Characterizing indoor-outdoor PM 2.5 concentrations using low-cost sensor measurements in residential homes in Dhaka, Bangladesh

We collected simultaneous indoor and outdoor PM 2.5 measurements from 17 homes in Dhaka, Bangladesh, to characterize spatio-temporal variations and identify factors influencing indoor and outdoor PM 2.5 levels. A pair of PurpleAir PM 2.5 sensors were deployed at each home, one indoors and the other outdoors, during the wet and dry seasons, and the locally calibrated data were used for analysis. Indoor and outdoor PM 2.5 levels were three times higher during the dry season (indoor 146 f 22 mu g/m3, outdoor 153 f 23 mu g/m3) than during the wet season (indoor 52 f 12 mu g/m3, outdoor 50 f 11 mu g/m3). Indoor to outdoor (I/O) ratios were close to 1 in both seasons (dry: 0.97 f 0.14, wet: 1.05 f 0.19). This suggests that regional background pollution levels significantly influence indoor levels observed in different households. Infiltration factors closer to 1 (dry: 0.83 f 0.12; wet: 0.87 f 0.14), determined through mixed-effects regression of indoor and outdoor time series data, further highlight the substantial impact of outdoor pollution on indoor levels. Data from individual households exhibited strong temporal correlation between indoor and outdoor levels in both seasons (Pearson R: 0.82 f 0.12 during the dry season and 0.83 f 0.14 during the wet season), whereas indoor-outdoor spatial correlations across measured households were moderate (R: 0.49 and 0.62 during dry and wet seasons, respectively). These spatial correlations and empirical regression modeling suggest that while the spatial variation of outdoor PM 2.5 levels significantly influences indoor levels' spatial variation, other factors such as indoor source activities and ventilation-related features play crucial roles in explaining variabilities in indoor PM 2.5 across homes. Overall, our study suggests that indoor environments in Dhaka city are nearly as polluted as outdoor settings, and this locally derived scientific evidence can be valuable for enhancing public awareness and developing mitigation measures to reduce PM 2.5 exposures in Bangladesh.