Air purifiers are highly suboptimal and could be >2.5x better.

Some things I learned while researching air purifiers for my house, to reduce COVID risk during jam nights.

• An air purifier is simply a fan blowing through a filter, delivering a certain CFM (airflow in cubic feet per minute). The higher the filter resistance and lower the filter area, the more pressure your fan needs to be designed for, and the more noise it produces.
• HEPA filters are inferior to MERV 13-14 filters except for a few applications like cleanrooms. The technical advantage of HEPA filters is filtering out 99.97% of particles of any size, but this doesn't matter when MERV 13-14 filters can filter 77-88% of infectious aerosol particles at much higher airflow. The correct metric is CADR (clean air delivery rate), equal to airflow * efficiency. [1, 2]
• Commercial air purifiers use HEPA filters for marketing reasons and to sell proprietary filters. But an even larger flaw is that they have very small filter areas for no apparent reason. Therefore they are forced to use very high pressure fans, dramatically increasing noise.
• Originally people devised the Corsi-Rosenthal Box to maximize CADR. They're cheap but rather loud and ugly, but later designs have fixed this.
• (85% confidence) Wirecutter recommendations (Coway AP-1512HH) have been beat by ~2.5x in CADR/$, CADR/noise, CADR/floor area, and CADR/watt at a given noise level, just by having higher filter area; the better purifiers are about 2.5x better at their jobs. [3]
  • At noise levels acceptable for a living room (~40 dB, Wirecutter's top pick on medium), CleanAirKits and Nukit sell purifier kits that use PC fans to push air through commercial MERV filters, getting 2.5x CADR at the same noise level, footprint, and energy usage [4]. These are basically handmade but still achieve cost parity with used Coways, 2.5x CADR/$ against new Coways, and use cheaper filters.
  • At higher noise levels (Wirecutter's top pick on high), there are kits and DIY options meant for garages and workshops that beat Wirecutter in cost too.
• However, there exist even better designs that no one is making.
  • jefftk devised a ceiling fan air purifier which is extremely quiet.
  • Someone on Twitter made a wall-mounted prototype with PC fans that blocks fan noise, reducing noise by another few dB and reducing the space requirement to near zero. If this were mass-produced flat-pack furniture (and had a few more fans), it would likely deliver ~300 CFM CADR (2.7x Wirecutter's top pick on medium, enough to meet ASHRAE 241 standards for infection control for 6 people in a residential common area or 9 in an office), be really cheap, and generally be unobtrusive enough in noise, space, and aesthetics to be run 24/7.
  • A seller on Taobao makes PC fan kits for much less than cleanairkits (reddit discussion). One model is sold on Amazon for a big markup, but it's not the best model, takes 4-7 weeks to ship, is often out of stock, and don't ship to CA where I live. If their taller (higher area) model shipped to CA I would get it over the cleanairkits one.
  • V-bank filters should have ~3x higher filter area for a given footprint, further increasing CADR by maybe 1.7x.
• If I'm right, the fact that these are not mass-produced is a major civilizational failing.

[1] For large rooms, another concern is getting air to circulate properly.

[2] One might worry that the 20% of particles that pass through MERV filters will be more likely to pass through again, which would put a ceiling on the achievable purification. But in practice, you can get to the air quality of a low-grade cleanroom with enough MERV 13 filtration, even if the filters are a few months old. Also, MERV filters get a slight efficiency boost from the slower airflow of a PC fan CR box.

[3] Most commercially available air purifiers have even worse CADR/$ or noise than the Wirecutter picks.

[4] The Wirecutter top pick was tested at 110 CFM on medium; the CleanAirKits Luggable XL was tested at 323 CFM at around the same noise level (not sure of exact values as measurements differ, but the Luggable is likely quieter) and footprint with slightly higher power usage.