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
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/journals/jam...
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.
jamanetwork.com
July 28, 2026 at 11:55 AM
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/journals/jam...
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
July 28, 2026 at 11:55 AM
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
Nor on staff absenteeism
11/n
11/n
July 28, 2026 at 11:55 AM
Nor on staff absenteeism
11/n
11/n
After the trial no effect is found on the primary outcome of resident respiratory infection episodes 10/n
July 28, 2026 at 11:55 AM
After the trial no effect is found on the primary outcome of resident respiratory infection episodes 10/n
Interestingly at the start of the trial there is a strong belief that HEPA filters can reduce infections 9/n
July 28, 2026 at 11:55 AM
Interestingly at the start of the trial there is a strong belief that HEPA filters can reduce infections 9/n
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 28, 2026 at 11:55 AM
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
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
July 27, 2026 at 1:24 PM
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
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
July 27, 2026 at 1:24 PM
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
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....
preprint available here /end papers.ssrn.com/sol3/papers....
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
papers.ssrn.com
July 24, 2026 at 7:15 AM
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....
preprint available here /end papers.ssrn.com/sol3/papers....
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
July 24, 2026 at 7:15 AM
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
No significant impact on frequency of symptoms either, in fact in some weeks there were far more symptoms in HEPA classrooms 5/n
July 24, 2026 at 7:15 AM
No significant impact on frequency of symptoms either, in fact in some weeks there were far more symptoms in HEPA classrooms 5/n
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
July 24, 2026 at 7:15 AM
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
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
July 24, 2026 at 7:15 AM
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
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
July 24, 2026 at 7:15 AM
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
thanks, I'll get in touch with them next week
February 27, 2026 at 4:23 PM
thanks, I'll get in touch with them next week
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
February 25, 2026 at 12:41 PM
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