The onset of the COVID-19 pandemic—caused by a disease that spreads, in part, when an infected person breathes out droplets and very small particles that contain the virus—gave rise to the need for quick innovation across industries, often spurred by scarcity of resources. From developments in personal protective equipment (referred to as PPE) to the transformation of remote education, the effects of the pandemic have uncovered (and provided for) needs we were largely unprepared to deal with.
One such innovation is the development of the Corsi-Rosenthal box. Nicknamed the Corsi cube, these DIY indoor air purifiers have been taking classrooms and other under-ventilated spaces by storm. Thanks to the work of two men, Jim Rosenthal and Richard Corsi, the combination of a handful of air filters, a box fan, and some duct tape has now become a well-known substitute for traditional, and expensive, air purifiers.
In the era of COVID-19, no idea too primitive can be discarded when it comes to protecting those around us. Corsi cubes are simple, inexpensive air cleaning devices that can actually remove substantial amounts of particles from the air. In August 2020, Richard Corsi, who was the incoming Dean of Engineering at the University of California, Davis, challenged STEM Twitter to devise a citizen science solution after previously demonstrated how a fan with filters on three or four other sides could reduce the “pressure drop” that can come from putting a filter directly in front of a fan and still move room-sized volumes of air while filtering them as effectively as some of the best, most expensive air purifiers available.
So, Jim Rosenthal, who had been Corsi’s collaborator and friend for over 15 years, decided to create the air cleaner he’d conceived. His design came to be known as a Corsi-Rosenthal box, or Corsi cube. (Read the initial media story that brought attention to the creation, written by Adam Rogers and published on Wired, for more information.)
Supplies needed: standard 20″ box fan, 5 MERV-13 furnace filters*, duct tape
Supplies optional: weather stripping, cardboard
*Note: You will want to purchase 20" sized MERV-13 filters. For the 3M Filtrete brand of furnace filters, this is "FPR 1900." If MERV-13 is not available, MERV-11 filters can be used but performance will not be as great. Important note: The FilterBuy brand MERV-13 filters, common on popular sites like Amazon, have been shown by multiple independent testers to not perform as expected; it’s best to avoid this brand if possible.
Steps:
Duct tape the filters together, forming an incomplete cube. Try to avoid taping over the filter media part of each filter. Important: You want each filter's airflow direction to point into the cube, not out; when taping the filters together, make sure to arrange each filter so that the air intake direction of the filter goes inward. The filters should indicate which direction the air is supposed to flow.
Slide the open face of the cube to the back of the box fan and securely fasten with tape.
Use additional tape as needed to ensure the items are securely attached.
Consider using weather-stripping in between connections (e.g., filter-filter, filter-cardboard, and/or filter-fan) before taping. This will help reduce leaks between filters, ensuring that the air flows through the filters instead of around them.
Consider adding a fan shroud to improve the rate of air flow. The shroud can be made by using additional duct tape or cardboard to create a frame on the front of your box fan. The idea is to cover the corners without covering any of the fan itself. The shroud helps to block any unwanted backwards airflow that would lower the overall airflow and effectiveness of the unit.
During extreme events such as a pandemic, the availability of off-the-shelf air cleaners (or most things, really) is limited; therefore, using alternative means of creating the same contraption ensures a greater likelihood of availability of supplies. Additionally, DIY air cleaners can be used in a variety of indoor spaces, such as schools, offices, and/or your personal residence.
