A month of HEPA filtration improved executive functioning
STUDY: Pellegrino N et al, Scientific Reports 2026
STUDY TYPE: Randomized, crossover trial, secondary outcome analysis
FUNDING: National Institute of Environmental Health Sciences; University of Connecticut School of Medicine
Background
Living near highways means regular exposure to traffic-related particulate matter, which is linked to poor cognition, heart disease, dementia, and Parkinson’s. This study looks asks whether reducing particulate pollutants with a HEPA purifiers can improve executive function.
The Study
- 119 adults 30 and older living within 200 meters of a highway near Boston, out of 156 enrolled.
- Randomized crossover design: each got 30 days with a HEPA purifier and 30 with a sham unit, separated by a washout.
- Outcome: Trail Making Test, a timed measure of processing speed and executive function, at each period’s start and end.
Results
HEPA and sham filtration produced no significant difference overall, but did improve cognition when stratified by age.
Those 40 and older finished the more difficult part of the Trail Making Test (B) test 12% faster after HEPA than after sham (54.0 versus 61.4 seconds, ratio of means=0.88, p=0.02). No benefit appeared under 40, and neither group differed on the simpler Part A test.
HEPA filters also improved air quality, cutting particulate matter (PM2.5) in half, and ultrafine particles by about a third.
Limitations
This was a secondary analysis by age, so treat it as an open-label finding, not a confirmation.
Sham units may have passively reduced pollution through air movement, understating HEPA’s benefit. Participants were mostly white and well-off, and none had cognitive impairment.
Practice Implications
- While this study isn’t definitive, HEPA filters have relevant health benefits, reducing asthma, inflammation, and hypertension (by 3.2 mmHg). They are already recommended by the American Heart Association, and here’s their advice on selecting one:
- Match the device’s clean air delivery rate (CADR) to the room, choose a true HEPA media (non-ionizing) unit, place it in the bedroom, run it on high when occupied (eg, when asleep) with windows and doors closed, and change filters on schedule. Airflow problems and room barriers mean the CADR often does not match up with reality, so err on getting larger than needed.
- Avoid electrostatic and ionizing “electronic” cleaners, which can generate ozone.
- Filters do not remove gaseous pollutants (like nicotine smoke) and have limited effect on animal allergens and pollen.
Related
Learn more about air ionization, an air purifier that treats depression.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
