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Abstract
We collected paired measurements of indoor and outdoor PM2.5 concentrations at 17 homes in Dhaka, Bangladesh, to quantify indoor-outdoor levels, their spatio-temporal variations, and influencing factors. A pair of PurpleAir PM2.5 sensors were deployed at each home, one indoors and the other outdoors, during the wet (June to August 2021) and dry (December 2021 to February 2022) seasons, and the locally calibrated (against a beta attenuation monitor) and quality-assured data were used for analysis. Indoor and outdoor PM2.5 levels were three times higher during the dry season (indoor 146 ± 22 µg/m³, outdoor 153 ± 23 µg/m³) than during the wet season (indoor 52 ± 12 µg/m³, outdoor 50 ± 11 µg/m³). Indoor to outdoor (I/O) ratios were close to 1 in both seasons (dry: 0.97 ± 0.14, wet: 1.05 ± 0.19). This suggests that regional background pollution levels significantly influence indoor levels observed in different households. Higher infiltration factors (dry: 0.83 ± 0.12; wet: 0.87 ± 0.14), determined through mixed effect regression of parallel indoor and outdoor timeseries 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 ± 0.12 during the dry season and 0.83 ± 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 PM2.5 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 PM2.5 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 PM2.5 exposures in Bangladesh.
Supplementary materials
Title
Supplementary Information (SI) of Characterizing Indoor-Outdoor PM2.5 Concentrations Using Low-Cost Sensor Measurements in Residential Homes in Dhaka, Bangladesh
Description
SI contains: Local co-location of low-cost sensors with reference monitor; Comparison of short-term and long-term PM2.5 concentrations from two outdoor Continuous Air Monitoring Stations in Dhaka; Example illustrating the process of determining the temporal correction factor for short-term measurements as a function of the hour of the day; Performance evaluation of modeled indoor PM2.5; Summary of surrounding outdoor land-use features and indoor features in measured homes; Meteorological conditions during measurements at various homes in dry and wet seasons; Performance of various regression models developed using co-located PurpleAir (PA) and BAM datasets; Pearson correlation coefficient (R) between indoor/outdoor PM2.5 concentrations and outdoor land-use features; Multiple linear regression model for predicting variabilities in indoor PM2.5 levels across sampled homes from dry and wet seasons.
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