BACKGROUND AND AIM[|]To test benefits of air purifiers in homes to reduce allergy symptoms.[¤]METHOD[|]This analysis was nested within a double-blinded randomized cross over trial of HEPA air purifiers in bedrooms and living rooms of 113 older adults free from serious illness living near a major highway in Massachusetts, USA. Participants had one month of real and one month of sham filtration randomized in order and separated by a one month wash out period. Data collection was at the start and end of each month (4x). Participants answered four allergy questions for runny or stuffy nose, itchy nose, sneezing, and itchy eyes in the previous month. We limited our analysis to 79 participants who reported at least one symptom. Responses were converted to number of days of symptoms in the last month: never=0; once or twice=1.5; a few times a week=12; every day=30. A linear mixed model with a random intercept, to account for within-person correlation as well as sequence and period effects, was used to compare the mean number of symptom days between sham and HEPA. The model was adjusted for symptom responses at the beginning of each period.[¤]RESULTS[|]The mean age (sd) was 42.8 (11.6) years. The sample was 54.4% female, 68.3% non-Hispanic White, 17.7% Hispanic, and 78.5% employed. Baseline symptom data at the first and third visits was between 3.5 and 8.8 days. Simply having a filtration unit within their home irrespective of sham/HEPA, participants reported approximately two fewer symptom days. There were small reductions in sneezing (-1.3; P=0.20), and itchy eyes (-0.8; P=0.26) with HEPA. Conversely, days of itchy nose were higher (+1.5; P=0.04) during HEPA.[¤]CONCLUSIONS[|]Our findings indicate a social desirability reaction to having an air purifier in the home as well as suggestive reductions of allergy symptoms during HEPA filtration.[¤]