Evaluation of MERV Filter Performance and Degradation Under Simulated Wildfire Smoke Exposure

Wildfire smoke poses significant risks to human health due to its complex composition and ability to travel long distances, degrading air quality in both outdoor and indoor environments. In-duct air filtration is a widely recommended mitigation strategy; however, the long-term performance of home furnace filters when exposed to wildfire smoke has not been extensively investigated. Moreover, current filter rating methods, such as ASHRAE Standard 52.2, rely on laboratory-generated potassium chloride (KCl) aerosols, which differ substantially from real wildfire smoke. This raises concerns regarding the applicability of standardized ratings, such as Minimum Efficiency Reporting Value (MERV), to real-world wildfire smoke conditions.
This study evaluates the performance of commercially available MERV 7 and MERV 13 filters when exposed to simulated wildfire smoke under controlled bench-scale conditions. In addition, filter performance against KCl aerosol is assessed to enable direct comparison with standard testing methods. Particle concentrations upstream and downstream of the filters were measured to determine filtration efficiency across particle size ranges, and scanning electron microscopy (SEM) was used to examine particle deposition characteristics.
Results show that filtration efficiency is lower for wildfire smoke compared to KCl aerosol. For MERV 13 filters, efficiency based on particle number was 66% for KCl and 52% for wildfire smoke under equivalent loading conditions. Under repeated wildfire smoke exposure, the mass-based efficiency of MERV 13 filter dropped from ~80% to ~30%. MERV 7 filters exhibited consistently lower performance, with mass-based efficiency declining from ~18% to ~8%.
SEM analysis revealed distinct deposition mechanisms: KCl formed crystalline structures on filter fibers, while wildfire smoke produced droplet-like “tar balls.” These morphological differences influence filtration behavior and contribute to performance degradation over time. Overall, the results highlight significant discrepancies between standardized filter ratings and real-world performance, emphasizing the need for improved testing methodologies and guidance for filtration strategies during wildfire smoke events.