Wildfire Smoke — A Rigorous Challenge for HVAC Air Filters

Source: Tanya Shirman, Hediyeh Zamani, Sissi Liu et al.; arXiv, May 2025 arXiv

Summary

This research examines the performance of over 17 different types of HVAC filter media (including synthetic charged, uncharged synthetic, and fiberglass media) under wildfire smoke exposure—using pine needle smoke as a proxy. The goal was to see how filter efficiency and airflow resistance change during smoke loading, and how well different media retain performance over time. arXiv

Main findings include:

• Charged polymer media (filters with electrostatic charge to augment particle capture) show a rapid decline in smoke removal efficiency within just minutes of exposure, even though airflow resistance doesn’t immediately increase. That suggests that although these filters initially perform well, they degrade quickly under realistic smoke loading. arXiv

• In contrast, mechanical media (filters relying purely on physical fiber capture rather than electrostatics) displayed more stable particle filtration over time but experienced increased airflow resistance (i.e. more pressure drop) as smoke particles accumulate. This can affect HVAC performance, energy use, and distribution of air. arXiv

• The study used scanning electron microscopy (SEM) to image how smoke particles deposit on fiber surfaces, showing that smoke causes particular patterns of deposition that affect both efficiency and pressure drop. These patterns differ by medium type. arXiv

• The authors argue that current commercial filter rating systems (like MERV in the U.S.) are designed around removal of inorganic particles, not smoke (organic carbon, fine combustion aerosols, etc.). Thus, filter ratings may overestimate performance in smoke conditions. They call for standards/tests specific to smoke particle challenges. arXiv

Understanding & Analysis

This study is vital because many homes/buildings rely on HVAC systems and assume their filters will deal with smoke/haze when wildfire events occur. However, the reality often diverges significantly.

Key reflections:

1. Electrostatic filters have limited long-term smoke resilience: Though appealing due to lower initial pressure drop and lightweight design, charged filters (electret filters) lose efficiency quickly under heavy smoke loading. Possibly the charge becomes neutralized by deposited particles, or pores clog in ways that degrade functionality more sharply than uncharged media. For someone buying filters for wildfire risk, purely electrostatic designs may give false assurance.

2. Mechanical filters are more robust but come with trade-offs: Physical capture by fibers tends to handle smoke better over time, but at the cost of increasing pressure drop (higher resistance), which means the HVAC fan must work harder. Over time, this means higher energy use and possibly decreased airflow (if fan power limited or not adjusted). Also, more frequent replacement or maintenance may be required to ensure airflow performance.

3. Importance of realistic testing and standardization: Ratings like MERV (or equivalents) often are based on particulate salt, dust, or standard challenge aerosols—not necessarily realistic smoke mixtures with organic/combustion material. Thus, a filter rated MERV-13 might seem great on paper, but under wildfire smoke, performance may degrade faster. The call for smoke-specific test methods is well justified. Building codes, HVAC component standards, and manufacturer claims need to factor in smoke exposure more explicitly.

4. Implications for building owners and occupants:

   • During wildfire seasons, it may be wise to replace or upgrade filters before smoke events. Use higher grade mechanical filters even if they cost more or drop airflow somewhat.

   • HVAC systems need to balance filtration vs airflow. If a high pressure drop filter is inserted without adjusting fan or platform, the airflow could drop so much that occupants feel less heating/cooling, or system strain increases.

   • Consider supplementary air cleaning (portable units, HEPA cleaners) in addition to HVAC filtration, since HVAC systems may not be designed for continuous smoke event mitigation.

5. Energy and cost trade-offs: More efficient mechanical filters consume more energy (fan power) as resistance grows. Also, maintenance cost goes up. These must be weighted against health benefits. In many cases, the cost of filtering smoke (in terms of health care, lost productivity) outweighs the energy cost; but users may not perceive that immediately.

Limitations & Further Research

• Duration of exposure: How do different filter media perform over extended, successive smoke events (days/weeks)? Do efficiencies decline cumulatively, and how dramatically?

• Real building systems: Beyond filter media alone, how do duct leakage, bypass flows, filter installation quality, and building envelope interactions affect real indoor smoke exposure?

• Hybrid filters: Electrostatic + mechanical + activated carbon or odor absorbing layers—how do these combined media perform?

• Cost-benefit analyses under varying climates/regions: For example, in places with frequent wildfire exposure, it may make sense to mandate certain filter standard ratings; elsewhere, it may not.

Conclusion

This research reveals that many filters commonly used in HVAC systems are not reliable in smoke-heavy conditions, especially for prolonged exposure. Mechanical media, though more resistant to efficiency decline, increase airflow resistance, necessitating design or operational adjustments. For building design, public health policy, and product manufacturing, there is a need to revise filter performance standards, develop smoke-specific test methods, and consider occupant health more explicitly in filtration design. In sum, this study adds urgency to investments in resilient indoor air systems as wildfire prevalence rises.