Residential indoor air monitoring to assess long-term exposure to air pollution in susceptible populations for epidemiology: overview and early findings.

While there has been accumulating evidence of the association between long-term exposure to air pollution and various health endpoints, most epidemiological studies relied on ambient air pollution which showed substantial deviation from indoor. This difference could lead to biased or imprecise health effect estimates, particularly for susceptible populations who spend most time indoors. Although a few studies focused on indoor air pollution, most monitoring was limited to one-time and/or relatively short-term periods. We aimed to provide an overview of an indoor monitoring campaign that was designed to assess long-term exposure to residential indoor PM2.5 for epidemiology and applied to 130 older adults with idiopathic pulmonary fibrosis in the Seoul Metropolitan Area, South Korea, for 2021-2023. Specifically, our design focused on (1) representation of long-term exposure, (2) identification of pollution sources, (3) assessment of infiltration, and (4) implementation of non-contact monitoring. Using RTI MicroPEMs, we measured concentrations of indoor PM2.5 and chemical components for five days including weekdays and weekends in four seasons as well as household information. We also collected matched outdoor samples to assess infiltration, and applied non-contact monitoring using postal mails for repeat visits. Our preliminary analyses of indoor hourly PM2.5 measurements (mean: 14.90 µg/m3 [standard deviation: 36.12]) showed temporal patterns with higher concentrations in winter (20.17 [50.54]) than in summer (11.19 [32.76]) and during the daytime than nighttime. Indoor concentrations were lower than outdoor (20.44 [SD: 61.63]) with consistent seasonal patterns but contrasted hourly patterns. Thirty-six chemical components of PM2.5 showed mostly similar temporal patterns to those of PM2.5 with lower concentrations and smaller differences between indoors and outdoors in summer that in other seasons. The infiltration factor characterized by median indoor to outdoor ratios of sulfur, nickel, and iron were 51-70%. Average PM2.5 concentrations tended to be high at households with cooking activities (15.14 vs. 10.96 µg/m3 at those without), humidifiers (19.19 vs. 13.16), pets (24.09 vs. 13.35), and smokers (16.76 vs. 11.79). Our indoor monitoring approach provides a practical guidance to future indoor monitoring studies that intend to assess the health effect of PM2.5.