Are Your AC Vents Spreading Smoke from Wildfires? What You Can Do

When wildfire plumes blanket the region, leaky ducts and low-grade filters can let microscopic soot circulate through supply registers. As Spokane’s smoke season intensifies, Air Duct Cleaning Spokane (Spokane, WA) helps homeowners and building managers turn HVAC systems from “smoke spreaders” into “smoke stoppers” with targeted filtration, sealing, and operating practices.

Why Vents Become Smoke Highways

Wildfire pollution is dominated by ultrafine and fine particles (especially PM2.5) that are light enough to stay suspended, slip through gaps, and travel deep into the respiratory tract. Central HVAC systems recirculate large volumes of air; if returns or filter racks leak—or if filtration is under-spec’d—those particles bypass protection and hitch a ride to every room. Indoors rarely stays pristine during smoke events: homes with poor sealing or open windows can approach outdoor particle levels, while homes running efficient filters with good duct integrity typically keep a far smaller share of the outdoor spike.

How Smoke Sneaks Into Your System

• Return-side leakage: Negative pressure on the return side can pull smoky air from attics, garages, or crawlspaces, contaminating the airstream before it even reaches the filter.

• Outdoor air paths: Economizers and fresh-air intakes are useful most days, but during smoke they can flood the system unless dampers are shut and actuators actually seal.

• Filter bypass and low MERV ratings: Gaps around the filter frame, flexed racks, and worn gaskets let unfiltered air slip around the media; filters below MERV 13 struggle to capture the smallest particles.

• Imbalanced exhaust: Strong bathroom or kitchen exhaust can depressurize the house, dragging smoky air through cracks and door undercuts.

Rapid-Response Plan the Moment AQI Rises

1) Follow the AQI and switch modes early

Check local air quality and act before indoor levels climb. As air becomes unhealthy, reduce outdoor air exchange and prepare a designated “clean room.”

2) Force recirculation and close outdoor-intake paths

Set the thermostat or BAS to recirculate. Verify that outdoor-air dampers and economizers are shut and actually sealing; many “closed” dampers still leak without maintenance.

3) Upgrade to the highest MERV your blower can handle

Aim for MERV 13–14 if the system allows. Tighten the filter rack with foam gaskets or clip-in seals to prevent bypass. Confirm static pressure and adjust fan speed as needed.

4) Deploy HEPA room purifiers sized to the space

Pick units with a smoke Clean Air Delivery Rate (CADR) that matches room size; larger rooms or high ceilings may need multiple units. In a pinch, a DIY box fan plus MERV 13 filters can substantially cut particles—reserve this for a dedicated clean room and run it on high.

5) Avoid ozone-producing gadgets

Skip ionizers and “ozone shock” devices. They don’t solve PM2.5 and can create new irritants. Choose certified low-ozone products only.

6) Reduce infiltration

Weather-strip doors, add door sweeps, and seal penetrations. Confirm that return grilles, filter doors, and air-handler access panels latch tightly. Small fixes here yield big wins during multi-day smoke intrusions.

7) Use proper respirators when leaving your clean space

For errands or cleanup, wear a well-fitted N95 or P100. Cloth and surgical masks aren’t designed for fine particle removal.

Pro-Level Hardening Before Smoke Season

Filtration that actually performs

• Media: Quality pleated filters rated MERV 13–14 catch far more wildfire PM than builder-grade pads.

• Fit: Add gasketing so the filter frame seals on all four sides. If the rack is warped, retrofit a tight-fitting housing.

• Frequency: Inspect weekly during smoke; replace when visibly loaded or when pressure drop rises—often sooner than the typical schedule.

Control the air you bring in

• OA dampers and economizers: Keep them closed during bad AQI and verify actuator travel and blade alignment so “closed” truly means closed.

• Continuous fan on recirc: Running the blower pushes more air through the high-efficiency filter, accelerating cleanup.

Build a high-performance clean room

Choose a bedroom or den with a tight door, close windows, and run a HEPA unit on high. Aim for multiple air changes per hour—enough that the room clears noticeably within minutes of closing it up.

Balance exhaust carefully

Use kitchen and bath exhaust only when necessary. Excess exhaust can create negative pressure that sucks smoky air in through the envelope.

After the event

Swap loaded filters, wipe or vacuum return cavities and coil faces with care (soft brush), and schedule duct leakage testing. Seal seams with mastic or UL-181 foil tape, prioritize return-side repairs, and re-check damper alignment.

Spokane-Specific Notes

Prolonged smoke waves are common regionally. Keep a small stock of MERV 13–14 filters and at least one HEPA unit per key room. When forecasts hint at a multi-day episode, preemptively close outdoor-air dampers, set recirculation, and stage a clean room so you’re not scrambling at the first smoky smell.

Quick Guide by AQI Category

• Unhealthy for Sensitive Groups (≈100–150): Close windows/doors; set HVAC to recirculate; run HEPA in occupied rooms; minimize door swings.

• Unhealthy (≈151–200): Activate the clean room; limit exhaust fan use; avoid indoor activities that add particles (candles, frying).

• Very Unhealthy/Hazardous (200+): Stay in the clean room as much as possible; use N95/P100 if you must go out; postpone nonessential projects.

Why This Works

Three levers consistently cut indoor smoke exposure: efficient filtration (MERV 13–14 or HEPA), airtight air paths with no bypass, and smart ventilation control that limits smoky outdoor air. Apply all three and your vents stop distributing soot—they start removing it.

CONCLUSION

Wildfire smoke infiltrates through leaky returns, open outdoor-air paths, and weak or bypassing filters. Tightening the system, upgrading to MERV 13–14 media, closing outside air during bad AQI, and running right-sized HEPA units—preferably in a prepared clean room—transforms your HVAC from a distribution network into a protective filtration stack.

Learn how clean ducts can help prevent summer insect infestations indoors—read the full guide here.

FAQs

1) Can every furnace handle MERV 13 or 14? 

Not always. Higher-efficiency filters add resistance. We verify fan capacity and static pressure, then adjust fan speed or retrofit a deeper filter cabinet so airflow and coil protection stay within spec.

2) How many HEPA units do I need? 

Start with one per frequently occupied room sized to the room’s square footage; larger living areas may need two. If the space takes a long time to clear or doors open often, add capacity.

3) Are duct cleanings worthwhile after smoke events? 

Yes, when filters loaded quickly or leakage allowed particles to deposit in returns and trunks. Post-event service—filter replacement, coil and plenum cleaning, and sealing measured leaks—helps the next smoke wave start from a clean baseline.