Abstract
As most persons spend most of their time indoors (>80% on most days), indoor air quality is potentially an important health concern, particularly in elderly populations at high risk for dementia. It is important to note that until recently, little attention has been given to indoor air quality and related standards; most regulatory thresholds remain limited to outdoor environments. Recognizing that the environments in which we live and work shape health, it is time to place indoor air quality at the center of brain health policy and research agendas, and to take intentional steps towards comprehensive in-home assessment.
Most persons in the United States 1 and Canada2,3 spend most of their time indoors (>80% on most days), reinforcing why indoor air quality is important for human health, and air pollution has been shown to be associated with higher dementia risk. 4 Although measuring the entire exposome 5 remains both highly burdensome and technologically challenging, Au-Yeung et al. 6 have advanced the field by leveraging a multi–data stream approach to characterize indoor and outdoor air quality exposures, illustrating a path toward more complete exposome assessment. Some observed indoor concentrations were higher than regulatory limits, even though regulatory standards are based solely on outdoor levels. The results presented underscore the importance of indoor exposures, particularly for older adults and those with dementia or mild cognitive impairment who spend more time indoors than younger adults. Indoor air quality thus represents a critical but underappreciated risk factor for Alzheimer's disease and cognitive aging, one that remains largely neglected in inquiries into air quality and dementia. The study's extensive indoor air quality dataset, with over 200 days of continuous bedroom monitoring for most participants, is a major strength and creates opportunities for innovative analyses including integration with other data (e.g., sleep measures).
We would like to highlight some important points that can help to inform future research.
Footnotes
Contribution to total exposome: from indoors
The detection of short-term spikes, 6 possibly from cooking, cleaning, or other lifestyle behaviors, highlights an underexplored pathway through which acute exposures may affect respiratory, neurological, and other health outcomes. 7 A single indoor monitor in the home provides valuable information, but it cannot fully capture the variability of air quality across different rooms or individual activities. For example, a monitor placed in the bedroom may not reflect exposures encountered in other areas of the home, such as the kitchen or bathroom. In other words, indoor air quality measured in one location does not necessarily represent the full range of exposures within the household. Heterogeneity in sleep routines and occupancy further complicates interpretation. While some data presented suggests a degree of indoor and outdoor correlation, this may reflect regression to the mean rather than predictive validity. That is, unusually high or low values in the short term tend to drift back toward the average over time, which can make it look like there's a meaningful link when it's maybe a statistical artifact. Importantly, recent work in this area is aiming to provide important indoor air standards 5 to parallel what exists for the outdoors (e.g., US Clean Air Act8,9).
Contribution to total exposome: from outdoors
A central finding of Au-Yeung et al. 6 is the modest correspondence between indoor measures and outdoor air quality estimates, whether from regional monitoring stations or the Hazardous Air Quality Ensemble System (HAQES). The considerable distances between homes and monitors, ranging from 7 to 53 km for EPA monitoring stations and 2 to 13 km for HAQES data sites, might be a reason for these weak associations. However, the association with distance to the stations/sites was poor, suggesting that other possible explanations should be investigated. That the HAQES data did not show a significantly stronger association with indoor air quality than did the regional air quality monitoring stations is disappointing. Even the best modeling will be limited by primary data and there are broad infrastructure gaps in U.S. air monitoring. 10 These limitations, combined with the likely generalizable finding of heterogeneous indoor air quality, show that outdoor-based estimates are not sufficient for capturing true personal exposure.
Exploratory future analyses of these data
We would like to suggest some additional analyses of the data presented in Au-Yeung et al. 6 that could provide more information regarding risk stratification and potential Hawthorne effects of being observed. Given the almost vertical bands of exposure seen in the HAQES data, 6 it would be interesting to know how many homes fell into each risk category. Exploratory analyses of current data could compare homes situated in high versus low HAQES exposure regions. Furthermore, using “days since start” as the x-axis in the time series could reveal whether awareness of monitoring influenced behavior.
Directions for the future
Several avenues for future work are apparent.
Expanding to larger cohorts, throughout the world, will also allow examination of the health effects of acute exposures and contextual influences such as wildfire events. Personal wearable sensors and/or fixed indoor sensors currently available commercially represent a promising complementary approach. These approaches would allow characterization of important indoor environments at temporal resolutions and are now being used in some studies.
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Quantifying exceedances of established regulatory pollution thresholds, rather than relying solely on mean levels, may better capture the health implications of short-term spikes for vulnerable populations. However, this raises questions about the appropriateness of applying outdoor standards to indoor environments. Although research on indoor air standards remains underdeveloped, emerging contributions
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highlight growing recognition of this need.
Recognizing that the environments in which we live and work shape health, it is time to place indoor air quality at the center of brain health policy and research agendas.
Acknowledgements
The authors have no acknowledgments to report.
Author contribution(s)
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of conflicting interests
The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Dr Roque declares funding on grants to his institution from the National Institute on Aging.
Dr Hosgood declares funding on grants to his institution and other institutions from the National Institute on Aging, the National Cancer Institute, and the National Center for Advancing Translational Sciences.
Dr Hall declares funding on grants and contracts to his institution and other institutions from the National Institute on Aging, the National Institute for Occupational Safety and Health, and the National Cancer Institute. Dr Hall is Chair of the Statistics and Data Science in Aging Interest Group of the American Statistical Association.