The Impacts of Wildfires on Indoor PM 2.5 in Western Canada

While current strategies for reducing wildfire PM 2.5 exposure focus on remaining indoors, the detailed impacts of wildfires on indoor air quality remain unclear. Here, low-cost PM 2.5 sensors were deployed in 21 residences across 4 locations commonly impacted by wildfires in western Canada. Surveys of the homes and the occupants’ activities were conducted, and wildfire influence was assessed using atmospheric back trajectories. Indoor and outdoor PM 2.5 concentrations were monitored for a one-year period, which included Canada’s most severe wildfire season on record in 2023. During the wildfire season, daily average indoor PM 2.5 levels were higher during wildfire episodes (7.1 μg m –3 ) than in the absence of fires (3.5 μg m –3 ), whereas the indoor-to-outdoor PM 2.5 ratios (0.42 vs 0.48) and infiltration factors were lower (0.25 vs 0.28 on fire and nonfire days, respectively). Increases in indoor PM 2.5 due to wildfires were comparable to the spread in PM 2.5 values between clean homes and those with many indoor particle generating activities, such as cooking. Overall, Canadian homes protected against PM 2.5 exposure during wildfire episodes on average, but the variability of indoor PM 2.5 exposure in the absence of wildfires across different homes was comparable in magnitude to the increases in PM 2.5 due to wildfires.