Effect of HEPA Filtration Air Purifiers on Cognitive Function: Secondary Outcome of a Pragmatic Randomized Crossover Trial

Effect of HEPA Filtration Air Purifiers on Cognitive Function: Secondary Outcome of a Pragmatic Randomized Crossover Trial | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Effect of HEPA Filtration Air Purifiers on Cognitive Function: Secondary Outcome of a Pragmatic Randomized Crossover Trial Nicholas Pellegrino, Misha Eliasziw, Richard Fortinsky, Hunter Gates, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8108219/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Apr, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Exposure to particulate matter is associated with adverse health outcomes such as cardiovascular and neurological diseases. However, there are few intervention studies that have examined short-term changes in air particulate exposure and cognitive outcomes. We analyzed data from 119 participants in the Home Air Filtration for Traffic-Related Air Pollution (HAFTRAP) study, a pragmatic randomized crossover trial. Participants were randomized to receive a high efficiency particulate arrestance (HEPA) air purifier for one month, a washout period of one month, then a sham unit for one month or vice versa. Time to complete Trail Making Test – Parts A and B were recorded at the beginning and end of each intervention period. Although there was no significant overall difference in completion times of the Trail Making Tests between HEPA and sham filtration, age was a significant moderator ( p = 0.02). Participants 40 years or older with HEPA filtration completed Part B 12% significantly faster than participants who had sham filtration in the preceding month (54.0 versus 61.4 seconds, ratio of means = 0.88, p = 0.02). No significant reduction in completion time was observed for participants < 40 years old. Among older healthy adults, there was an improvement in cognition following one month of in-home HEPA filtration. Further research is needed on the short-term effects of air pollution among individuals with some level of cognitive impairment. Health sciences/Diseases Earth and environmental sciences/Environmental sciences Health sciences/Health care Health sciences/Medical research Biological sciences/Neuroscience Health sciences/Risk factors air pollution particulate matter cognitive outcome air filtration Figures Figure 1 Figure 2 Introduction Living or working near highways and dense traffic has been linked to increased mortality and morbidity in adults via multiple pathways, including respiratory illness and cardiovascular disease 1 . It has also been associated with neurological outcomes, such as Alzheimer’s and Parkinson’s diseases 2 . Air pollution near highways is a mixture of particles, such as ultrafine particles (UFP, ≤ 0.1 micrometers in aerodynamic diameter) and black carbon; and gasses, such as nitrogen dioxide 3 . Exposure to particulate matter (PM) and other air pollution in urban areas is largely due to roadway emissions and indoor activities such as cooking 4 , exposures that can be difficult to avoid. High efficiency particulate arrestance (HEPA) air filtration systems are one approach to reducing exposure to PM 5,6 . Previous studies have reported that highway-related PM is associated with adverse cognitive outcomes. A randomized crossover trial that assessed PM concentrations in cars showed worse ability to multitask and react to other cars on the road when the air filter was not reducing concentrations of UFP and other PM 7 . Short-term observational exposure studies have also reported that cognitive outcomes were associated with higher concentrations of PM 2.5 in both young adult and elderly populations 8 , 9 . To date, few studies have examined the potential benefit of in-home air filtration units that reduce highway-related PM on cognitive outcomes. As air purifiers are increasingly being used and recommended for homeowners 10 to reduce a wide range of PM 11 , it is of interest to examine whether these units are capable of improving cognitive function for residents living near highways. The aim of this secondary outcome analysis of a pragmatic randomized crossover trial was to assess whether there was a benefit to cognitive function from HEPA air filtration for urban-dwelling, near-highway residents. Methods Study Participants The Home Air Filtration for Traffic-Related Air Pollution (HAFTRAP) trial 6 enrolled individuals who were 30 years of age or older, lived full time in residences within 200 meters of highways in Somerville, Massachusetts, had the cognitive capacity to fill out questionnaires, and spoke English or Spanish. The trial excluded individuals who were taking anti-hypertensive medications or had a history of major cardiovascular events. Individuals were also excluded if they currently smoked/vaped or allowed indoor smoking/vaping, because air purifiers have limited efficacy against these indoor sources. Additionally, participants were not enrolled if they had occupational or regular exposures to traffic pollution outside of the home, or significant combustion sources inside the home, other than cooking. Participants were enrolled between September and April, 2020–2024. The study sought to enroll participants in cooler months when near highway concentrations of UFP tend to be higher 12 . Participant households were randomized to 30 days of either HEPA or sham filtration, with air purifiers placed in the living room and bedroom, followed by a 30-day washout period, and then a subsequent 30-day period of the alternative intervention. Assessment of cognitive function was a secondary outcome in the trial whose primary aim was reduction in blood pressure 13 . The trial received approval from the UConn Health Institutional Review Board Human Subjects Protection Program and was conducted according to the Declaration of Helsinki. All participants provided written informed consent. Trail Making Test Participants completed the Trail Making Test, which is used to assess cognitive function and has been shown to be effective in quantifying cognitive impairment in healthy adults 14 over short time periods 15 . Participants completed the test in the early morning at their home at the beginning and end of each intervention period, resulting in the test being completed four times by each participant. Participants were timed for sequentially connecting numbers (Part A) and sequentially connecting alternating numbers and letters (Part B). Part A of the Trail Making Test assesses visual memory and motor speed skills, while Part B assesses executive function and mental flexibility 16 . For Part A, a completion time of 29 seconds is considered average and > 78 seconds is deficient. For Part B, a completion time of 75 seconds is considered average and > 273 seconds is deficient 17 . Statistical Analyses Over-dispersed Poisson regression models were used to compare the Trail Making Test – Part A and Part B completion times between the HEPA and sham filtration groups at the end of the intervention periods, as time was considered a count variable and the distribution of times were right-skewed (Fig. 1 ). The model included sequence and period effects and was adjusted for the corresponding Trail Making Test completion time at the start of each intervention period. Results were also adjusted for number of hours per day spent indoors at home a week prior to the Trail Making Test, and Perceived Stress Scale (PSS-4) score and outdoor temperature at the time of completing the Trail Making Test. Adjusting the results for the number of hours per day spent indoors at home accounted for some participants spending more time inside the home and thus receiving potentially more benefit from air purification. Perceived Stress Scale (PSS-4) scores were included in the models as perceived stress has been linked to worse cognitive outcomes among healthy younger and older adults 19 , 20 . Similarly, outdoor temperature has been associated with a significant effect on scores for Part A of the Trail Making Test 21 . The Trail Making Test learning effect was assessed using the first three consecutive Trail Making Test completion times from the 67 participants who were randomly assigned to receive the sham then HEPA sequence. Because these participants received one month of sham filtration first, followed by a one-month washout period, their first three consecutive Trail Making Test results could not be affected by HEPA filtration. Thus, improvements in completion times should primarily due to retaking the test rather than filtration. These 67 participants’ Trail Making Test – Part A and Part B completion times were compared across the three timepoints using over-dispersed Poisson regression models. As it was previously shown that performance on the Trail Making Test decreases with increasing age 13 , 17 , as well as reported in a systematic review that a general decline in cognition due to PM exposure begins at age 40 18 , the moderating effect of age was assessed by dichotomizing participant’s age as < 40 years versus 40 years or older and a cross-product term was included in the models to assess its significance. The cross-product term consisted of the intervention (HEPA vs sham) and the dichotomized age (< 40 years versus ≥ 40 years). All statistical analyses were carried out using SAS 9.4 (SAS Institute Inc., Cary, NC), and results with p-values < 0.05 were deemed statistically significant. Results Of the 156 participants enrolled in the trial, 37 did not complete the Trail Making Test at all four home visits; 2 moved outside the study area, 1 had a phone survey, 23 had paper forms misplaced, and 11 opted not to complete the Trail Making Test at one or more of the home visits. The remaining 119 participants had complete data for the Trail Making Test at all four home visits and are included in the analyses. There were no appreciable differences in participant characteristics between the 26 who are excluded for administrative reasons and the 119 who are included in the analyses (Table 1 ). However, the 11 participants who opted not to complete the Trail Making Test were generally older, less educated, and more likely to be unemployed, which may explain their longer Trail Making Test completion times. Table 1 Characteristics of participants at trial entry. Administrative (N = 26) Incomplete (N = 11) Analyzed (N = 119) Age in years, mean (sd) 41.5 (10.9) 48.0 (15.0) 40.8 (9.4) Age category, n (%) < 40 years 40 years or older 14 (53.8) 12 (46.2) 2 (18.2) 9 (81.8) 69 (58.0) 50 (42.0) Sex, n (%) Male Female 10 (38.5) 16 (61.5) 3 (27.3) 8 (72.7) 50 (42.0) 69 (58.0) Ethnicity and race, n (%) Hispanic White, non-Hispanic Black, non-Hispanic Asian, non-Hispanic 5 (19.2) 18 (69.2) 1 (3.9) 2 (7.7) 3 (27.3) 8 (72.7) 0 (5.0) 0 (5.0) 21 (17.6) 81 (68.1) 7 (5.9) 10 (8.4) Highest level of education, n (%) Grade or high school Some college College or university degree Graduate degree 5 (19.2) 1 (3.9) 7 (26.9) 13 (50.0) 2 (18.2) 2 (18.2) 5 (45.4) 2 (18.2) 11 (9.2) 15 (12.6) 31 (26.1) 62 (52.1) Work status, n (%) Unemployed Part-time working Full-time working 2 (7.7) 6 (23.1) 18 (69.2) 6 (54.5) 1 (9.1) 4 (36.4) 19 (16.0) 12 (10.1) 88 (73.9) Total annual household income, n (%) < $48,000 $48,000 to $84,999 $85,000 or greater Declined to answer 0 (0.0) 1 (3.8) 19 (73.1) 6 (23.1 1 (9.1) 1 (9.1) 5 (45.4) 4 (36.4) 10 (8.4) 17 (14.3) 74 (62.2) 18 (15.1) Weight status, n (%) Healthy Overweight Obesity 9 (34.6) 8 (30.8) 9 (34.6) 5 (45.4) 4 (36.4) 2 (18.2) 52 (43.7) 36 (30.2) 31 (26.1) Time spent inside home (hours per day), mean (sd) 18.0 (3.3) 17.3 (3.5) 19.1 (3.6) Perceived Stress Scale (PSS-4), mean (sd) 4.6 (3.6) 4.1 (2.9) 3.2 (2.6) Outdoor temperature in Fahrenheit, mean (sd) 45.1 (8.3) 47.4 (12.8) 39.8 (12.8) Trail Making Test – Part A (seconds), mean (sd) ----- 49.3 (49.8) 24.3 (12.6) Trail Making Test – Part B (seconds), mean (sd) ----- 118.6 (94.8) 70.7 (42.9) Characteristics of younger (< 40 years) and older ( ≥ 40 years) participants are shown in Table 2 . The difference in mean age was almost 15 years (34.5 versus 49.4 years). Older participants were generally less educated, were less likely to work full-time, more likely to be female, and to have obesity. Although the mean Trail Making Test completion time for older participants was 4 seconds longer for Part A and 14 seconds longer for Part B, the mean times at the start of the intervention period were within the range of adults with normal cognitive function 14 , 22 . Table 2 Characteristics of analyzed participants at the start of the intervention period by age category. < 40 years (N = 138) 40 years or older (N = 100) Randomized to sequence, n (%) HEPA then sham filtration Sham then HEPA filtration 62 (44.9) 76 (55.1) 42 (42.0) 58 (58.0) Age in years, mean (sd) 34.5 (2.8) 49.4 (8.4) Sex, n (%) Male Female 66 (47.8) 72 (52.2) 34 (34.0) 66 (66.0) Ethnicity and race, n (%) Hispanic White, non-Hispanic Black, non-Hispanic Asian, non-Hispanic 28 (20.3) 84 (60.9) 10 (7.2) 16 (11.6) 14 (14.0) 78 (78.0) 4 (4.0) 4 (4.0) Highest level of education, n (%) Grade or high school Some college College or university degree Graduate degree 10 (7.2) 6 (4.4) 46 (33.3) 76 (55.1) 12 (12.0) 24 (25.0) 16 (16.0) 48 (48.0) Work status, n (%) Unemployed Part-time working Full-time working 14 (10.1) 8 (5.8) 116 (84.1) 24 (24.0) 16 (15.0) 60 (60.0) Total annual household income, n (%) < $48,000 $48,000 to $84,999 $85,000 or greater Declined to answer 8 (5.8) 12 (8.7) 98 (71.0) 20 (14.5) 12 (12.0) 22 (22.0) 50 (50.0) 16 (16.0) Weight status, n (%) Healthy Overweight Obesity 74 (53.6) 38 (27.6) 26 (18.8) 30 (30.0) 34 (34.0) 36 (36.0) Time spent inside home (hours per day), mean (sd) 18.9 (3.8) 18.7 (3.5) Perceived Stress Scale (PSS-4), mean (sd) 3.2 (2.9) 2.9 (2.7) Outdoor temperature in Fahrenheit, mean (sd) 43.6 (13.2) 44.3 (13.9) Trail Making Test – Part A (seconds), mean (sd) 21.2 (9.1) 25.2 (16.2) Trail Making Test – Part B (seconds), mean (sd) 58.3 (28.9) 72.7 (50.0) Mean completion times at the end of the intervention period for Parts A and B of the Trail Making Test between HEPA and sham filtration are shown in Table 3 . No significant differences between HEPA and sham filtration were observed for Trail Making Test – Part A, and the mean completion times of approximately 20 seconds were similar between the two filtration groups and between younger and older participants (Fig. 2 ). Table 3 Trail Making Test – Part A and Part B completion times at the end of the intervention period Part A (seconds) HEPA Sham Ratio of Means P-value N HEPA , N Sham mean (se) mean (se) (95% CI) Overall 119, 119 20.3 (0.5) 19.7 (0.5) 1.03 (0.96, 1.10) 0.38 < 40 years of age 69, 69 19.5 (0.6) 18.7 (0.6) 1.04 (0.95, 1.15) 0.34 40 years or older 50, 50 21.4 (0.8) 21.2 (0.8) 1.01 (0.92, 1.11) 0.82 Part B (seconds) HEPA Sham Ratio of Means P-value N HEPA , N Sham mean (se) mean (se) (95% CI) Overall 119, 119 53.6 (1.6) 55.8 (1.6) 0.96 (0.89, 1.04) 0.33 < 40 years of age 69, 69 53.7 (2.0) 51.4 (2.0) 1.04 (0.94, 1.16) 0.43 40 years or older 50, 50 54.0 (2.4) 61.4 (2.4) 0.88 (0.79, 0.98) 0.02 Results were adjusted for the corresponding Trail Making Test completion time at the start of each intervention period. Results were also adjusted for number of hours per day spent indoors at home a week prior to the Trail Making Test, and Perceived Stress Scale (PSS-4) score and outdoor temperature at the time of completing the Trail Making Test. For Trail Making Test – Part B, the overall reduction in mean completion times between HEPA and sham filtration did not reach statistical significance but was significantly moderated by age ( p = 0.02, Fig. 2 ). Participants 40 years or older completed Part B 12% significantly faster after a month of HEPA filtration, on average, than participants who had experienced sham filtration (54.0 versus 61.4 seconds, ratio of means = 0.88, p = 0.02). No significant reduction in completion time was observed for younger participants. As expected, the Trail Making Test had a learning effect among the 67 participants who were randomly assigned to receive the sham then HEPA sequence. For both Part A and for Part B, these participants' completion times significantly improved with each subsequent visit; mean completion times for Part A were 24.4, 22.8, and 20.9 seconds, respectively and mean completion times for Part B were 70.0, 62.4, and 57.8 seconds, respectively. Discussion In this secondary outcome analysis of a pragmatic randomized crossover trial, with no restrictions on participants’ daily activities, we observed a statistically significant improvement in cognitive performance on Part B of the Trail Making Test for participants 40 years of age or older. This test assesses executive function and mental flexibility, and the effect was observed after a month of HEPA filtration compared to sham filtration. Participants 40 years or older completed Part B 12% faster after a month of HEPA compared to a month of sham filtration. Other crossover trials of air filtration that examined the effect of air pollution on cognition have also reported that higher exposure to particulate matter was associated with worse executive function and attention switching. 7 , 23 , 24 These studies used different cognitive tests for these domains, namely the CANTAB test and Strategic Management Simulation. Studies have found no difference in visual memory or motor speed 25 , 26 when using the psychomotor vigilance task and the simple reaction time test, the domains of cognition that was assessed in Trail Making Test – Part A. There is neurobiological evidence that particulate matter exposure may reduce white matter in the brain 27 , specifically in the frontal and temporal lobes 28 . These lobes are responsible for the mental flexibility and executive function skills assessed by Part B of the Trail Making Test 29 . Thus, our finding of benefit of HEPA filtration on Part B is consistent with underlying neurobiological processes. Additionally, our result of benefit observed among participants 40 years or older is consistent with reports that the cognitive effects of air pollution are more pronounced beginning around age 40 18 . Our results are also largely consistent with other intervention studies that observed the effects of reducing particulate matter exposure on cognitive outcomes among healthy adults. A randomized crossover trial among healthy office workers of a similar age range to ours reported that HEPA filtration improved executive functioning, with no differences between HEPA and sham filtration for visual or working memory 30 . Another randomized crossover trial comparing HEPA and sham filtration was performed among college-aged students. Students were provided air purifiers to use while they slept and were then given a cognitive test in the morning. Sham filtration was associated with slower memory, worse executive function, and lower global cognition 23 . Although the latter study was with younger people than ours, both crossover trials provide evidence of a possible benefit of HEPA filtration on cognition among healthy adults. Our analysis has several strengths and limitations. A strength of our analysis is that the data arose from a crossover trial design in which participants were compared to themselves, eliminating time-invariant confounding. Our analysis also controlled time-varying confounders, such as time spent inside the home, ambient temperature and level of stress, which can affect results. One limitation to the use of the Trail Making Test to assess cognitive function is that it has a learning effect, whereby participants’ completion times improve simply from retaking the test multiple times 31 , as in our crossover trial, Effects of this sort have been shown to be more common in younger adults 32 and more common in short-term periods similar to the time-frame of our intervention periods 15 , 33 . Although the Trail Making Test has a learning effect, we expect that the improvement in completion times would be similar between the participants randomly assigned to the sham then HEPA sequence and those randomly assigned to the HEPA then sham sequence. Another limitation was the use of sham filtration which could have reduced UFP exposure 34 through diffusional deposition, resulting in concentrations of UFP being reduced by air movement alone 35 . Reducing participants’ UFP exposure during the sham filtration period could have attenuated the effect between the HEPA and sham intervention periods. Our results are also limited in terms of generalizability. As none of the participants had preexisting cognitive impairments, our analysis is not relevant to the effectiveness of HEPA filtration among cognitively impaired individuals. Conclusions Our results suggest that, among older healthy adults living near highways in the Northeastern US, there was an improvement in cognition following one month of in-home HEPA filtration. Despite the strength of our trial design, there is a need for further research that employs methods with strong causal inference of short-term exposure to air particulates and cognitive outcomes, especially among individuals who have cognitive impairment. Declarations Conflicts of interest: None of the authors have relationships with industry. The air purifiers were custom adapted by the manufacturer, Austin Air, and sold at a discount based on bulk purchase. The manufacturer was not involved in any way in the design, conduct or analysis. Funding: NIEHS. Grant ID: R01 ES030289 and the School of Medicine at the University of Connecticut Author Contribution NP, ME, and DB were responsible for drafting the manuscript. HG scored and prepared the Trail Making Tests for analysis. ME was responsible for the analysis of the data. RF contributed to the interpretation of the results. Acknowledgement We would like to thank our project manager, Teresa Vazquez-Dodero for administering the trail making tests and the trial participants for their time and being receptive to in-home visits. Data Availability The data used in this study is available upon request. Please contact the corresponding author Dr. Doug Brugge at [email protected] . References Boogaard, H. et al. Long-term exposure to traffic-related air pollution and selected health outcomes: A systematic review and meta-analysis. Environ. Int. 164 , 107262. 10.1016/j.envint.2022.107262 (2022). Costa, L. G. et al. Effects of air pollution on the nervous system and its possible role in neurodevelopmental and neurodegenerative disorders. Pharmacol. Ther. 210 , 107523. 10.1016/j.pharmthera.2020.107523 (2020). Brugge, D., Durant, J. L. & Rioux, C. 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Supplementary Files HAFTRAPCognitivePaperTablesandFigures.docx Cite Share Download PDF Status: Published Journal Publication published 16 Apr, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 06 Jan, 2026 Reviews received at journal 05 Jan, 2026 Reviews received at journal 03 Jan, 2026 Reviewers agreed at journal 15 Dec, 2025 Reviewers agreed at journal 04 Dec, 2025 Reviewers invited by journal 03 Dec, 2025 Editor assigned by journal 03 Dec, 2025 Editor invited by journal 19 Nov, 2025 Submission checks completed at journal 18 Nov, 2025 First submitted to journal 18 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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10:41:18","extension":"html","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":119328,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8108219/v1/41f26cf842d9187b2306b27d.html"},{"id":97893367,"identity":"20165514-dd74-4208-8ddf-10fda6399af8","added_by":"auto","created_at":"2025-12-10 15:30:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":318515,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDistribution of Trail Making Test – Part A and Part B completion times at the end of the intervention periods by age category.\u003c/strong\u003e \u003cstrong\u003e(A)\u003c/strong\u003e Part A \u0026lt; 40 years (N = 138); \u003cstrong\u003e(B)\u003c/strong\u003e Part A \u003cu\u003e\u0026gt;\u003c/u\u003e 40 years (N = 100); \u003cstrong\u003e(C)\u003c/strong\u003e Part B \u0026lt; 40 years (N = 138); \u003cstrong\u003e(D)\u003c/strong\u003e Part B \u003cu\u003e\u0026gt;\u003c/u\u003e 40 years (N = 100). Each bar shows the number of tests completed within that time interval (seconds).\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8108219/v1/1ff0c298ce212b823edfac5f.png"},{"id":97688827,"identity":"152b69c4-f29b-4bbc-b7d4-f3d1b1f145c8","added_by":"auto","created_at":"2025-12-08 10:41:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":451127,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComparison of Trail Making Test – Part A and Part B completion times at the end of the intervention periods between HEPA and sham filtration.\u003c/strong\u003e \u003cstrong\u003e(A)\u003c/strong\u003e Trail Making Test – Part A; \u003cstrong\u003e(B)\u003c/strong\u003eTrail Making Test – Part B. Means with 95% confidence intervals of completion times between HEPA and sham filtration for younger (\u0026lt; 40 years) and older (\u003cu\u003e\u0026gt;\u003c/u\u003e 40 years)participants. A significant benefit was only observed among older participants for Trail Making Test – Part B. No significant benefit for younger participants and none for Trail Making Test – Part A.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8108219/v1/9c37e9877409b7fbf3a2d950.png"},{"id":107350843,"identity":"2cd2f89c-1d4c-4ab8-9414-aa35cd0528d5","added_by":"auto","created_at":"2026-04-20 16:05:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1286705,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8108219/v1/d5475ee7-1b2e-4d75-abd8-922f89f447ee.pdf"},{"id":97688828,"identity":"b4b0c8df-899b-4441-9f75-52943a8c85ac","added_by":"auto","created_at":"2025-12-08 10:41:17","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":662273,"visible":true,"origin":"","legend":"","description":"","filename":"HAFTRAPCognitivePaperTablesandFigures.docx","url":"https://assets-eu.researchsquare.com/files/rs-8108219/v1/e6c57e583b4cf4bd073daae0.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of HEPA Filtration Air Purifiers on Cognitive Function: Secondary Outcome of a Pragmatic Randomized Crossover Trial","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLiving or working near highways and dense traffic has been linked to increased mortality and morbidity in adults via multiple pathways, including respiratory illness and cardiovascular disease\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. It has also been associated with neurological outcomes, such as Alzheimer\u0026rsquo;s and Parkinson\u0026rsquo;s diseases\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Air pollution near highways is a mixture of particles, such as ultrafine particles (UFP, \u0026le; 0.1 micrometers in aerodynamic diameter) and black carbon; and gasses, such as nitrogen dioxide\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Exposure to particulate matter (PM) and other air pollution in urban areas is largely due to roadway emissions and indoor activities such as cooking\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e, exposures that can be difficult to avoid. High efficiency particulate arrestance (HEPA) air filtration systems are one approach to reducing exposure to PM\u003csup\u003e5,6\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003ePrevious studies have reported that highway-related PM is associated with adverse cognitive outcomes. A randomized crossover trial that assessed PM concentrations in cars showed worse ability to multitask and react to other cars on the road when the air filter was not reducing concentrations of UFP and other PM\u003csup\u003e7\u003c/sup\u003e. Short-term observational exposure studies have also reported that cognitive outcomes were associated with higher concentrations of PM\u003csub\u003e2.5\u003c/sub\u003e in both young adult and elderly populations\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eTo date, few studies have examined the potential benefit of in-home air filtration units that reduce highway-related PM on cognitive outcomes. As air purifiers are increasingly being used and recommended for homeowners\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e to reduce a wide range of PM\u003csup\u003e11\u003c/sup\u003e, it is of interest to examine whether these units are capable of improving cognitive function for residents living near highways. The aim of this secondary outcome analysis of a pragmatic randomized crossover trial was to assess whether there was a benefit to cognitive function from HEPA air filtration for urban-dwelling, near-highway residents.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eStudy Participants\u003c/h2\u003e\u003cp\u003eThe Home Air Filtration for Traffic-Related Air Pollution (HAFTRAP) trial\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e enrolled individuals who were 30 years of age or older, lived full time in residences within 200 meters of highways in Somerville, Massachusetts, had the cognitive capacity to fill out questionnaires, and spoke English or Spanish. The trial excluded individuals who were taking anti-hypertensive medications or had a history of major cardiovascular events. Individuals were also excluded if they currently smoked/vaped or allowed indoor smoking/vaping, because air purifiers have limited efficacy against these indoor sources. Additionally, participants were not enrolled if they had occupational or regular exposures to traffic pollution outside of the home, or significant combustion sources inside the home, other than cooking. Participants were enrolled between September and April, 2020\u0026ndash;2024. The study sought to enroll participants in cooler months when near highway concentrations of UFP tend to be higher\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eParticipant households were randomized to 30 days of either HEPA or sham filtration, with air purifiers placed in the living room and bedroom, followed by a 30-day washout period, and then a subsequent 30-day period of the alternative intervention. Assessment of cognitive function was a secondary outcome in the trial whose primary aim was reduction in blood pressure\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. The trial received approval from the UConn Health Institutional Review Board Human Subjects Protection Program and was conducted according to the Declaration of Helsinki. All participants provided written informed consent.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eTrail Making Test\u003c/h3\u003e\n\u003cp\u003eParticipants completed the Trail Making Test, which is used to assess cognitive function and has been shown to be effective in quantifying cognitive impairment in healthy adults\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e over short time periods\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Participants completed the test in the early morning at their home at the beginning and end of each intervention period, resulting in the test being completed four times by each participant. Participants were timed for sequentially connecting numbers (Part A) and sequentially connecting alternating numbers and letters (Part B). Part A of the Trail Making Test assesses visual memory and motor speed skills, while Part B assesses executive function and mental flexibility\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. For Part A, a completion time of 29 seconds is considered average and \u0026gt;\u0026thinsp;78 seconds is deficient. For Part B, a completion time of 75 seconds is considered average and \u0026gt;\u0026thinsp;273 seconds is deficient\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eStatistical Analyses\u003c/h3\u003e\n\u003cp\u003eOver-dispersed Poisson regression models were used to compare the Trail Making Test \u0026ndash; Part A and Part B completion times between the HEPA and sham filtration groups at the end of the intervention periods, as time was considered a count variable and the distribution of times were right-skewed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The model included sequence and period effects and was adjusted for the corresponding Trail Making Test completion time at the start of each intervention period. Results were also adjusted for number of hours per day spent indoors at home a week prior to the Trail Making Test, and Perceived Stress Scale (PSS-4) score and outdoor temperature at the time of completing the Trail Making Test. Adjusting the results for the number of hours per day spent indoors at home accounted for some participants spending more time inside the home and thus receiving potentially more benefit from air purification. Perceived Stress Scale (PSS-4) scores were included in the models as perceived stress has been linked to worse cognitive outcomes among healthy younger and older adults\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Similarly, outdoor temperature has been associated with a significant effect on scores for Part A of the Trail Making Test\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe Trail Making Test learning effect was assessed using the first three consecutive Trail Making Test completion times from the 67 participants who were randomly assigned to receive the sham then HEPA sequence. Because these participants received one month of sham filtration first, followed by a one-month washout period, their first three consecutive Trail Making Test results could not be affected by HEPA filtration. Thus, improvements in completion times should primarily due to retaking the test rather than filtration. These 67 participants\u0026rsquo; Trail Making Test \u0026ndash; Part A and Part B completion times were compared across the three timepoints using over-dispersed Poisson regression models.\u003c/p\u003e\u003cp\u003eAs it was previously shown that performance on the Trail Making Test decreases with increasing age\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, as well as reported in a systematic review that a general decline in cognition due to PM exposure begins at age 40\u003csup\u003e18\u003c/sup\u003e, the moderating effect of age was assessed by dichotomizing participant\u0026rsquo;s age as \u0026lt;\u0026thinsp;40 years versus 40 years or older and a cross-product term was included in the models to assess its significance. The cross-product term consisted of the intervention (HEPA vs sham) and the dichotomized age (\u0026lt;\u0026thinsp;40 years versus \u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e\u0026ge;\u003c/span\u003e\u0026thinsp;40 years).\u003c/p\u003e\u003cp\u003eAll statistical analyses were carried out using SAS 9.4 (SAS Institute Inc., Cary, NC), and results with p-values\u0026thinsp;\u0026lt;\u0026thinsp;0.05 were deemed statistically significant.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eOf the 156 participants enrolled in the trial, 37 did not complete the Trail Making Test at all four home visits; 2 moved outside the study area, 1 had a phone survey, 23 had paper forms misplaced, and 11 opted not to complete the Trail Making Test at one or more of the home visits. The remaining 119 participants had complete data for the Trail Making Test at all four home visits and are included in the analyses. There were no appreciable differences in participant characteristics between the 26 who are excluded for administrative reasons and the 119 who are included in the analyses (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). However, the 11 participants who opted not to complete the Trail Making Test were generally older, less educated, and more likely to be unemployed, which may explain their longer Trail Making Test completion times.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eCharacteristics of participants at trial entry.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAdministrative\u003c/p\u003e\n\u003cp\u003e(N\u0026thinsp;=\u0026thinsp;26)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eIncomplete\u003c/p\u003e\n\u003cp\u003e(N\u0026thinsp;=\u0026thinsp;11)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAnalyzed\u003c/p\u003e\n\u003cp\u003e(N\u0026thinsp;=\u0026thinsp;119)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAge in years, mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e41.5 (10.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e48.0 (15.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e40.8 (9.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAge category, n (%)\u003c/p\u003e\n\u003cp\u003e\u0026lt; 40 years\u003c/p\u003e\n\u003cp\u003e40 years or older\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e14 (53.8)\u003c/p\u003e\n\u003cp\u003e12 (46.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2 (18.2)\u003c/p\u003e\n\u003cp\u003e9 (81.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e69 (58.0)\u003c/p\u003e\n\u003cp\u003e50 (42.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSex, n (%)\u003c/p\u003e\n\u003cp\u003eMale\u003c/p\u003e\n\u003cp\u003eFemale\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e10 (38.5)\u003c/p\u003e\n\u003cp\u003e16 (61.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e3 (27.3)\u003c/p\u003e\n\u003cp\u003e8 (72.7)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e50 (42.0)\u003c/p\u003e\n\u003cp\u003e69 (58.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEthnicity and race, n (%)\u003c/p\u003e\n\u003cp\u003eHispanic\u003c/p\u003e\n\u003cp\u003eWhite, non-Hispanic\u003c/p\u003e\n\u003cp\u003eBlack, non-Hispanic\u003c/p\u003e\n\u003cp\u003eAsian, non-Hispanic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e5 (19.2)\u003c/p\u003e\n\u003cp\u003e18 (69.2)\u003c/p\u003e\n\u003cp\u003e1 (3.9)\u003c/p\u003e\n\u003cp\u003e2 (7.7)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e3 (27.3)\u003c/p\u003e\n\u003cp\u003e8 (72.7)\u003c/p\u003e\n\u003cp\u003e0 (5.0)\u003c/p\u003e\n\u003cp\u003e0 (5.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e21 (17.6)\u003c/p\u003e\n\u003cp\u003e81 (68.1)\u003c/p\u003e\n\u003cp\u003e7 (5.9)\u003c/p\u003e\n\u003cp\u003e10 (8.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eHighest level of education, n (%)\u003c/p\u003e\n\u003cp\u003eGrade or high school\u003c/p\u003e\n\u003cp\u003eSome college\u003c/p\u003e\n\u003cp\u003eCollege or university degree\u003c/p\u003e\n\u003cp\u003eGraduate degree\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5 (19.2)\u003c/p\u003e\n\u003cp\u003e1 (3.9)\u003c/p\u003e\n\u003cp\u003e7 (26.9)\u003c/p\u003e\n\u003cp\u003e13 (50.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2 (18.2)\u003c/p\u003e\n\u003cp\u003e2 (18.2)\u003c/p\u003e\n\u003cp\u003e5 (45.4)\u003c/p\u003e\n\u003cp\u003e2 (18.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e11 (9.2)\u003c/p\u003e\n\u003cp\u003e15 (12.6)\u003c/p\u003e\n\u003cp\u003e31 (26.1)\u003c/p\u003e\n\u003cp\u003e62 (52.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWork status, n (%)\u003c/p\u003e\n\u003cp\u003eUnemployed\u003c/p\u003e\n\u003cp\u003ePart-time working\u003c/p\u003e\n\u003cp\u003eFull-time working\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e2 (7.7)\u003c/p\u003e\n\u003cp\u003e6 (23.1)\u003c/p\u003e\n\u003cp\u003e18 (69.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e6 (54.5)\u003c/p\u003e\n\u003cp\u003e1 (9.1)\u003c/p\u003e\n\u003cp\u003e4 (36.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e19 (16.0)\u003c/p\u003e\n\u003cp\u003e12 (10.1)\u003c/p\u003e\n\u003cp\u003e88 (73.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTotal annual household income, n (%)\u003c/p\u003e\n\u003cp\u003e\u0026lt; $48,000\u003c/p\u003e\n\u003cp\u003e$48,000 to $84,999\u003c/p\u003e\n\u003cp\u003e$85,000 or greater\u003c/p\u003e\n\u003cp\u003eDeclined to answer\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e0 (0.0)\u003c/p\u003e\n\u003cp\u003e1 (3.8)\u003c/p\u003e\n\u003cp\u003e19 (73.1)\u003c/p\u003e\n\u003cp\u003e6 (23.1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e1 (9.1)\u003c/p\u003e\n\u003cp\u003e1 (9.1)\u003c/p\u003e\n\u003cp\u003e5 (45.4)\u003c/p\u003e\n\u003cp\u003e4 (36.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e10 (8.4)\u003c/p\u003e\n\u003cp\u003e17 (14.3)\u003c/p\u003e\n\u003cp\u003e74 (62.2)\u003c/p\u003e\n\u003cp\u003e18 (15.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWeight status, n (%)\u003c/p\u003e\n\u003cp\u003eHealthy\u003c/p\u003e\n\u003cp\u003eOverweight\u003c/p\u003e\n\u003cp\u003eObesity\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e9 (34.6)\u003c/p\u003e\n\u003cp\u003e8 (30.8)\u003c/p\u003e\n\u003cp\u003e9 (34.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e5 (45.4)\u003c/p\u003e\n\u003cp\u003e4 (36.4)\u003c/p\u003e\n\u003cp\u003e2 (18.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e52 (43.7)\u003c/p\u003e\n\u003cp\u003e36 (30.2)\u003c/p\u003e\n\u003cp\u003e31 (26.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTime spent inside home (hours per day), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e18.0 (3.3)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e17.3 (3.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e19.1 (3.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePerceived Stress Scale (PSS-4), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4.6 (3.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.1 (2.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.2 (2.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eOutdoor temperature in Fahrenheit, mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e45.1 (8.3)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e47.4 (12.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e39.8 (12.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTrail Making Test \u0026ndash; Part A (seconds), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-----\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e49.3 (49.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e24.3 (12.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTrail Making Test \u0026ndash; Part B (seconds), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-----\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e118.6 (94.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e70.7 (42.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eCharacteristics of younger (\u0026lt;\u0026thinsp;40 years) and older (\u003cspan class=\"Underline\"\u003e\u0026ge;\u003c/span\u003e\u0026thinsp;40 years) participants are shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The difference in mean age was almost 15 years (34.5 versus 49.4 years). Older participants were generally less educated, were less likely to work full-time, more likely to be female, and to have obesity. Although the mean Trail Making Test completion time for older participants was 4 seconds longer for Part A and 14 seconds longer for Part B, the mean times at the start of the intervention period were within the range of adults with normal cognitive function\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eCharacteristics of analyzed participants at the start of the intervention period by age category.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u0026lt;\u0026thinsp;40 years\u003c/p\u003e\n\u003cp\u003e(N\u0026thinsp;=\u0026thinsp;138)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e40 years or older\u003c/p\u003e\n\u003cp\u003e(N\u0026thinsp;=\u0026thinsp;100)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eRandomized to sequence, n (%)\u003c/p\u003e\n\u003cp\u003eHEPA then sham filtration\u003c/p\u003e\n\u003cp\u003eSham then HEPA filtration\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e62 (44.9)\u003c/p\u003e\n\u003cp\u003e76 (55.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e42 (42.0)\u003c/p\u003e\n\u003cp\u003e58 (58.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAge in years, mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e34.5 (2.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e49.4 (8.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eSex, n (%)\u003c/p\u003e\n\u003cp\u003eMale\u003c/p\u003e\n\u003cp\u003eFemale\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e66 (47.8)\u003c/p\u003e\n\u003cp\u003e72 (52.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e34 (34.0)\u003c/p\u003e\n\u003cp\u003e66 (66.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEthnicity and race, n (%)\u003c/p\u003e\n\u003cp\u003eHispanic\u003c/p\u003e\n\u003cp\u003eWhite, non-Hispanic\u003c/p\u003e\n\u003cp\u003eBlack, non-Hispanic\u003c/p\u003e\n\u003cp\u003eAsian, non-Hispanic\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e28 (20.3)\u003c/p\u003e\n\u003cp\u003e84 (60.9)\u003c/p\u003e\n\u003cp\u003e10 (7.2)\u003c/p\u003e\n\u003cp\u003e16 (11.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e14 (14.0)\u003c/p\u003e\n\u003cp\u003e78 (78.0)\u003c/p\u003e\n\u003cp\u003e4 (4.0)\u003c/p\u003e\n\u003cp\u003e4 (4.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eHighest level of education, n (%)\u003c/p\u003e\n\u003cp\u003eGrade or high school\u003c/p\u003e\n\u003cp\u003eSome college\u003c/p\u003e\n\u003cp\u003eCollege or university degree\u003c/p\u003e\n\u003cp\u003eGraduate degree\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e10 (7.2)\u003c/p\u003e\n\u003cp\u003e6 (4.4)\u003c/p\u003e\n\u003cp\u003e46 (33.3)\u003c/p\u003e\n\u003cp\u003e76 (55.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12 (12.0)\u003c/p\u003e\n\u003cp\u003e24 (25.0)\u003c/p\u003e\n\u003cp\u003e16 (16.0)\u003c/p\u003e\n\u003cp\u003e48 (48.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWork status, n (%)\u003c/p\u003e\n\u003cp\u003eUnemployed\u003c/p\u003e\n\u003cp\u003ePart-time working\u003c/p\u003e\n\u003cp\u003eFull-time working\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e14 (10.1)\u003c/p\u003e\n\u003cp\u003e8 (5.8)\u003c/p\u003e\n\u003cp\u003e116 (84.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e24 (24.0)\u003c/p\u003e\n\u003cp\u003e16 (15.0)\u003c/p\u003e\n\u003cp\u003e60 (60.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTotal annual household income, n (%)\u003c/p\u003e\n\u003cp\u003e\u0026lt; $48,000\u003c/p\u003e\n\u003cp\u003e$48,000 to $84,999\u003c/p\u003e\n\u003cp\u003e$85,000 or greater\u003c/p\u003e\n\u003cp\u003eDeclined to answer\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e8 (5.8)\u003c/p\u003e\n\u003cp\u003e12 (8.7)\u003c/p\u003e\n\u003cp\u003e98 (71.0)\u003c/p\u003e\n\u003cp\u003e20 (14.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e12 (12.0)\u003c/p\u003e\n\u003cp\u003e22 (22.0)\u003c/p\u003e\n\u003cp\u003e50 (50.0)\u003c/p\u003e\n\u003cp\u003e16 (16.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWeight status, n (%)\u003c/p\u003e\n\u003cp\u003eHealthy\u003c/p\u003e\n\u003cp\u003eOverweight\u003c/p\u003e\n\u003cp\u003eObesity\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e74 (53.6)\u003c/p\u003e\n\u003cp\u003e38 (27.6)\u003c/p\u003e\n\u003cp\u003e26 (18.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e30 (30.0)\u003c/p\u003e\n\u003cp\u003e34 (34.0)\u003c/p\u003e\n\u003cp\u003e36 (36.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTime spent inside home (hours per day), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e18.9 (3.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e18.7 (3.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePerceived Stress Scale (PSS-4), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3.2 (2.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2.9 (2.7)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eOutdoor temperature in Fahrenheit, mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e43.6 (13.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e44.3 (13.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTrail Making Test \u0026ndash; Part A (seconds), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e21.2 (9.1)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e25.2 (16.2)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eTrail Making Test \u0026ndash; Part B (seconds), mean (sd)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e58.3 (28.9)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e72.7 (50.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eMean completion times at the end of the intervention period for Parts A and B of the Trail Making Test between HEPA and sham filtration are shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. No significant differences between HEPA and sham filtration were observed for Trail Making Test \u0026ndash; Part A, and the mean completion times of approximately 20 seconds were similar between the two filtration groups and between younger and older participants (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab3\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eTrail Making Test \u0026ndash; Part A and Part B completion times at the end of the intervention period\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ePart A (seconds)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eHEPA\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSham\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eRatio of\u003c/p\u003e\n\u003cp\u003eMeans\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eP-value\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eN\u003csub\u003eHEPA\u003c/sub\u003e, N\u003csub\u003eSham\u003c/sub\u003e\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003emean (se)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003emean (se)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e(95% CI)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eOverall\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e119, 119\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e20.3 (0.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e19.7 (0.5)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.03\u003c/p\u003e\n\u003cp\u003e(0.96, 1.10)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.38\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026lt;\u0026thinsp;40 years of age\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e69, 69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e19.5 (0.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e18.7 (0.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.04\u003c/p\u003e\n\u003cp\u003e(0.95, 1.15)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.34\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e40 years or older\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e50, 50\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e21.4 (0.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e21.2 (0.8)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.01\u003c/p\u003e\n\u003cp\u003e(0.92, 1.11)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.82\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003ePart B (seconds)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eHEPA\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eSham\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eRatio of\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeans\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eP-value\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eN\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eHEPA\u003c/strong\u003e\u003c/sub\u003e, \u003cstrong\u003eN\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eSham\u003c/strong\u003e\u003c/sub\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003emean (se)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003emean (se)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e(95% CI)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eOverall\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e119, 119\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e53.6 (1.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e55.8 (1.6)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.96\u003c/p\u003e\n\u003cp\u003e(0.89, 1.04)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.33\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026lt;\u0026thinsp;40 years of age\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e69, 69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e53.7 (2.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e51.4 (2.0)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1.04\u003c/p\u003e\n\u003cp\u003e(0.94, 1.16)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.43\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e40 years or older\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e50, 50\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e54.0 (2.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e61.4 (2.4)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.88\u003c/p\u003e\n\u003cp\u003e(0.79, 0.98)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e0.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003ctfoot\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"6\"\u003eResults were adjusted for the corresponding Trail Making Test completion time at the start of each intervention period. Results were also adjusted for number of hours per day spent indoors at home a week prior to the Trail Making Test, and Perceived Stress Scale (PSS-4) score and outdoor temperature at the time of completing the Trail Making Test.\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tfoot\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eFor Trail Making Test \u0026ndash; Part B, the overall reduction in mean completion times between HEPA and sham filtration did not reach statistical significance but was significantly moderated by age (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.02, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). Participants 40 years or older completed Part B 12% significantly faster after a month of HEPA filtration, on average, than participants who had experienced sham filtration (54.0 versus 61.4 seconds, ratio of means\u0026thinsp;=\u0026thinsp;0.88, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.02). No significant reduction in completion time was observed for younger participants.\u003c/p\u003e\n\u003cp\u003eAs expected, the Trail Making Test had a learning effect among the 67 participants who were randomly assigned to receive the sham then HEPA sequence. For both Part A and for Part B, these participants' completion times significantly improved with each subsequent visit; mean completion times for Part A were 24.4, 22.8, and 20.9 seconds, respectively and mean completion times for Part B were 70.0, 62.4, and 57.8 seconds, respectively.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn this secondary outcome analysis of a pragmatic randomized crossover trial, with no restrictions on participants\u0026rsquo; daily activities, we observed a statistically significant improvement in cognitive performance on Part B of the Trail Making Test for participants 40 years of age or older. This test assesses executive function and mental flexibility, and the effect was observed after a month of HEPA filtration compared to sham filtration. Participants 40 years or older completed Part B 12% faster after a month of HEPA compared to a month of sham filtration.\u003c/p\u003e\u003cp\u003eOther crossover trials of air filtration that examined the effect of air pollution on cognition have also reported that higher exposure to particulate matter was associated with worse executive function and attention switching.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e These studies used different cognitive tests for these domains, namely the CANTAB test and Strategic Management Simulation. Studies have found no difference in visual memory or motor speed\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e when using the psychomotor vigilance task and the simple reaction time test, the domains of cognition that was assessed in Trail Making Test \u0026ndash; Part A.\u003c/p\u003e\u003cp\u003eThere is neurobiological evidence that particulate matter exposure may reduce white matter in the brain\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, specifically in the frontal and temporal lobes\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. These lobes are responsible for the mental flexibility and executive function skills assessed by Part B of the Trail Making Test\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Thus, our finding of benefit of HEPA filtration on Part B is consistent with underlying neurobiological processes. Additionally, our result of benefit observed among participants 40 years or older is consistent with reports that the cognitive effects of air pollution are more pronounced beginning around age 40\u003csup\u003e18\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eOur results are also largely consistent with other intervention studies that observed the effects of reducing particulate matter exposure on cognitive outcomes among healthy adults. A randomized crossover trial among healthy office workers of a similar age range to ours reported that HEPA filtration improved executive functioning, with no differences between HEPA and sham filtration for visual or working memory\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Another randomized crossover trial comparing HEPA and sham filtration was performed among college-aged students. Students were provided air purifiers to use while they slept and were then given a cognitive test in the morning. Sham filtration was associated with slower memory, worse executive function, and lower global cognition\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Although the latter study was with younger people than ours, both crossover trials provide evidence of a possible benefit of HEPA filtration on cognition among healthy adults.\u003c/p\u003e\u003cp\u003eOur analysis has several strengths and limitations. A strength of our analysis is that the data arose from a crossover trial design in which participants were compared to themselves, eliminating time-invariant confounding. Our analysis also controlled time-varying confounders, such as time spent inside the home, ambient temperature and level of stress, which can affect results.\u003c/p\u003e\u003cp\u003eOne limitation to the use of the Trail Making Test to assess cognitive function is that it has a learning effect, whereby participants\u0026rsquo; completion times improve simply from retaking the test multiple times\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, as in our crossover trial, Effects of this sort have been shown to be more common in younger adults\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e and more common in short-term periods similar to the time-frame of our intervention periods\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. Although the Trail Making Test has a learning effect, we expect that the improvement in completion times would be similar between the participants randomly assigned to the sham then HEPA sequence and those randomly assigned to the HEPA then sham sequence.\u003c/p\u003e\u003cp\u003eAnother limitation was the use of sham filtration which could have reduced UFP exposure\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e through diffusional deposition, resulting in concentrations of UFP being reduced by air movement alone\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Reducing participants\u0026rsquo; UFP exposure during the sham filtration period could have attenuated the effect between the HEPA and sham intervention periods. Our results are also limited in terms of generalizability. As none of the participants had preexisting cognitive impairments, our analysis is not relevant to the effectiveness of HEPA filtration among cognitively impaired individuals.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOur results suggest that, among older healthy adults living near highways in the Northeastern US, there was an improvement in cognition following one month of in-home HEPA filtration. Despite the strength of our trial design, there is a need for further research that employs methods with strong causal inference of short-term exposure to air particulates and cognitive outcomes, especially among individuals who have cognitive impairment.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eConflicts of interest:\u003c/h2\u003e\u003cp\u003eNone of the authors have relationships with industry. The air purifiers were custom adapted by the manufacturer, Austin Air, and sold at a discount based on bulk purchase. The manufacturer was not involved in any way in the design, conduct or analysis.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e\u003cp\u003eNIEHS. Grant ID: R01 ES030289 and the School of Medicine at the University of Connecticut\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eNP, ME, and DB were responsible for drafting the manuscript. HG scored and prepared the Trail Making Tests for analysis. ME was responsible for the analysis of the data. RF contributed to the interpretation of the results.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe would like to thank our project manager, Teresa Vazquez-Dodero for administering the trail making tests and the trial participants for their time and being receptive to in-home visits.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data used in this study is available upon request. Please contact the corresponding author Dr. Doug Brugge at [email protected].\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBoogaard, H. et al. Long-term exposure to traffic-related air pollution and selected health outcomes: A systematic review and meta-analysis. \u003cem\u003eEnviron. Int.\u003c/em\u003e \u003cb\u003e164\u003c/b\u003e, 107262. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.envint.2022.107262\u003c/span\u003e\u003cspan address=\"10.1016/j.envint.2022.107262\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCosta, L. G. et al. Effects of air pollution on the nervous system and its possible role in neurodevelopmental and neurodegenerative disorders. \u003cem\u003ePharmacol. 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Phys.\u003c/em\u003e \u003cb\u003e4\u003c/b\u003e (11), 723\u0026ndash;734. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/s42254-022-00506-7\u003c/span\u003e\u003cspan address=\"10.1038/s42254-022-00506-7\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"air pollution, particulate matter, cognitive outcome, air filtration","lastPublishedDoi":"10.21203/rs.3.rs-8108219/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8108219/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eExposure to particulate matter is associated with adverse health outcomes such as cardiovascular and neurological diseases. However, there are few intervention studies that have examined short-term changes in air particulate exposure and cognitive outcomes. We analyzed data from 119 participants in the Home Air Filtration for Traffic-Related Air Pollution (HAFTRAP) study, a pragmatic randomized crossover trial. Participants were randomized to receive a high efficiency particulate arrestance (HEPA) air purifier for one month, a washout period of one month, then a sham unit for one month or vice versa. Time to complete Trail Making Test \u0026ndash; Parts A and B were recorded at the beginning and end of each intervention period. Although there was no significant overall difference in completion times of the Trail Making Tests between HEPA and sham filtration, age was a significant moderator (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.02). Participants 40 years or older with HEPA filtration completed Part B 12% significantly faster than participants who had sham filtration in the preceding month (54.0 versus 61.4 seconds, ratio of means\u0026thinsp;=\u0026thinsp;0.88, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.02). No significant reduction in completion time was observed for participants\u0026thinsp;\u0026lt;\u0026thinsp;40 years old. Among older healthy adults, there was an improvement in cognition following one month of in-home HEPA filtration. Further research is needed on the short-term effects of air pollution among individuals with some level of cognitive impairment.\u003c/p\u003e","manuscriptTitle":"Effect of HEPA Filtration Air Purifiers on Cognitive Function: Secondary Outcome of a Pragmatic Randomized Crossover Trial","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-08 10:41:12","doi":"10.21203/rs.3.rs-8108219/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-06T08:26:50+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-05T09:20:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-03T23:56:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"250157195525811221122207846906072624880","date":"2025-12-15T22:04:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"67214762370501128525392994618174999833","date":"2025-12-04T14:42:05+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-03T16:17:30+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-03T16:15:36+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-19T17:18:01+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-18T16:58:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-18T16:55:56+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"655fbbaa-c150-47cd-9610-306eed7b543e","owner":[],"postedDate":"December 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":59104824,"name":"Health sciences/Diseases"},{"id":59104825,"name":"Earth and environmental sciences/Environmental sciences"},{"id":59104826,"name":"Health sciences/Health care"},{"id":59104827,"name":"Health sciences/Medical research"},{"id":59104828,"name":"Biological sciences/Neuroscience"},{"id":59104829,"name":"Health sciences/Risk factors"}],"tags":[],"updatedAt":"2026-04-20T16:02:42+00:00","versionOfRecord":{"articleIdentity":"rs-8108219","link":"https://doi.org/10.1038/s41598-026-48063-8","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-04-16 15:58:38","publishedOnDateReadable":"April 16th, 2026"},"versionCreatedAt":"2025-12-08 10:41:12","video":"","vorDoi":"10.1038/s41598-026-48063-8","vorDoiUrl":"https://doi.org/10.1038/s41598-026-48063-8","workflowStages":[]},"version":"v1","identity":"rs-8108219","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8108219","identity":"rs-8108219","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}