Impact of Wildfire Smoke on HVAC Filter Performance and Indoor Air Quality
SHIBO WANG, Qisheng Ou, Dongryul Park, Keonwang Lee, Hyungho Park, David Y. H. Pui, University of Minnesota
Abstract Number: 56
Working Group: Indoor Aerosols
Abstract
Introduction
As wildfires increase in frequency and intensity across many regions, their emissions pose a growing and serious threat to indoor air quality (IAQ), particularly in buildings that rely heavily on mechanical ventilation systems. HVAC filters are widely used to remove airborne particles and are often the primary defense against outdoor pollution. However, existing filtration standards—such as ASHRAE 52.2 or ISO 16890—are based on standardized test aerosols like potassium chloride or DEHS, which do not reflect wildfire smoke's physical or chemical complexity. Wildfire-derived aerosols are typically composed of fine soot, sticky tar-like agglomerates, and semi-volatile organic compounds, all of which may interact with filter fibers in ways that compromise filtration efficiency. A clearer understanding of filter aging under wildfire exposure is urgently needed to assess indoor exposure risks and inform public health guidance and building operation strategies during smoke events.
Method
This study evaluated the performance of three commercially available MERV-13 HVAC filters under multiple particle loading scenarios. Laboratory experiments were conducted using smoldering wood smoke, potassium chloride (KCl), and DEHS oil aerosols. In parallel, field tests were performed during prescribed burns in savanna environments to capture real wildfire smoke loading. Filtration efficiency for 300 nm particles—a representative submicron size—was continuously monitored as a function of cumulative loading mass (mg/cm²). Scanning electron microscopy (SEM) was employed to assess surface morphology changes and observe fiber-particle interactions. Based on the measured degradation behavior, a dynamic mass balance model was constructed, incorporating η as a function of loading mass, to simulate indoor PM₂.₅ concentrations under a range of conditions including varying outdoor smoke concentrations, HVAC runtimes, and infiltration rates. Scenarios with and without portable air cleaners were also simulated to explore mitigation strategies.
Results
Filters that relied primarily on electrostatic capture mechanisms showed rapid degradation in filtration efficiency under wildfire smoke exposure, especially when loaded with smoldering or flaming smoke. In contrast, filters with greater mechanical filtration capacity demonstrated more stable performance over time. SEM analysis revealed progressive surface coverage by oil-like droplets, consistent with charge neutralization effects and reduction in active filtration sites. Field-collected filters exhibited similar droplet accumulation and fiber coverage patterns, validating the laboratory-generated smoldering smoke as a realistic proxy for real-world loading. Simulation results showed that under severe wildfire smoke conditions (e.g., 200 µg/m³) and continuous HVAC operation, indoor PM2.5 concentrations could exceed 90–120 µg/m³ within a few days—far above WHO and EPA recommended exposure limits. Filter lifespan was shown to decrease from 60 days to under 5 days under such conditions. However, simulations also demonstrated that improving natural particle decay—such as by using portable air cleaners—could reduce indoor concentrations by more than 50 percent.
Conclusions
Wildfire smoke causes accelerated degradation of HVAC filter performance due to sticky particle deposition and electret charge loss. This degradation significantly increases indoor PM2.5 levels, even in buildings with active filtration. Experimental and modeling results suggest that replacement strategies and IAQ mitigation approaches must be adapted to account for the realities of wildfire aerosol behavior and loading. Conventional test methods may underestimate these effects and should be reconsidered in light of wildfire-specific risks.