Running a HEPA air purifier at home for a single month measurably sped up performance on tests of mental flexibility and executive function in adults over 40 living near heavy traffic, according to a randomized trial published in Scientific Reports — early evidence that filtering indoor air could offer a low-cost way to protect cognition in polluted neighborhoods.
The study, conducted by researchers including Nicholas Pellegrino and Doug Brugge with colleagues at the University of Connecticut and Tufts University, recruited 119 adults aged 30 to 74 living near Interstate 93 and Route 28 in Somerville, Massachusetts. Full results appear in the peer-reviewed paper, with additional detail summarized by ScienceDaily.
A crossover design, and a 12% gain
Each participant used a real HEPA purifier for one month, took a month-long break, then used a visually identical sham device for a third month, with the order randomized across participants. After each phase, participants took the Trail Making Test, a standard neuropsychological measure of processing speed, attention and mental flexibility that involves rapidly connecting sequences of numbers and letters. Among the roughly 42% of participants who were 40 or older, completion times on the executive-function portion of the test were about 12% faster after the real-purifier month than after the sham month — a difference the authors describe as comparable in size to cognitive gains reported from increased physical exercise. Participants younger than 40 showed no measurable change either way.
The result held up after researchers controlled for how much time participants spent indoors and how stressed they reported feeling during testing, the authors said, strengthening confidence that the effect wasn't simply explained by scheduling or mood. The brain regions governing mental flexibility and executive function are among those most consistently linked to harm from long-term air pollution exposure in prior epidemiological research, which the authors say makes the pattern biologically plausible, even though this trial cannot pin down the exact mechanism.
Traffic-adjacent air pollution — a mix of fine particulates, ultrafine particles and combustion byproducts — has been associated in previous observational research with accelerated cognitive decline and higher dementia risk, particularly in older adults. What distinguishes the new work is its randomized, crossover design, which lets each participant serve as their own control and reduces the chance that the result simply reflects who chooses to use an air purifier.
The authors, however, flagged real constraints on how far the finding can be generalized. Fewer than 10 participants were over age 60, limiting statistical power in the group most vulnerable to pollution-linked cognitive decline. The trial tested only one month of purifier use, so it says nothing about whether benefits persist, grow, or fade with continued or discontinued use, and the study cannot identify the specific biological pathway — reduced systemic inflammation and lower particulate exposure to the brain are candidate explanations, not confirmed ones.
The research team said longer-duration trials, larger cohorts of older adults, and studies pairing cognitive testing with biomarkers of inflammation are needed before HEPA filtration could be recommended as a targeted cognitive-health intervention rather than a general air-quality measure.
