Why Do Ceiling Fans Get Dusty?

You'd think, says reader Loren Larson, that the constantly turning blades would throw off any incidental dust that accumulates on a ceiling fan, particularly the blades of ceiling fans. But you'd be wrong. Ceiling fans seem to be dust magnets.

Your house or apartment, we say without insult, is full of dust. In the hair-raising first chapter of the marvelous The Secret House, David Bodanis notes that tens of thousands of human skin flakes fall off our body every minute.

"Luckily" for us, there are millions of microscopic mites in our abodes, insects that dine on the skin that we shed. Bodanis estimates that just within the average double bed mattress, two million dust mites live on our discarded skin and hair. Each mite defecates perhaps twenty times a day; their fecal pellets are so small that they float in the air, circulating around the house. Despite the millions of insects who depend upon our shedding skin for their survival, human skin and hair is by far the largest component in the dust found on ceiling fans and throughout the house. Makes you want to run out and get an air filter, doesn't it?

Ceiling fans create a tremendous amount of air flow, and dust is thrown around the room. But much lands on the fan and its blade, and just seems to sit there. Charles Ausburn, of Casablanca Fan Company, pleads guilty, but with an explanation:

"The air always has a great deal of dust in it -- larger particles that you can see, and also microscopic ones. Over time, a large volume of the circulating air hits and collects on the blades of the fan. People often ask why spider webs and dust can be seen on the fans. But they must understand that there is a lot of dust in the circulating air."

But the accumulation of dust on a given object is not random. Most dust particles carry an electrical charge, and therefore can be attracted to one another (a dust ball is simply an accumulation of charged dust particles that have a fatal attraction). Physicist Chris Ballas, of Vanderbilt University, explains:

"The charged dust particles are attracted and cling to any surface that develops a charge. This can be electrical equipment, which directly carries electric current) or a surface subjected to frictional forces, which result in a static electricity build-up. The latter is the case for ceiling fans. As the blades rotate, they experience frictional forces as they `rub' against the air; this knocks electrons around, causing the blades to build up a net charge. The charged dust particles then stick to the charged areas of the blades.

"The leading edge of the blades usually develops the thickest layer of dust. That's because the leading edge [the edge first cutting the air as the blade spins] encounters the most friction and develops the largest charge.

"So the dust doesn't collect on the blades simply by `falling' or landing on them. The electrical-attraction effect also plays a large part. This same effect explains why some vertical surfaces also get quite dusty (television and stereo equipment, for example). The dust doesn't just fall off these surfaces -- it sticks due to the electrical attraction."

Submitted by Loren A. Larson of Orlando, Florida. Thanks also to Crystal Lloyd of Perryville, Kentucky.

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