Assemble Your DIY SAFE Box in Five Fast Steps

What is a SAFE box?

"SAFE" stands for Supplementary Air Filtered Exchanges. It’s based on a peer-reviewed method of air filtration that increases the amount of clean air in classrooms. If you’re familiar with Corsi-Rosenthal boxes, you’ll find that the SAFE box is developed using similar principles. You can learn more in this deep dive from Devabhaktuni Srikrishna, the engineer who designed and tested this open source approach for homemade air purifiers.

What do you need to get started?

  • 1 20-inch box fan and its cardboard box
  • 1 circle cutter
  • 16–24 hook and loop Velcro strips, 3 1/2 x 3/4 inches (24 for pizza screen)
  • 1 pair of scissors or 1 box cutter
  • 1 5-inch MERV-16 Lennox air filter
  • 1 16-inch pizza screen (optional but recommended)

Where should you put your SAFE box?

Each unit should be placed in different parts of the room and be as distributed as possible, working within constraints of power outlets. Give each SAFE box at least 6 inches of clearance in front and behind to allow for proper airflow.

For safety, keep units on the ground with the fan standing upright. Also, the optional aluminum pizza screen can keep young kids from playing with or poking pencils into the fan.

How many SAFE boxes should you use?

The California Department of Public Health recommends at least 6 air changes per hour (ACH) and ideally as many as 12. SAFE box air purifiers used in combination with existing HVAC systems can ensure 5 to 12 total ACH.

For a typical classroom of 9000 cubic feet, one SAFE box purifier adds 2 ACH, so 6 units can achieve at least 12 ACH.

How do you build your SAFE box?

Step 1 - Trace and Use Scissors or a Box Cutter to Remove the Cardboard Piece

Step 1. Open your box fan, attach the legs and test the fan to make sure it’s working properly.

Step 2. Using the empty fan box, cut out the cardboard fan shroud.

  • Cut off the tops and sides of the cardboard box so that all you have left is the front piece of the box.
  • Trace the shape of your fan on the cardboard and trim it down until it is the same size as the fan (about 20 x 20 inches).
Step 2 - Use Circle Cutter to Remove the Middle Piece
  • Take 2 diagonal measurements in an "X" pattern to find the center of the new cardboard piece and mark the center of the box
  • Set your circle cutter so that the center of the tool is at 14 inches.
  • Place the point of the circle cutter at your "X" mark in the center of the piece of cardboard and cut out a 14-inch circle.
  • The cardboard circle that has been cut out is no longer needed—remove it from your workspace.
Step 3 - Add Velcro Straps to Fan and Filter, Attach Filter to Back of Fan

Step 3. Attach the filter to the back of the box fan.

  • Attach 4 strips of Velcro (one per edge) to the back of the box fan.
  • Attach 4 strips of Velcro (one per edge) to the the front of the filter.
  • Make sure the arrow of the filter points towards the fan and that the strips on the fan and filter line up with each other.
  • Connect the Velcro strips on the back of the fan and front of the filter to attach the two pieces together.
Step 4 - Add Velcro Straps to the Cardboard Shroud, Fan, and Optional Pizza Screen

Step 4. Add the optional pizza screen.

  • Attach four Velcro strips evenly spaced around the edge of the pizza screen.
  • Attach the 4 more strips to the back of the cardboard shroud so that they will line up with the screen.
  • Attach the pizza screen to the cardboard shroud.
Step 5 - Attach in Order the Cardboard Shroud, Optional Pizza Screen, and Front of Fan

Step 5. Attach the cardboard shroud to the front of the box fan.

  • Attach 4 Velcro strips to the four corners of the outside (decorative side) of the cardboard.
  • Attach the last 4 Velcro strips to the four corners of the front of the box fan so that they will line up with the strips on the cardboard.
  • Attach the cardboard shroud to the front of the box fan.
Step 6 - SAFE Box Is Complete
  • Your SAFE Box is now complete—enjoy cleaner air!


  • Never leave box-fan air filters unattended while turned on.
  • Adult supervision is recommended while using or assembling.
  • Check the filters periodically to see if they need to be changed.