How Wildfires Affect Indoor Air Quality in the DMV

The DMV area has experienced multiple wildfire smoke events in recent years, with Canadian wildfires in particular sending massive smoke plumes that have blanketed Washington DC, Maryland, and Virginia in hazy, acrid air. What was once considered a strictly West Coast problem has become a recurring concern for residents throughout the mid-Atlantic region. Wildfire smoke contains a complex mixture of fine particulate matter (PM2.5), carbon monoxide, volatile organic compounds, and other hazardous pollutants. PM2.5 particles are especially dangerous because they are small enough to penetrate deep into the lungs and even enter the bloodstream. During severe smoke events, DMV air quality has reached "unhealthy" and even "hazardous" levels on the Air Quality Index (AQI), rivaling some of the worst air quality days recorded in major cities worldwide. Unlike local pollution sources that DMV residents are accustomed to managing, wildfire smoke events are unpredictable, can last days or weeks, and are largely outside local control. This makes personal and household preparation essential for protecting indoor air quality during these events.

Even with windows and doors closed, wildfire smoke infiltrates homes through numerous pathways. Air leaks around windows and doors, gaps in building envelopes, dryer vents, bathroom exhaust vents, and the HVAC system's outdoor air intake all provide entry points. Older DMV homes with less air-tight construction are particularly vulnerable to smoke infiltration. Your HVAC system can be a major pathway for smoke entry. Most residential systems in the DMV draw some outside air through the return duct system, and the standard MERV 8 filters installed in many homes are not effective at capturing the fine PM2.5 particles that make wildfire smoke so hazardous. These tiny particles pass right through basic filters and are then distributed throughout every room via the supply ducts. Even in tightly sealed homes, activities like opening doors to enter and exit, using exhaust fans, and operating fireplaces or gas appliances can draw smoky outdoor air inside. During a smoke event, every door opening allows a pulse of contaminated air into the home, and it takes time for indoor levels to return to baseline.

Your HVAC system's air filter is the first line of defense against smoke particles entering your home through the duct system. During a wildfire smoke event, upgrade to the highest MERV-rated filter your system can handle—ideally MERV 13 or higher. MERV 13 filters capture approximately 85% of PM2.5 particles, compared to less than 20% for a standard MERV 8 filter. Before upgrading your filter, check your HVAC system's specifications or consult with a professional. Higher-MERV filters restrict more airflow, and some older systems in DMV homes cannot handle MERV 13 without straining the blower motor. In these cases, a MERV 11 filter is a reasonable compromise, or you can supplement with portable HEPA air purifiers in occupied rooms. Set your HVAC fan to run continuously (the "on" setting rather than "auto") during smoke events. This keeps air circulating through the filter even when the system is not actively heating or cooling. Check and replace the filter more frequently during extended smoke events—a filter that normally lasts three months may become loaded in one to two weeks during heavy smoke conditions.

During severe smoke events when outdoor AQI is consistently above 150, the EPA recommends creating a "clean air room" in your home—a single room where you can maintain the best possible air quality. Choose a room where you spend the most time, ideally a bedroom, and take steps to make it as smoke-free as possible. Seal the clean air room by closing all windows and doors, placing damp towels at the base of doors, and covering any visible gaps or vents with plastic sheeting and tape. Place a portable HEPA air purifier in the room, sized appropriately for the room's square footage. Run the purifier on its highest setting to establish clean air, then reduce to a medium setting to maintain it. A DIY alternative for air purification is the Corsi-Rosenthal box—a cube made from four or five MERV 13 furnace filters taped to a box fan. These improvised purifiers are surprisingly effective at reducing PM2.5 levels and can be built quickly from materials available at any hardware store in the DMV area. They are particularly useful when commercial HEPA purifiers are sold out during smoke events.

Once a wildfire smoke event subsides and outdoor air quality returns to normal, take steps to flush the accumulated smoke residue from your home. Open all windows and doors to ventilate thoroughly. Run the HVAC fan continuously for several hours to cycle fresh air through the duct system. This initial flush removes much of the residual particulate matter trapped in carpets, upholstery, and ductwork. Replace your HVAC filter after a significant smoke event. Even if the filter looks clean, it has likely captured a substantial load of fine particles that restrict airflow and can continue to release odors. Returning to your standard filter type is fine once air quality is consistently good, but keep higher-rated filters available for the next event. For homes that experienced extended or severe smoke exposure, professional duct cleaning can remove accumulated smoke residue from inside the ductwork. Smoke particles are sticky and adhere to duct surfaces, where they can continue to release odors and particles for weeks or months after the event. This is especially worthwhile for residents who noticed persistent smoky smells even after ventilating their home.