Influence of Building Characteristics on Wildfire Smoke Impacts on Indoor Air Quality

Indoor air quality was monitored in a variety of public buildings during the wildfire seasons of 2019 and 2020 in Missoula, MT, to better understand what factors impact smoke infiltration indoors. PurpleAir sensors were used indoors and outdoors to calculate indoor/outdoor (I/O) PM_2.5 ratios that were compared with building characteristics. Wildfire smoke concentrations measured indoors during a 7 day smoke event were always lower than outdoors, but some buildings had up to 4 days of PM_2.5 above 55 μg/m³, corresponding to an unhealthy air quality index. Locations with heating, ventilation, and air conditioning (HVAC) systems in excellent condition and with tightly fitting filters had lower I/O ratios than locations with HVACs in poor condition or locations without an HVAC system. On average, I/O ratios were 15% higher during building open hours compared to closed hours, which may have been due to increased HVAC operation and more frequent door opening during open hours. The I/O ratios ranged from 0.29 to 0.97, varying across locations and during different conditions (presence of smoke or cold weather). No single building factor was identified as being most important in reducing indoor PM_2.5; therefore, indoor PM_2.5 measurements are essential for identifying when additional mitigation measures are needed.