build
Build a Corsi-Rosenthal box-fan filter tonight
Four 20-inch MERV-13 (or local equivalent: ISO 16890 ePM1 ≥50%) furnace filters taped to a 20-inch box fan. Tonight, the cost of one fan plus four filters, no skills required. Cuts indoor by 50–80% in a typical bedroom or classroom. Open-source design — anyone can build it.