Protecting Indoor Air During Wildfire Smoke Events

Why Wildfire Smoke Is Dangerous

Wildfire smoke is a complex mixture of fine particulate matter (PM2.5), carbon monoxide, volatile organic compounds, and other toxic chemicals produced by burning vegetation, structures, and materials. PM2.5 particles are the primary health concern because they are small enough to penetrate deep into the lungs, enter the bloodstream, and cause systemic inflammation. Short term exposure to elevated PM2.5 irritates the eyes, nose, and throat, triggers asthma attacks, and worsens existing heart and lung conditions. Long term or repeated exposure is associated with increased risk of heart disease, stroke, and respiratory illness.

During major smoke events, outdoor PM2.5 levels can exceed 200 to 500 micrograms per cubic meter, compared to the EPA's 24 hour health standard of 35 micrograms per cubic meter. At these levels, the EPA classifies air quality as "very unhealthy" or "hazardous" for everyone, not just sensitive groups. Without any mitigation, indoor PM2.5 levels typically reach 50 to 80 percent of outdoor levels within a few hours as smoke infiltrates through the building shell. With proper filtration and sealing, indoor levels can be held below 10 to 25 micrograms per cubic meter even when outdoor levels are in the hazardous range.

Your HVAC System During Smoke Events

Your HVAC system can be either your best defense or a significant vulnerability during wildfire smoke, depending on its configuration. The key factors are the filter quality, the duct system integrity, and the fresh air intake settings.

Upgrade to MERV 13 or higher before smoke season. A MERV 13 filter captures about 85 percent of PM2.5 particles on each pass, and with the fan running continuously, air cycles through the filter multiple times per hour. Over the course of a day, this dramatically reduces indoor particle levels. If your system cannot handle MERV 13 (due to high static pressure), a MERV 11 or 12 still provides meaningful protection and is better than the MERV 8 or lower that many homes have.

Run the fan continuously in circulation mode. Set the thermostat fan to "on" rather than "auto" so air passes through the filter continuously, not just during heating or cooling cycles. During smoke events, you want maximum filtration passes per hour. If your system has a variable speed blower, running it on low provides constant filtration without the noise and energy cost of full speed operation.

Close the fresh air intake if your system has one. ERV and HRV systems, fresh air dampers, and economizer controls that bring outdoor air into the HVAC system should be shut off during smoke events. You are normally ventilating to improve air quality, but when the outdoor air is hazardous, bringing it in defeats the purpose. Most ERV and HRV units have an off switch or a recirculation mode for exactly this situation. Close the outdoor air damper and resume normal ventilation once the air quality index returns to acceptable levels.

Seal duct leaks where possible. Return duct leaks in attics, crawl spaces, and exterior walls draw smoky outdoor air into the system, bypassing the filter entirely. While sealing all duct leaks is a longer term project, you can temporarily seal obvious gaps with foil tape during a smoke event. Better yet, have your ductwork professionally sealed before smoke season as a preventive measure.

Portable HEPA Purifiers for Clean Air Rooms

Even with HVAC filtration running, creating one or two "clean air rooms" with portable HEPA purifiers provides the highest level of protection for sleeping and extended time indoors. A portable HEPA purifier sized for the bedroom (typically 200 to 400 square feet) costs $100 to $300 and reduces PM2.5 in that room to near zero levels when the door is kept closed.

Choose a purifier rated for the room size (look for the CADR, Clean Air Delivery Rate, which should be at least two thirds of the room's square footage), and run it on the highest setting you can tolerate for noise. Keep the bedroom door closed to maintain a pocket of clean air. Sleep with the purifier running, and keep it running during waking hours if you are spending extended time in the room.

For homeowners who do not own a portable HEPA purifier and cannot purchase one during a smoke event (they sell out quickly when smoke arrives), the Corsi-Rosenthal box is a proven DIY alternative. It consists of four to five MERV 13 furnace filters taped together in a cube shape with a box fan on top, pulling air through the filter walls. Studies from multiple universities have confirmed that these DIY units achieve 60 to 80 percent of the particle removal of commercial HEPA purifiers at a fraction of the cost ($30 to $50 in materials).

Sealing Your Home Against Smoke Infiltration

Smoke enters through every gap in the building shell, including door sweeps, window seals, mail slots, pet doors, fireplace dampers, recessed lights, plumbing penetrations, and the gaps around attic hatches. While perfect sealing is impossible in an existing home, addressing the biggest leakage points makes a meaningful difference.

Close all windows and exterior doors. Close the fireplace damper and seal the fireplace opening if the damper does not seal tightly (a sheet of heavy duty aluminum foil works temporarily). Place rolled towels at the base of exterior doors if the weatherstripping is worn. Tape over mail slots and close any pet doors. Turn off exhaust fans (bathroom fans, range hoods, dryer vents) because they create negative pressure that pulls smoky air in through every crack. If you must cook, use an electric range or microwave rather than gas, and avoid grilling or any activity that requires opening doors.

Long term weatherization improvements (caulking, weatherstripping, attic air sealing) provide the best smoke protection because they permanently reduce air infiltration. These improvements also save energy year round, making them a dual purpose investment. A well sealed home with a MERV 13 filter can maintain indoor PM2.5 below 25 micrograms per cubic meter even when outdoor levels reach 300 or more.