Ask Dr. Knowledge

What are mothballs made of?

December 14, 2009

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What are mothballs made of? They don’t smell the way I remember them from childhood. Has something changed?

In days gone by, mothballs were usually made of camphor. Waxy and white or clear, camphor comes from a few different plants, most notably the Asian camphor laurel. It can also be made from turpentine. Its fumes discourage insects, which is no doubt why plants with it evolved in the first place.

Camphor’s vapors also inhibit rust, making it a useful thing in a toolbox. When it burns, it does so with a cool flame, which eventually goes out, leaving almost no residue. It also turns up in some medicines for coughs and in Vicks VapoRub, since it can help relieve congestion.

Another historically important choice was naphthalene, a white solid derived from coal tar. Its odor is quite different from camphor’s, and it has crystalline sort of look. Most people would agree it’s nowhere near as pleasant-smelling as camphor, but it’s cheaper.

Naphthaline is quite flammable, so nowadays a lot of mothballs are made of the synthetic chemical pdichlorobenzene, which has a lower fire risk. I think it has the least agreeable odor. If you’re a man, you’ve almost certainly seen (and smelled) it: It’s used to make urinal deodorizing cakes.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box