How to Filter Out Smoke? | Keep Indoor Air Clean

The fastest way to filter smoke from a wildfire or nearby fire is to create a “clean room.” Pick one room, seal every gap, close windows and doors, and run a HEPA air purifier or a DIY box-fan filter continuously. This isn’t complicated, but skipping one step—like leaving a window cracked—defeats the whole strategy. Below are the exact methods that work, in the order that matters most.

Seal Your Home First

Before any filter can work, you must stop smoke from flowing in. Close all windows and doors. Use weather stripping or tape on gaps around window frames and door bottoms. If you have a window air conditioner, close its outdoor air damper and seal the gap around the unit with tape or foam. Turn off whole-house fans, swamp coolers with outside intakes, and any fresh-air intake on your HVAC system—they pull smoky air inside. During smoke events, also avoid burning candles, incense, or using fireplaces and wood stoves, because they add particles to already compromised air.

Portable HEPA Air Cleaners: The Reliable Tool

A portable HEPA air cleaner removes the very small particles in wildfire smoke effectively. Place it in the room you spend the most time in—typically a bedroom or living room—and run it continuously on high. Make sure the unit is sized for the room’s square footage (check the CADR rating on the box). Close the door to keep filtered air inside. Replace the filter when it looks dirty or according to the manufacturer’s schedule; a clogged filter stops cleaning. See our tested picks for the best air filter for smoke if you’re ready to buy.

DIY Box-Fan Filter: Cheap and Effective

The EPA has researched and recommends a DIY air cleaner made from a box fan and a MERV 13 filter. It’s a cost-effective way to reduce smoke concentrations significantly. Use only a newer, certified box fan (2012 models or newer) because older fans lack safety features. Attach a MERV 13 filter to the back of the fan with bungee cords or tape—the 4-inch-thick filters last longer and work better than 1-inch versions. For even more performance, build a cardboard shroud around the filter to direct all air through it, or use a design that sandwiches 2 to 5 filters together in a cube shape. Keep extra filters on hand and swap them out when they turn gray. The EPA’s research page provides detailed build guides.

Using Your Central HVAC System

Your central HVAC can help, but only with the right filter and settings. Install a MERV 13 or higher filter if your system’s static pressure can handle it—check your owner’s manual or ask an HVAC professional if unsure. Run the fan continuously (set the thermostat to “Fan On” rather than “Auto”) to circulate air through the filter many times per hour. During smoke, turn off any fresh-air intake so you’re recirculating indoor air, not pulling in smoke. If your HVAC can’t support a MERV 13 filter without restricting airflow, stick with a portable purifier or the DIY fan filter instead.

Damp-mop floors and wipe surfaces to capture settled smoke particles. Avoid vacuuming unless your vacuum has a HEPA filter—a standard vacuum will blow fine particles back into the air. If you must go outside in smoky conditions, wear a tight-fitting N95 or P100 respirator; surgical and cloth masks do nothing for smoke particles.