A Comparative Analysis on Indoor and Outdoor PM 2.5 and Their Hourly Associations with Acute Respiratory Inflammation Among College Students in Lhasa

Ambient concentrations are commonly used as proxies for personal PM 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 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 concentrations in individual dormitories ranged from 3.2 to 30 μg/m in the summer and from 3.6 to 57 μg/m in the fall, in most cases exceeding the outdoor level (4.3 and 4.9 μg/m , respectively). The hourly mean indoor PM 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 . Overnight PM 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 . The results demonstrate that disregarding indoor exposure can result in biased estimates of acute health effects of PM in low PM areas.