A Comparative Analysis on Indoor and Outdoor PM_2.5 and Their Hourly Associations with Acute Respiratory Inflammation...

Ambient concentrations are commonly used as proxies for personal PM_2.5 exposure in epidemiological studies, despite indoor settings being the places where people spend most of their time. In a panel study of 110 nonsmoking, healthy college students in Lhasa, Tibet, indoor PM_2.5 was monitored using calibrated low-cost sensors for two multiweek periods, in over 40 dormitories where participants resided. We also repeatedly measured fractional exhaled nitric oxide (FeNO), an acute respiratory inflammation biomarker, for each participant. Time-averaged indoor PM_2.5 concentrations in individual dormitories ranged from 3.2 to 30 μg/m³ in the summer and from 3.6 to 57 μg/m³ in the fall, in most cases exceeding the outdoor level (4.3 and 4.9 μg/m³, respectively). The hourly mean indoor PM_2.5 concentrations displayed a clear trimodal diel pattern, with peaks coincident with periods of increased activities. Further questionnaire-based analysis suggests that incense burning and smoking contributed to elevated levels of indoor PM_2.5. Overnight PM_2.5 levels in the dormitories were significantly associated with increased FeNO the following morning, with the effects attenuated as the hourly lag increased. In contrast, inconclusive associations were observed for ambient PM_2.5. The results demonstrate that disregarding indoor exposure can result in biased estimates of acute health effects of PM_2.5 in low PM_2.5 areas.