Holy fuck
This works best if you keep windows closed.
Another design is using 2 20x25x1 filters, taping them to the sides of the box fan and then to each other so they sort of make a triangle, then cutting cardboard to make a top and bottom to the triangle.
This was discovered as a more effective design during the 2020 US west coast fires.
https://tombuildsstuff.blogspot.com/2013/06/better-box-fan-air-purifier.html
Better Box Fan Air Purifier A better more efficient and odor eliminating homemade air purifier than just taping a 20x20x1 filter to a box fan. Sometimes you need to tombuildsstuff.blogspot.comIf you live on the west coast of the United States, fire season is coming and this is vital.