Fine particulate matter infiltration at Western Montana residences during wildfire season.

Background/aims: Wildfire air pollution is a growing public health concern as wildfires increase in size, intensity, and duration in the United States. The public is often encouraged to stay indoors during wildfire smoke events to reduce exposure. However, there is limited information on how much wildfire smoke infiltrates indoors at residences and what household/behavioral characteristics contribute to higher infiltration. We assessed fine particulate matter (PM) infiltration into Western Montana residences during wildfire season.

Methods: We measured continuous outdoor and indoor PM concentrations from July-October 2022 at 20 residences in Western Montana during wildfire season using low-cost PM sensors. We used paired outdoor/indoor PM data from each household to calculate infiltration efficiency (F; range 0-1; higher values indicate more outdoor PM infiltration to the indoor environment) using previously validated methods. Analyses were conducted for all households combined and for various household subgroups.

Results: Median (25th percentile, 75th percentile) daily outdoor PM at the households was 3.7 μg/m (2.1, 7.1) during the entire study period and 29.0 μg/m (19.0, 49.4) during a 2-week period in September impacted by wildfire smoke. Median daily indoor PM at the households was 2.5 μg/m (1.3, 5.5) overall and 10.4 μg/m (5.6, 21.0) during the wildfire period. Overall F was 0.34 (95 % Confidence Interval [95%CI]: 0.33, 0.35) with lower values during the wildfire period (0.32; 95%CI: 0.28, 0.36) versus non-wildfire period (0.39; 95%CI: 0.37, 0.42). Indoor PM concentrations and F varied substantially across household subgroups such as household income, age of the home, presence of air conditioning units, and use of portable air cleaners.

Conclusions: Indoor PM was substantially higher during wildfire-impacted periods versus the rest of the study. Indoor PM and F were highly variable across households. Our results highlight potentially modifiable behaviors and characteristics that can be used in targeted intervention strategies.