The gist of this post is that ‘better’ isn’t necessary; any will suffice. So long as there’s airflow traveling through the filter pleats, your air will get cleaned.

If you’re in a wildfire scenario, you should — if you can afford to and can still find and purchase the parts — absolutely go the extra mile, because you need to maintain a filtration level sufficient to overcome the particulates brought in with outdoor air.

Regardless, any time you have a box fan, just tying a filter to it will clean your air as effectively — but not as rapidly — as a commercial filter, under typical home scenarios.

They are indeed awesome and I made one with a simple box fan. It works great but it’s really loud. We can’t really have it on when company comes over.

We got one of these instead and have been really happy with it. Pros: super-quiet low-energy computer fans. Uses standard air filters you can buy at Lowe’s. Cons more than a box fan.

https://www.cleanairkits.com/products/brisk-box

They are awesome and cheap but take up a lot of room.