Monitoring for Clean Air Spaces During Wildfire Events
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BACKGROUND: Many communities are exposed to extended periods of smoke from wildland fires. A common recommendation to reduce exposure during smoke events is to go indoors. However, building characteristics that translate to clean air spaces during such events are poorly understood. The purpose of this study was to compare outdoor and indoor fine particulate (PM2.5) at commercial buildings during smoke events.
METHODS: Purple Air Sensors (PA-II-SD) were co-located with regulatory grade reference monitors to generate individual sensor correction equations. Outdoor and indoor sensors were co-located at 18 buildings in Missoula, MT from July to September 2019. Outdoor to indoor daily PM2.5 averages were compared for assessment of building air handling systems. Spikes in indoor PM2.5, identified as any hour where the standard deviation of the 2-minute data was over 10% of the hourly average, were flagged and removed from the comparisons.
RESULTS: Individual correction equations and strength of agreement were similar across the set of sensors (slope: 0.36 –0.47 and R2: 0.91 to 0.95). Censoring of data flagged as indoor generated PM2.5 accounted for <5% to 14% of observations across buildings. Fourteen buildings had MERV8 filter systems, two buildings had higher MERV rating filters, and two buildings had no air handling systems. All buildings with air handling systems had reductions in indoor PM2.5 compared to outdoor concentrations but building-to-building observations were highly variable (6% to 44% reduction for MERV8 buildings). Outdoor-to-indoor reductions were similar during smoke impacted periods compared to non-smoke periods.
CONCLUSIONS: This study demonstrates the utility of low-cost PM2.5 sensors for identifying poor indoor air quality. The variable findings for buildings with similar air handling filter ratings indicate that other building features and operation behaviors are important factors and need to be understood to inform exposure mitigation strategies during smoke events.