How to make a box fan filter to clean indoor air of smoke

a couple walks between two trees in thick smoke.

Wildfires up and down the west coast fill the air with smoke.

Box fan filters are simple, relatively inexpensive, and effective at cleaning indoor air. Our infographic shows you how to make one.

an infographic showing how to make a box fan filterAs wildfire smoke fills the sky up and down the west coast, experts advise people to stay inside to avoid the health effects of smoke exposure. But how clean is your indoor air? Many older houses lack insulation, have drafty single-paned windows, and/or airspace around their doors where smoke can get in. In houses without air conditioning, windows may need to be opened periodically to manage heat.

One of the most effective ways to clean smoke from indoor environments is to attach a MERV 13 furnace filter to a box fan. The Puget Sound Clean Air Agency tested this method with air monitors and found that a box fan filter reduced airborne particulate matter dramatically.

Box fan filters are easy and relatively inexpensive to make. The Puget Sound Clean Air Agency provides instructions on their website.

But, what if you don't have the drill and hardware or the brackets?

No problem. You can still make an effective filter by holding the filter to the back of the running fan and letting suction hold the filter in place. Once the fan is turned off the filter will detach.

Alternately, you can use duct tape or bungee cords to hold the filter to the fan. Our simple infographic explains how.



Public Health-Seattle King County has more advice for keeping indoor air clean. The San Francisco Department of Health has tips for those who can't stay indoors including wearing a N95 or P100 respirator if you are a healthy adult.