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) PM2.5 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 PM2.5 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 PM2.5; therefore, indoor PM2.5 measurements are essential for
identifying when additional mitigation measures are needed.