The Corsi-Rosenthal Box is a simple, do-it-yourself air cleaner made from four furnace filters, a box fan, and some tape. But don’t let the simplicity fool you—it’s highly effective at cleaning the air in a room. Designed by a university engineer and popularized by the clean air community, this low-cost purifier is often just as powerful as commercial HEPA units that cost hundreds more.

It works by pulling air through high-efficiency MERV 13 or MERV 14 filters, trapping dust, mold spores, allergens, smoke, and even some airborne viruses. The large surface area of the filters and the strong pull from the box fan allow it to move a lot of air quickly, making it great for both small and large spaces.

If you’re looking for a way to clean your indoor air without breaking the bank—or need multiple purifiers in a large home—this DIY solution is one of the most affordable and reliable options out there.