Cleaning the air you breathe is a really great idea. Having good filtration:

• Reduces the spread of COVID-19 and other respiratory diseases.
• Keeps you safe from lung damage due to wildfire smoke, if your area is affected by this.
• Helps with allergies.

Buying a filter

• This is a cheap model that comes highly recommended (via Twitter).
• The Coway Airmega is what Wirecutter recommends, and my relatives have one and love it.

DIY-ing a filter

Corsi-Rosenthal Box (Do this!)

“Clean Air Crew” has an awesome page on how to build and use Corsi-Rosenthal boxes. I highly recommend that you build one of these!

Here are some other links about CR Boxes:

• A thread on how to build them from David Elfstrom.
• Comparison of different air purifiers from Marwa Zaatari.
• Marina Creed points out that CR Boxes are listed in Johns Hopkins’ “Ventilation: Vital Tool to Reduce Spread” paper.

Not-as-good DIY version with a single filter

Here’s what I did during 2020, before I’d come across the CR Box design— if I had to DIY it again, I’d just make a CR Box! Right now I have one I bought and I run it in my bedroom to help with dust allergies. The main difference between my DIY attempt and a CR Box is that I just use a single filter here, whereas a CR Box is a cube with 4-5 sides that are filters, and the remaining side is a box fan. That results in way higher air flow, which is basically the most important metric in determining the efficacy of a filter.

• I ended up going over to Home Depot and buying:
  • 20” Box fan ($20) link
  • 20” x 20” x 1” HVAC Air filter ($20) link
  • And a roll of duct tape
• Then I taped the filter over the front of the fan very carefully. You need to make sure it’s really airtight all around; Because the fan’s corners are rounded and the filters’ corners aren’t, I went over the fan corners first, leaving a gap near the fan body, then came back and did another layer of tape on each corner to seal it well.
• I added a fan shroud on the back with just duct tape.

Discussion of air purifiers, how to use them, what you should care about, etc.

• Here’s a cool video from UMich.
• As I dug around a bit more, I found this site, which has a blog with some good write-ups
• Here’s a test of whether DIY purifiers work.
• And the source for the design they test there
• Here’s an alternate design, that uses 2 filters and a bit of cardboard:
• Some things about making your filter efficient, regardless of which design you choose:
  • Adding a fan shroud should improve efficiency
  • Doesn’t really matter whether you put the filter on the front or back of the fan
  • How close to the wall can you put it? (You can get pretty dang close without affecting performance!)
    • Smart Air Filter’s writeup of a test.
    • Matthias Wandel has a good video on this too, from his shop air filter series.
• And then a good analysis of whether you should keep your purifier on all day. The answer is that air quality is highly responsive to your purifier being on— if it’s on, your air quickly becomes clean, and then as soon as you turn it off air quality rapidly degrades. In this test, the air got to its resting “purifier running” state of air cleanliness in 20min; after turning it off, air returned to its resting “purifier off” state in 80min, but it got halfway there in 15min.
• This guy has also done some great experiments and is a doctor, if you care about credentials.
  • Any air pollution is really quite bad for you
  • Are there any actual studies on air purifiers and health? (Not really, unfortunately)
• And then this person literally just strapped some HEPA purifiers to a box fan… and it seems to have worked? They don’t even cover the whole back of the fan… but they measured particulate levels and it seems to have worked just fine. Confusing to me, but okay. Original post, HN post