The largest randomized trial yet conducted on portable air purifiers in residential care found no benefit, and the argument that has broken out over the result is more useful to readers than the result itself.
British researchers randomized 95 care homes in England, roughly half receiving portable HEPA filtration units for communal areas and residents' bedrooms and half continuing with usual care, and tracked more than 1,100 older residents across three winters. Writing in JAMA Internal Medicine, the authors reported that they "found no evidence that HEPA filters reduced resident winter respiratory infection episodes, other infections, antibiotic use, or staff absenteeism."
Several air quality scientists who were not involved say the trial did not test what its headline implies. Their objection is blunt: an air purifier that is switched off, turned to its lowest setting, or too small for the room is not an intervention.
Ninety-Five Care Homes Across Three Winters
The AFRI-c trial ran from September 2021 to May 2024 and was designed as a pragmatic study, meaning it aimed to measure what happens in ordinary conditions rather than under ideal ones.
The primary outcome was the rate of symptomatic winter respiratory infection episodes recorded by care home staff, not laboratory-confirmed influenza or COVID specifically. Secondary outcomes included other infections, laboratory-confirmed infections, hospitalizations, falls, and staff sickness. Residents were old and frail, with a median age of 87 to 88, more than half living with dementia, and vaccination coverage above 94 percent for both influenza and COVID.
Lead author Alastair Hay, a professor of primary care at the University of Bristol, told CIDRAP News the design reflected the moment. Care home staff in 2021 agreed to take part only if the research added no burden, and the trial ran "without research staff visiting the sites," with interviews conducted by phone or video.
Hay argues that constraint is the point. A pragmatic trial estimates how well the devices are likely to perform in real care homes, including all the difficulties those homes face keeping equipment running.
The Adherence Problem at the Center of the Result
The trial's own paper acknowledges what it calls low intervention adherence. A University of Bristol press release described high compliance. Those two statements are difficult to reconcile, and the gap is where the disagreement lives.
Homes were counted as compliant with the protocol if the filters ran at least 20 percent of the time. Joseph Allen, a professor of exposure assessment science at the Harvard T.H. Chan School of Public Health, called that a very low bar and noted the devices may have been on for only a fifth of the study period.
Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, read the adherence language more directly, saying it means that "in many cases, the filters were not turned on."
The reasons are mundane and entirely believable to anyone who has worked in long-term care. Units were mostly run on their lowest setting at staff request because the noise was intolerable on higher settings. Residents, many living with dementia, moved or tampered with the machines. Staff reported one resident who regularly switched off the nearest purifier because he objected to the electricity being used.
Units Sized for Smaller Rooms Than They Were Placed In
A second criticism concerns engineering rather than behavior.
The paper does not report room dimensions or airflow rates, the figures aerosol scientists use to judge whether a filter can clean a given space. Working from the limited data available, Allen and Marr independently estimated that the units were exchanging room air roughly once or twice per hour.
The CDC's ventilation guidance recommends aiming for at least five air changes per hour in occupied spaces. A device delivering one or two is not underperforming at the margin. It is operating well below the threshold at which any benefit would be expected.
Placement compounds this. Lisa Brosseau, a research consultant at the Center for Infectious Disease Research and Policy, noted that a filter positioned in a corner cleans that corner efficiently and little else, and that meaningful particle removal near a sick person requires the unit to sit close to them, such as between a resident's bed and a visitor's chair.
Given all of this, Marr said she was not surprised the trial found no benefit.
A Paper and a Press Release That Do Not Match
This is where the study becomes a lesson in reading health research rather than a verdict on air filtration.
The trial is genuine and substantial. It is randomized, it is large by the standards of this field, it collected data across nearly 200,000 resident days, and it was peer reviewed in a major journal. Its finding, read narrowly, is that distributing portable HEPA units to care homes under real-world conditions, with the noise complaints and tampering and staffing pressure that come with them, did not reduce recorded respiratory infections.
That is a useful and policy-relevant finding for anyone deciding whether to buy filters for a facility without also budgeting for maintenance, monitoring, and staff buy-in.
What the trial does not establish is that HEPA filtration fails to remove airborne virus. Laboratory work has shown these filters can substantially reduce viral particles in air. Allen's objection is that the published conclusion outruns the data, and that read strictly, the finding is that a small filter which is turned off does not work.
The methodological limitation belongs in the same breath as the finding rather than at the end of the article, which is why it appears here. A null result in one pragmatic trial does not settle the underlying question, and the authors' own recommendation is narrow: care homes should keep following nationally recommended infection prevention measures.
Purchasing Decisions This Does and Does Not Settle
For a family evaluating a care facility or an individual considering a purifier at home, a few things follow from this trial and a few do not.
It does not follow that air purifiers are useless. It does follow that buying one and putting it in a corner on its quietest setting is unlikely to accomplish much. Anyone purchasing a unit should match its rated clean air delivery rate to the room's actual size rather than to a marketing claim, run it continuously on a setting that moves meaningful air, and position it near where people spend time.
It also does not follow that filtration substitutes for the measures with stronger evidence behind them. Vaccination, staying home while sick, hand hygiene, and prompt treatment remain the interventions with the clearest support in older adults, and residents in this trial were already highly vaccinated.
Families with a relative in long-term care can reasonably ask a facility what its respiratory infection prevention plan includes, whether any air cleaning equipment is actually running and maintained, and how the facility handles isolation during an outbreak. Those questions produce more useful answers than asking whether the building has purifiers.
Further analysis from the trial team, including a health economic evaluation and a published process evaluation examining how the intervention was implemented, is available alongside the main paper. MedicalDaily will report on subsequent trials that address the airflow and adherence questions this one leaves open.
Frequently Asked Questions
What did the trial actually measure? The rate of symptomatic winter respiratory infection episodes recorded by care home staff, not laboratory-confirmed flu or COVID specifically. Laboratory-confirmed infections were a secondary outcome.
How large was the study? Ninety-five care homes in England and more than 1,100 residents, followed across three winters between September 2021 and May 2024, with roughly 195,000 resident days of symptom data.
Does this prove air purifiers do not work? No. It shows that distributing portable HEPA units to care homes under real-world conditions did not reduce recorded infections in this trial. Independent experts argue the devices were frequently off, run on low, or undersized for the rooms.
What is the air changes per hour standard? CDC guidance recommends aiming for at least five air changes per hour in occupied spaces. Outside experts calculated the units in this trial delivered roughly one to two.
Was the study done in U.S. nursing homes? No. It was conducted in residential care homes in England. Staffing, room sizes, and ventilation standards differ from U.S. facilities, which limits how directly the findings transfer.
Should I buy an air purifier for my home? This trial does not answer that. If you do, match the unit's clean air delivery rate to the room size, run it continuously at a setting that moves real air, and place it near where people spend time rather than in a corner.
What should I ask a care facility? Ask what the respiratory infection prevention plan includes, whether any air cleaning equipment is running and maintained, and how outbreaks and isolation are handled. Vaccination and sick-leave policies matter more than equipment.
