The fan/filter is a DIY and low-cost strategy to effectively remove particles in the air. Our team recommends using a MERV-13 20×20 4″ filter for desired filtration and longevity. Through our Low-Cost Monitoring Project participants, we have seen a significant reduction of particles (0.5+ microns) in homes with the use of a fan/filter.

People making fan filters using box fans

What You’ll Need

The fan/filter can be placed in a window to filter outdoor air as it comes into your home, or it can be used within a space. Fan/filters are particularly useful if homes rely on open windows during the summer months for comfort.


  • MERV 13 20×20 4” furnace filter
  • 20×20 box fan
  • Foil-faced duct tape or cellophane tape
  • Scissors/utility knife

Screencap of filter with caption MERV 13 Filter 4 inches thick captures particles down to .3 microns


  1. Find the arrow indicating airflow on the filter; make sure this matches the direction of the fan’s airflow.
  2. Place the filter on the box fan. We recommend placing the filter on the front of the box fan if placed in a window. The filter can be placed on the back of the box fan if used to recirculate the air within a room.
  3. Secure the filter to the fan by using tape; completely seal the filter to the fan with tape.
  4. Plug in the fan/filter and run on low. Your DIY fan/filter is complete!
  5. Change the filters as needed. A 4″-deep filter should last for 6 months of continuous use if used within a room rather than to filter air coming in a window.

Screengrab of woman holding filter with caption July August September