23/09/2026
Living near a highway means chronic exposure to traffic related particulate matter (PM), long linked to cardiovascular disease and more recently neurological decline. Whether simply cleaning indoor air could measurably protect cognition, however had barely been tested.
This secondary analysis of the HAFTRAP trial a pragmatic randomized crossover study followed 119 adults (ages 30–74) living within 200m of highways in Somerville, Massachusetts. Each participant used a real HEPA purifier for one month and a sham (non-filtering) unit for another month, in random order, with a washout period between. Cognitive function was measured using the Trail Making Test (Parts A and B), which assesses processing speed and executive function.
Overall, there was no significant difference between HEPA and sham filtration. However, age significantly moderated the effect: adults 40 and older completed the executive-function task 12% faster after HEPA filtration than after sham (54.0s vs. 61.4s, p = 0.02). No benefit was seen in younger participants.
Limitations include a predominantly white, well-educated sample, a known learning effect on repeated testing and no participants with pre-existing cognitive impairment leaving open whether the effect holds for at-risk populations.
