Los Angeles homeowners have grown accustomed to checking air quality apps during fire season, but far fewer think about what happens to their HVAC system once the smoke clears. Every major regional wildfire — from the 2018 Woolsey Fire to the devastating January 2025 Eaton and Palisades Fires — has left behind the same hidden problem: HVAC systems contaminated with smoke particulate and ash that continue circulating long after the visible smoke is gone.
How Smoke Actually Enters Your HVAC System
Wildfire smoke doesn't need an open window to get inside your home's ductwork. There are several entry points:
- Return air vents — the primary pathway. Any time the system runs during a smoke event, the return vents pull contaminated room air back through the system, and a portion of that air originated from outside infiltration.
- Window and door seals — no home is perfectly sealed. Smoke and fine particulate work their way in through small gaps, especially during the high-wind conditions that typically accompany major wildfire events in Southern California.
- Fresh air intakes — some HVAC systems are designed to bring in a percentage of outside air for ventilation purposes, which becomes a direct pipeline for smoke and ash during a wildfire event.
What Smoke Actually Deposits Inside Your System
Once airborne smoke and ash enter the ductwork, they don't just pass through — they deposit. The main contaminants include:
- PM2.5 and PM10 particulate matter — ultrafine particles capable of penetrating deep into lung tissue, which settle on duct interior surfaces and the coil.
- Volatile organic compounds (VOCs) — released by burning vegetation and, in structure fires, synthetic materials and plastics.
- Toxic heavy metals — when homes and structures burn, as occurred extensively in the Eaton Fire, ash can contain lead, arsenic, and mercury released from building materials, paint, and electronics.
The Infiltration Stages: Filter → Ducts → Coil → Blower
Contamination doesn't stop at the filter — it progresses through the system in stages:
- Filter — captures some particulate, but standard residential filters are not designed to stop the finest wildfire particles, and the filter becomes saturated quickly during heavy smoke events.
- Ductwork — particulate that passes the filter settles along duct walls throughout the system, especially at bends and branch connections where airflow slows.
- Evaporator coil — fine particulate that reaches the air handler coats the coil fins, which reduces heat transfer efficiency and creates a surface where moisture and ash combine.
- Blower wheel — the blower wheel's curved blades are especially prone to accumulating sticky ash residue, which can throw the wheel out of balance and reduce airflow throughout the entire system.
Why Standard Filters Aren't Enough
Many homeowners assume that simply upgrading to a higher-MERV filter solves the problem. A better filter helps reduce ongoing infiltration, but it does nothing for contamination that has already settled inside the ducts, coil, and blower before the filter was upgraded. Even a high-efficiency filter only captures particulate that physically passes through it — it cannot reach material already deposited deeper in the system.
Short-Term vs. Long-Term Damage
Short-Term Effects
- A burning or smoky odor whenever the system runs
- Reduced airflow as filters clog more quickly than normal
- Immediate respiratory irritation, particularly for sensitive household members
Long-Term Effects
- Corrosion of metal components from acidic ash residue settling on coils and ductwork
- Persistent odor that resists masking and returns even after surface cleaning
- Reduced HVAC system efficiency and shortened equipment lifespan
- Continued redistribution of toxic particulate every time the system cycles on
Health Impact of Continued Exposure
A contaminated HVAC system doesn't just smell bad — it actively undermines indoor air quality every time it runs. Continued exposure to circulating ash and smoke particulate has been linked to worsened asthma and allergy symptoms, headaches, eye and throat irritation, and — with prolonged exposure to heavy-metal-laden ash from structure fires — more serious long-term health risks. Children, older adults, and anyone with existing respiratory conditions face the highest risk.
A History of LA Wildfires That Have Affected HVAC Systems
| Fire | Year |
|---|---|
| Woolsey Fire | 2018 |
| Bobcat Fire | 2020 |
| Eaton Fire | 2025 |
| Palisades Fire | 2025 |
Each of these events left thousands of homes — many well outside the direct burn perimeter — with HVAC systems carrying smoke particulate that most homeowners never had professionally addressed.
What To Do During and After a Wildfire
During an active wildfire: set your HVAC system to recirculate mode if available to minimize pulling in outside air, install the highest MERV-rated filter your system supports, and limit system run time to only what's necessary for comfort and safety.
After the fire passes: do not resume normal operation without an inspection if your home experienced visible smoke or ash exposure. Schedule a professional inspection before running the system through its normal daily cycles, especially if you're in or near a recently burned area.
Our 6-Step Cleaning Process
- Pre-cleaning inspection and documentation for insurance purposes
- System isolation to seal registers and prevent cross-contamination
- Negative pressure extraction through every duct run using truck-mounted equipment
- Air handler and evaporator coil cleaning, where the highest concentration of ash typically accumulates
- EPA-registered antimicrobial sanitization to address mold risk from fire-suppression moisture
- Complete insurance documentation package with before/after photos and a completion certificate
Frequently Asked Questions
Smoke and ash enter primarily through return air vents while the system is running, but also infiltrate through imperfect seals around windows and doors, and through any fresh-air intake the system may have. Once airborne particulate is inside the return ductwork, normal system operation distributes it throughout the entire home.
Wildfire smoke deposits fine particulate matter (PM2.5 and PM10), volatile organic compounds (VOCs), and — when structures burn, as in the Eaton and Palisades fires — toxic heavy metals like lead, arsenic, and mercury released from burning building materials, paint, and electronics.
Standard residential filters are designed to capture larger particles like dust and pet hair, not the ultrafine PM2.5 particulate found in wildfire smoke. Even higher-MERV filters only capture a percentage of what passes through, and none of them address particulate that has already settled on duct walls, the blower wheel, or the coil before reaching the filter.
Short-term effects include a burning odor when the system runs, reduced airflow from a clogged filter, and immediate respiratory irritation. Long-term effects include corrosion of metal components from acidic ash residue, persistent odor that resists masking, reduced HVAC efficiency and lifespan, and continued redistribution of toxic particulate every time the system operates.
Several major events have affected the region's HVAC systems, including the 2018 Woolsey Fire in the Malibu and Thousand Oaks area, the 2020 Bobcat Fire near the San Gabriel Mountains, and the catastrophic January 2025 Eaton Fire in Altadena and Pasadena and Palisades Fire on the Westside, both of which deposited heavy ash and smoke across large portions of LA County.
Set your system to recirculate mode if it has one to avoid pulling in outside air, replace your air filter with the highest MERV-rated filter your system supports, and avoid running the system more than necessary. After the fire passes, do not resume normal operation without an inspection if there was visible smoke or ash exposure.
Our process includes a documented pre-cleaning inspection, full system isolation to prevent cross-contamination, truck-mounted negative pressure extraction through every duct run, air handler and evaporator coil cleaning, EPA-registered antimicrobial fogging, and a complete insurance documentation package with before/after photos and a completion certificate.
As soon as it's safe to return home and before resuming normal HVAC operation. The longer a contaminated system runs, the more particulate gets redistributed throughout the home, and moisture introduced during firefighting efforts can begin to create mold risk inside the ductwork within just a few days.