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

TL;DR from the AFRI-c trial: Portable HEPA filters didn't reduce resident winter respiratory infections, staff sickness, or antibiotic use in UK care homes. Core IPC measures (ventilation, hygiene, vaccines) remain. /end jamanetwork.com/journals/jam...

jamanetwork.com

High-Efficiency Particulate Air Filters to Prevent Winter Respiratory Infections in Care Homes

This randomized clinical trial investigates whether high-efficiency particulate air filters in communal rooms and private bedrooms reduce respiratory infection episodes in care home residents.

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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

This effect would be even smaller in larger communal areas or better ventilated communal areas. So whilst we can be confident that HEPA remove virus from the air, the magnitude matters and that is primarily dependent on how much virus an infected person emits 7/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

So if we assume this poor ventilation scenario this HEPA filter provides more than 2x the equivalent ventilation, assuming a 1ach for bio decay and deposition this would give a relative reduction in concentration of c50% 5/n

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

The CADR is pretty meagre at around 44l/s for a communal room. Let's consider a small lounge at 40m2 and 3m floor to ceiling (120m3) with a low background ventilation of 0.5ach (16l/s). [NB this is very low, if we assume 10 occupants at 5l/s/p that would be 50l/s] 4/n

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

HEPA filters were placed in communal areas and bedrooms and the settings were agreed with the care homes, meaning that low level flows were used to reduce noise. Also no continued monitoring of usage, tho staff report air cleaners are turned on 3/n

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

The trial was sized to find at least 50% effect size, so it is possible that a smaller effect is present, but the study is not sufficiently powered to determine it. Indeed if the effect is small it would need such a large study cohort that it would be impractical to carry out 2/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 · 27/07/2026

and even *if* this response is real and not a statistical artefact, then completing the test in 54s (clinical impairment is defined as >273s) is shaving 7 seconds off a paper-and-pencil line-drawing task. This does not translate to noticeable improvements in work, focus, or daily decision-making

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Chris Iddon @chrisiddon.bsky.social · 27/07/2026

First, the overall study found no statistically significant effect on cognitive performance (p = 0.33). The "benefit" only appeared after the authors split participants at an arbitrary age cut-off (>40). This age subgrouping seems to be a post-hoc analysis on a secondary outcome

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

and by other transmission routes. All cause absenteeism may hide the impact on respiratory infections. Many respiratory infections lead to low symptoms and pupils don't take absence for them. preprint available here /end papers.ssrn.com/sol3/papers....

papers.ssrn.com

Elementary school classroom attendance rates in a randomized crossover trial with portable HEPA air cleaners

The goal of this study was to assess the benefits of improved air filtration in elementary schools via the use of standalone High-Efficiency Particulate Air (HE

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

HEPA are good at removing particulate matter in classrooms, and this likely includes airborne pathogens. But the amount of pathogens in the air is dependent on the emission rate (which varies by many orders of magnitude), and kids also exposed in other environments 6/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

They use a Blueair Classic 605 air cleaner but don't report CADR, I googled a datasheet which claims 235-300l/s, which is pretty good. The authors have previously reported that PM2.5 were reduced by around 40% compared to control (filters can filter 🤷) 3/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 · 24/07/2026

barely visit here these days, too much to keep track across platforms, might post some interesting studies...

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Chris Iddon @chrisiddon.bsky.social · 27/02/2026

thanks, I'll get in touch with them next week

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Chris Iddon @chrisiddon.bsky.social · 25/02/2026

this looks like a really interesting study. Do you have a gauge of how many of the pfu released into the chamber will have landed on the petri dishes? I'd be interested to know the ratio of pfu the cells on the petri dish were exposed to compared to the number leading to pfu in the assay

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Reposted by Chris Iddon

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 · 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

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...

journals.plos.org

Evaluating modes of influenza transmission (EMIT-2): Insights from lack of transmission in a controlled transmission trial with naturally infected donors

Author summary Human-to-human influenza virus transmission under controlled conditions could provide insights leading to better control of epidemics and pandemics. However, a previous study using labo...

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

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

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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

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...

x.com

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

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Chris Iddon @chrisiddon.bsky.social · 21/01/2026

thanks for the comprehensive explanations :)

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Chris Iddon @chrisiddon.bsky.social · 21/01/2026

16+ venue, my kids will be disappointed. Any other gigs in the pipeline?

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

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...

proquest.com

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

The 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.

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

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.

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

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).

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

And importantly: ⚠️ In Winter, some models show higher absenteeism in HEPA treated classrooms compared to controls—especially at higher equivalent ventilation rates.

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

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.

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

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).

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

But the big question was absenteeism. Do cleaner classrooms = fewer sick days? Short answer: the evidence is mixed and weak.

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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 · 08/01/2026

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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Chris Iddon @chrisiddon.bsky.social · 08/01/2026

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.

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Chris Iddon @chrisiddon.bsky.social · 08/01/2026

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 🤷‍♀️

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Chris Iddon @chrisiddon.bsky.social · 07/01/2026

They will provide air purifiers with HEPA filters running at 486m3/h (approx 3ach) or at 48m3/h.

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