An Introduction to Making a Box Fan Air Filter for Wildfire Smoke

@novelistparty － April 2021

UPDATE August 2022: This is half-obsolete thanks to new techniques and products that were developed for the pandemic or are now much more readily available (again because of the pandemic). I recommend searching for commerical filters (like you'll see at a dentist office). They're quiet and have some guarantee of filtering ability. The box-fan filter is still often easiest to acquire and definitely cheapest, but are quite loud and only as good as you make it, but even then maybe not too good.

Introduction

I'm not a doctor. Talk with your doctor about your risk profile and specific precautions you should take. Follow CDC advice.

This page is only about an air filter for a room or rooms in your house. For more all-around advice about dealing with wildfire smoke, refer to health departments resources, like the CDC (RECOMMENDED) and this guide from Clark County, WA.

When you're choosing an air filter and looking at $60 for one, think about it this way: that filter could be doing 24/7 harm reduction for you for perhaps a couple months. That $60 spread out over 24/7 operation over a couple months is very economical. Try not to scrimp on costs now. That said, get what you can. Any reduction of smoke you can manage is helpful. Don't worry if you can't reach perfection. Do the best you can with what you have.

The Basics

UPDATE 2021: This twitter thread has instructions for building a box fan filter that is quieter and moves more air than the designs linked below. I've built one and I like it a lot.

(video) Build a do-it-yourself air purifier for about $25

Explainer of MERV ratings. Large particulates in wildfire smoke can be filtered by many techniques, but for the micro-particulates, the ones that can go straight into your bloodstream, you'll need a higher rated filter, MERV 12 as a minimum. MERV 14 is highly effective without the higher price of a MERV 15. Filters with Carbon in them can also reduce harmful gases in the air.

When it comes to filter depth, deeper is generally better because a deeper filter (20x20x5 vs 20x20x2) will load the box fan motor less than a shallower filter. However, a two inch filter will still effectively filter particulates from the air. I've been told the deeper filters can last longer because they clog more slowly (again, b/c of the larger surface area).

The average box fan in the US is about 20"x20". But not all of them are. Measure your box fan or the one you'll buy and order the filter that most closely matches it.

Supplies

If you can afford it, try to have a backup box fan and a backup filter. You never know when your fan might give out or your cat might shred the filter. And if you can afford more, get a few sets to give to neighbors and friends. When the summer wildfires start, most people will be caught off-guard and the filters will already be sold-out. Talk to your friends, family, and neighbors and ask about their preparations.

More Resources

This is only a collection of what I've gathered from my own searching. There is always more to learn and ways to improve air filtering. If you hear of any helpful resources or tips, or if I'm totally wrong about something, please let me know! You can find me on twitter as @novelistparty.