Abstract

Air pollution is a leading global cause of morbidity and mortality, and indoor environments play an important role in personal exposure because people spend more of their time indoors. Outdoor monitoring data is frequently used to assess health impacts -using metrics such as the Disability-Adjusted Life Year (DALY)- but it may not closely represent indoor exposures influenced by building characteristics, ventilation practices, and occupant activities. In fact, personal exposures may align more closely with indoor exposures, indicating that current health impact analyses should account for indoor air quality. This study takes a top-down approach using indoor-outdoor (I/O) ratios to examine exposure variability and influencing factors. Indoor concentrations were measured every ten minutes in eight dwellings in Santiago (Chile) in 2023 and 2024, and were compared to hourly outdoor data from nearby government monitoring stations. The analysis shows spatial and temporal variations in I/O ratios, which were then grouped by shared factors such as building characteristics, dwelling profiles, and occupant activities. Ratios often exceeded unity during the heating season, mainly due to combustion-based heating systems, including gas and kerosene heaters. These findings emphasize the important role of structural and behavioral factors in shaping indoor air quality. This research highlights the limitations of using outdoor air quality data for assessing health impacts because it does not capture the variable and prolonged exposures experienced indoors. The findings enhance our understanding of particle dynamics in urban housing and reinforce the need to consider indoor air pollution in public health policy. © 2025 ASHRAE.