Corsi-Rosenthal Box: $70 of box fan and furnace filters beats costly HEPA purifiers

In August 2021, with the Delta variant surging as the school year began, ER nurse practitioner Hillary Creech's back-to-school list was a box fan, high-quality air filters and lots of duct tape — parts for a homemade purifier for the high school where her husband teaches. The design, the Corsi-Rosenthal Box, is four standard furnace filters taped into a cube with a 20-inch box fan on top: about 10–20 minutes to assemble, $70–120 in materials, and it should last an entire school year.

Richard Corsi, incoming dean of engineering at the University of California, Davis, floated the idea on Twitter; Jim Rosenthal, owner of Tex-Air Filters, built the first box, and the two agreed to share credit in the name. Corsi cited people reporting clean-air delivery rates of 600 cubic feet per minute — 'better than a lot of the more expensive HEPA-based portable air cleaners' — while warning that many devices being heavily marketed to school districts hadn't been rigorously tested.

The backdrop: a Government Accountability Office study had found tens of thousands of schools with heating, ventilation and air conditioning systems needing attention, no national requirement keeps school air healthy, and Harvard's Joseph Allen noted nothing checks indoor air the way fire alarms and elevators are checked. Studies had long tied better-ventilated classrooms to higher test scores and fewer sick days.

600 cubic feet per minute of clean-air delivery was being reported from DIY builds — beating many much pricier HEPA portables.

Parts cost $70–120 and assembly takes 10–20 minutes, so a parent rather than a procurement office can deploy one.

It uses standard furnace filters and an ordinary box fan — parts any hardware shop stocks.

It arrived where institutions had failed: no national requirement, no oversight, and tens of thousands of schools with ailing HVAC.

A named, open design made of commodity parts can outrun official procurement: when districts can't buy clean air, parents can tape one together in an afternoon.

Don Blair pulled resources and step-by-step instructions together on a website for parents and teachers, and builders reporting results to Corsi were getting 600 cfm. NPR framed the pandemic's silver lining as raised awareness of indoor air quality in classrooms, plus new tools to act on it. Creech's box went to the local high school where her husband teaches.