People attach paper or filters to fans for one main reason: to turn a cheap box fan into a surprisingly effective air purifier. This trick gained massive popularity during the COVID-19 pandemic and wildfire seasons, but it’s also used for simpler purposes like cooling air, testing airflow direction, or spreading scent through a room.
DIY Air Filtration: The Most Common Reason
The most widespread version of this is taping a furnace filter (the pleated paper-like panels you’d normally slide into your home’s HVAC system) onto the back of a box fan. When the fan runs, it pulls air through the filter material, trapping dust, smoke particles, pollen, and other airborne irritants. The simplest setup costs about $35: one 20-inch box fan and a single MERV 13 furnace filter secured with duct tape or bungee cords.
A more advanced version, called a Corsi-Rosenthal box, uses four MERV 13 filters arranged in a cube shape with a cardboard base and the fan mounted on top blowing upward. The cube design pulls air inward through all four sides, giving the device far more filter surface area and better performance. This design was developed during the pandemic by engineers at UC Davis and has since been endorsed by the U.S. Environmental Protection Agency as a legitimate option for reducing wildfire smoke indoors.
How Well These Actually Work
The performance numbers are genuinely impressive. A DIY setup using four two-inch MERV 13 filters on a box fan running at its lowest speed delivered a clean air delivery rate of 293 cubic feet per minute. For context, a top-tier commercial HEPA purifier costing $549 (the Coway Airmega 300) delivered 282 cubic feet per minute at maximum speed. The DIY version cost roughly $35 and outperformed it while running quieter at low speed.
Thicker filters improve results further. A five-inch filter configuration reached 405 cubic feet per minute, though it costs more (around $120 total). The tradeoff between filter options comes down to thickness: a four-inch filter creates about 25% to 50% less airflow resistance than a one-inch filter, which means your fan doesn’t have to work as hard and moves more clean air. Thicker filters also last longer before they clog, since dust has more material to spread across before blocking airflow.
One important limitation: MERV 13 filters are effective for most household particles, but they’re not perfect for the smallest smoke particles. Filters rated below MERV 13, like MERV 11, capture only about 20% of particles smaller than 1 micrometer. So filter rating matters. MERV 13 is the minimum worth using for smoke or fine particulate matter.
Safety Considerations
The EPA recommends using only box fans that carry UL or ETL safety certification markings and were manufactured in 2012 or newer. Older fans may lack thermal safety features that prevent overheating if airflow is restricted. A filter does add resistance to the fan motor, so using a certified modern fan reduces fire risk significantly.
Dust buildup on fan blades or inside the motor forces it to work harder and produce more heat. Ironically, the filter itself helps prevent this by catching debris before it reaches the fan’s internals. Still, you should keep extra filters on hand and replace them when they become visibly dirty or discolored. Other signs a filter needs changing: noticeably weaker airflow from the fan, a musty smell, or the fan sounding louder than usual.
Evaporative Cooling With Wet Paper
A different use involves draping a wet towel, cloth, or paper over or behind a fan to cool the air. This works through evaporation: water needs heat energy to transition from liquid to vapor, and it pulls that heat from the surrounding air. The result is a slight drop in air temperature as the breeze passes through the wet material.
This method works best in hot, dry climates. In humid conditions, water evaporates slowly, so the cooling effect is minimal. It’s essentially the same principle behind swamp coolers. The paper or cloth increases the surface area exposed to moving air, which speeds up evaporation compared to just setting out a bowl of water.
Testing Airflow Direction
Technicians and homeowners sometimes hold a thin piece of tissue paper or toilet paper near a fan, vent, or duct opening to check whether air is flowing and in which direction. Lightweight paper responds visibly to even small air currents. This works especially well for bathroom exhaust fans or HVAC vents, where the airflow volume might be too low to feel with your hand but strong enough to move a strip of tissue. The smaller the opening, the faster air moves through it, making paper a reliable low-tech diagnostic tool.
Spreading Scent Through a Room
Some people attach scented dryer sheets to fan blades or tape them to the back of a fan to distribute fragrance throughout a room. As the fan circulates air, it carries the scent from the sheet much farther than it would travel on its own. The same approach works with ceiling fans: wiping the blades with a scented dryer sheet leaves a light fragrance that disperses each time the fan runs, with the added benefit of reducing dust buildup on the blades.