. So a therapeutic approach to countering poor IAQ if you can't replace kerosene lamps is to use microbiome-targeted therapies 🤯 7/n

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

The abundance of some bacteria went down and some went up. The abundance of some viruses went down and some went up 5/n

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

What if we also collect some 💩 and just look at all the bacteria and phages (viruses that infect bacteria) that we find and try and find some differences. No difference in the diversity of bacteria, but some differences in the types of virus found 4/n

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

And in this new study they show significant reductions in particulate pollution when using solar lamps as well as a reduction in respiratory symptoms esp dyspnea (shortness of breath). Reduced exposure to pollution correlates with reduced respiratory symptoms - great! but.. 3/n

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Chris Iddon @chrisiddon.bsky.social · 06/06/2025

This is an extension of previous work that looks at the impact of changing from using kerosene lamps to solar lamps on indoor air quality in dwellings in rural Uganda. Burning of kerosene lamps creates a lot of particulate pollution as one might expect pmc.ncbi.nlm.nih.gov/articles/PMC... 2/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

This is represented for the different flu strains, suggesting that there is some difference between strains in their ability to confer infection via the aerosol route 10/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

Triangles are direct inoculated and circles aerosols. As time from inoculation increases the amount of detected virus in supernatant increases, but at a faster rate in the direct inoculated 9/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

More virus is detected quicker in cell lines inoculated directly compared to by the aerosol route and this is most likely because the cell lines are exposed to more virus in the direct inoculation compared to aerosol inoculation. Here serial dilutions of supernatant tested 8/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

imo the virus still undergoes a stage where it moves from aerosol into the cell culture medium and then infects the cells, so I can't imagine any mechanical difference compared to the direct inoculation (tho i cld well be wrong) 5/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

A nebuliser is used to aerolise a viral solution, some of these aerosols containing virus deposit onto the solution covering the cells, and virus then infect the cells. Again incubated for a while before cells are washed and covered with virus free soln 3/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

Here's a rough sketch of what was done, two types of cell lines are cultured on the bottom of a well in a plate. These cells need a solution with nutrients/antibiotics etc in to survive. In direct inoculation this fluid contains virus. Incubated for a while before washing 2/n

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Chris Iddon @chrisiddon.bsky.social · 05/06/2025

New paper describes an in vitro method to infect cell culture with different flu strains by direct inoculation and by aerolised viral solution. Whilst it is an interesting novel methodology, I think it is limited in what it can tell us about the nature of aerosol transmission 1/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

Also concerns that short term methods of increasing ventilation in the winter (opening doors/windows) would impact customer thermal comfort 12/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

Other regulatory requirements took precedence, especially for bars and nightclubs where increased ventilation openings (windows/doors) would lead to noise escape putting their licences at risk 11/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

Many responders mentioned they were too busy with customers during their opening hours to take time to respond to CO2 monitors or make changes to ventilation 10/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

some evidence that education might lead to continued behavioural changes to ensure good ventilation ventilation is continued, but not many venues thought it worth buying a CO2 monitor 9/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

but there are many confounders so it is not strong evidence eg no data on occupancy or differences in weather etc 8/n

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Chris Iddon @chrisiddon.bsky.social · 16/05/2025

Following interventions and education of venue hosts there was some evidence of improved ventilation practice the following winter 22/23 7/n

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