← All articles

Protection

Building a Corsi-Rosenthal Box Fan Air Cleaner

By Jason Curtis · 4 min read · Updated 2026-05-21

Person observing a small electric fan in a clean, minimalist indoor setting
Photo: cottonbro studio / Pexels

For about $65 in parts and 15 minutes of duct-taping, you can build an air cleaner that moves more clean air than a $400 HEPA unit. The Corsi-Rosenthal Box (CR Box) was invented in 2020 by Richard Corsi and Jim Rosenthal during the pandemic, and has held up in independent testing as one of the best dollar-per-CFM air cleaners you can make.

Why it matters

A standard 20-inch box fan plus four MERV 13 furnace filters delivers roughly 600 to 850 CFM of clean air when assembled into a cube. A UC Davis test of a five-filter version measured CADR climbing from about 600 to 850 CFM as fan speed increased. That's two to three times the CADR of most consumer HEPA units in the same price range, and the filters trap 90%+ of PM2.5 from wildfire smoke.

What to do

Materials (about $65 total)

  • One 20-inch box fan (Lasko, Hurricane, Utilitech). Newer fans with UL Standard 507 listing are preferred since they shut off on overheat.
  • Four 20" x 20" x 2" MERV 13 filters (Filtrete, Nordic Pure, or similar). Two-inch filters outperform one-inch.
  • One roll of heavy duct tape (3M or Gorilla brand).
  • A piece of cardboard for the bottom (the fan box itself works).

Assembly

  1. Stand the four filters on edge in a square, arrows on the frames pointing inward (air gets pulled through the filter, not blown).
  2. Duct tape every vertical seam, top and bottom. Press firmly. Any air gap is a leak.
  3. Cut a square of cardboard the size of the cube's base and tape it on as the floor.
  4. Place the box fan on top, blowing upward, so it pulls air through all four filter walls and exhausts out the top.
  5. Tape the fan to the top of the cube around the perimeter. Seal any gaps between fan and filter frames.

That's it. Plug in, run on medium or high.

Tweaks that boost performance

  • A "shroud" (cardboard ring) over the fan inlet reduces dead-zone losses and adds roughly 20% CADR. UC Davis tested this directly.
  • Five filters in a pentagon (instead of four in a cube) increases surface area and pushes CADR higher, at the cost of footprint.
  • Run on the lowest speed that gives you the noise level you can live with. Even low speed beats most plug-in purifiers.

What to avoid

  • Old box fans without a thermal cutoff. If you smell anything hot, unplug it.
  • MERV 8 or MERV 11 filters. They pass too much PM2.5. MERV 13 is the floor.
  • Reversing the airflow arrows. Filters are directional and lose efficiency the wrong way.
  • Leaving gaps in the duct tape. Even a thumb-sized hole drops CADR noticeably.
  • Running it unattended overnight for the first few nights. Verify it stays cool first.

Quick checklist

  • One 20-inch box fan (UL 507 listed preferred).
  • Four 20" x 20" x 2" MERV 13 filters, arrows pointing in.
  • Duct tape every seam, top to bottom.
  • Cardboard floor, fan on top blowing out.
  • Replace filters when visibly gray, typically every 3 to 6 months (faster during smoke season).
  • Build one per main room you spend time in.

Sources

This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.