Box-fan and air filter can reduce pollution in small, tightly-sealed room by 90% in 30 minutes

In 2011, Jeffrey E. Terrell, M.D. of the University of Michigan demonstrated the effectiveness of using a cheap box fan with furnace filter to improve indoor air quality. In his video he reduces particulate matter in a room by 90% using nothing more than a box-fan and filter.

The recent poor air quality in the bay area has caused many to wonder if this really works. So I've run a replication of that experiment.


A simple fan and MERV >=12 filter can significantly reduce indoor particulate matter by 90% in small tightly-sealed room. Compared to having no filter at all, this is a dramatic improvement.

The principal downside of this filter compared to a more expensive filter is that a box-fan filter will have a hard time keeping up with polluted air leaking in. A door being open for just a few seconds will very quickly reduce the air quality significantly. With normal traffic in and out of room, the box fan may never bring the air to a "good" quality level.



  1. The room was filled with dirty air fromFill room with dirty air. I opened the window and used the box fan to fill the room with outside air until the Dylos 0.5 um reading was at ~30k.
  2. All openings were sealed as much as possible. Windows and doors closed. Vents closed and blocked. Cracks under doors blocked.
  3. The air filter was placed on the back of the fan and it was set to high.
  4. 0.5 um count readings were taken from the Dylos sensor every 30 seconds.



Comparison to Other Purifiers

I accidentally left the window cracked open for the AustinAir trial. Darn.

See Also