HEPA air filters linked to faster test performance in adults 40 and older

Researchers studied whether reducing indoor particle pollution might affect performance on brief thinking tests. The work used data from 119 participants in the Home Air Filtration for Traffic-Related Air Pollution study, a pragmatic randomized crossover trial.

In a crossover trial, each person receives both study conditions at different times. This can help researchers compare how the same participants perform under each condition. Participants were randomly assigned to use either a HEPA air purifier for one month followed by a one-month break and then a sham purifier for one month, or to follow the same sequence in reverse.

A HEPA purifier is designed to filter tiny particles from the air. The sham unit looked like the real purifier but did not provide HEPA filtration. The comparison was meant to help separate the effect of filtration from other factors, such as expectations about using an air cleaner.

At the beginning and end of each intervention period, participants completed the Trail Making Test. Part A measures how quickly someone can connect numbers in sequence. Part B asks people to alternate between numbers and letters in sequence. Part B is often used to assess executive function, a group of skills that includes mental flexibility, attention, and shifting between tasks.

When the researchers looked at all participants together, they found no statistically significant overall difference in completion times between the HEPA and sham periods. Age, however, changed the pattern of results.

Among participants aged 40 and older, people completed Part B significantly faster after HEPA filtration than after sham filtration in the preceding month. The reported average times were 54.0 seconds with HEPA filtration and 61.4 seconds after sham filtration. That difference was about 12%, with a ratio of means of 0.88. The result met the study’s threshold for statistical significance, with a reported p value of 0.02.

The study did not find a significant reduction in completion time among participants younger than 40. The abstract also does not report evidence that the purifier improved every part of the cognitive testing. The clearest reported finding was for Part B among people 40 and older.

This was a completed randomized trial, which is stronger evidence than an observational study that simply compares people who choose to use air filters with those who do not. Still, the finding is limited to a relatively short, one-month intervention and a specific test result in a subgroup. It cannot show whether using a purifier slows cognitive decline over years or protects against conditions such as Alzheimer’s or Parkinson’s disease.

The participants lived in an area affected by traffic-related air pollution, so the results may not apply in the same way to every home or community. The supplied study information also does not show whether the improvement continued after the filter was removed, or whether the effect would be seen in people with memory problems or other cognitive impairment.

The researchers say further work is needed, especially on short-term effects of air pollution in people who already have some level of cognitive impairment. Future studies would need to test whether the finding is repeated in larger and more varied groups, whether it affects other measures of thinking, and how long any benefit lasts.