Dumpster Fire, Forest Fire, Easy DIY Air Purifier

It has been a YEAR.  Trash Panda hopes you've been staying sane and safe. Well, as much as you can be in the year of our Everlasting Dumpster Fire 2020 (Update July 2021: And the year of our Great Anxiety 2021).  

Speaking of fires - today's post is about how to make a cheap & effective DIY air purifier. Depending on the rating of the filter you use, it’s good for wildfire smoke, allergens, or even viruses.

Parts needed: 

   + 20"x20" box fan

   + Furnace air filter, preferably 20"x20" and with a rating of MERV 13 or higher (more on this later)

   + Tape or bungee cord

Tape or use a bungee cord to attach the filter on the back/air intake of the box fan and make sure to have the little arrow on the side of the filter facing towards the fan. That's it. Just use enough tape to keep the filter on there, and try not to cover too much of the filter area with tape.

You can screw on little metal brackets on the side of the fan to create a more permanent way to slide the filter on the back, if you're feeling fancy.

Notes on use: 

When you’ve got bad air quality, keep your windows and doors closed while you run your DIY air purifier. That will keep at least some of the particulates from coming inside. As the air in the room recirculates through the purifier, it will keep removing those nasty little particulates and your indoor air will get cleaner and cleaner.

I’d recommend having one DIY air purifier per room, and giving the poor little buddy a break by turning it off while you’re not home.

A warning: 

Box fan manufacturers warn against blocking the intake of their fans in any way for fear of the risk of the itty bitty fan motor overheating and catching something on fire.  I've been running my trash panda air purifier for two days straight and the fan has not gotten any warmer. But I'm a trash panda. So if you're more cautious than a blogging racoon, only use your box fan air purifier when you're there to keep an eye on it, and don't run it when you're asleep.

Update July 2021: I’ve been using this DIY air purifier for almost a year. No malfunctions, worked like a charm for the Season of Smoke and even helped my dust mite allergies. I just picked up a new MERV 13 filter to tape on the back because my first filter (a mere MERV 10) was looking real dusty. And gods help us, it’s almost fire season again on the West Coast. 

On fans: 

I can guarantee that if you live in the American West, every handy person in your area already executed on this air purifier idea before I even starting typing this post.  This means that box fans might be hard to find, especially since this summer has been hot as hell. A quick check shows Grainger has them available in Seattle now, but Home Depot and Lowe's won't have them til the end of next week. 

On air filters: 

I can guarantee that if you live in the American West, every handy person in your area already executed on this air purifier idea before I even starting typing this post.  This means that 20"x20" furnace air filters with a rating of MERV 13 or higher are sold out almost everywhere. 

First off, feel free to improvise a bit with the size of filter you use for your fan. You could tape on two 10"x20" filters right next to each other. You can definitely use a filter larger than 20"x20", though you might need to get a little inventive attaching it to the back of your box fan. You can even use one slightly smaller filter. Your DIY air purifier will still be (relatively) effective as long as most of the fan area is covered by filter. A quick search of Grainger's website showed that they had plenty of usable filters in stock as long as you're willing to be flexible with your size choice. Note: Use a one inch thick filter rather than a two inch thick one if you can help it - the thicker filter really restricts air flow.

Secondly, even MERV 10 rated filters can be useful for our purposes, i.e. wildfire smoke where the big problem is air particles 2.5 microns or smaller (P2.5).  I will say that the Puget Sound Clean Air Agency recommends using a filter rated MERV 11 to 14.

As you can see on this useful chart from the kind people at LakeAir*, filters rated MERV 10 and higher will trap 50% or more of the air particles between 1.0 and 3.0 microns. Please note that even a MERV 10 rated air filter will remove pollen, dust mites and mold spores from the air, making your DIY air purifier useful all year round.

You're going to want a filter MERV 13 or higher for sure if you want something effective at filtering COVID particulates out of the air. MERV 13 is good for removing sneeze droplets from the air and higher ratings are rated for viruses. And of course higher ratings are even better at removing wildfire smoke particulates. 

Visual Learner? Some Videos: 

The Washington Department of Ecology has a quick and easy instruction video: 

https://youtu.be/4qr1Aj6Di7w

A very nice woman from the Colville Indian Nation walks you through the process of making a one filter and a two filter DIY box fan air purifier: 

The University of Michigan made the original video ages ago on the how and why of this particular air purifier design:

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*not an affiliate link or anything, they just have a very useful chart - thanks LakeAir!