The Impacts of Wildfires on Indoor PM 2.5 in Western Canada

While current strategies for reducing wildfire PM2.5 exposure focus on remaining indoors, the detailed impacts of wildfires on indoor air quality remain unclear. Here, low-cost PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 due to wildfires were comparable to the spread in PM2.5 values between clean homes and those with many indoor particle generating activities, such as cooking. Overall, Canadian homes protected against PM2.5 exposure during wildfire episodes on average, but the variability of indoor PM2.5 exposure in the absence of wildfires across different homes was comparable in magnitude to the increases in PM2.5 due to wildfires.