Are camp lanterns radioactive?

December 5, 2003

Dear Cecil:

My wife is about to take a group of Girl Scouts camping and needed a propane lantern. When I went to install the wicks, a warning label informed me that the wicks were radioactive. Being a nuclear medicine tech, I had access to a Geiger counter, and brother, they weren't kidding. Is Saddam taking his vengeance by slowly wiping out campers in this country? What gives?

Cecil replies:

Life is full of little surprises, isn't it? Your wife was probably looking forward to a night under the stars, far from the worries of modern society. Now she finds out that the camp lantern hissing cheerfully on the picnic table is pumping out alpha particles. Makes one long for the good old days, when all a Girl Scout had to worry about was poison ivy and curious bears.

But you know what? Radioactive lantern wicks aren't a modern threat. Properly known as thorium gas mantles, they were perfected in 1891 by Austrian chemist Carl Auer von Welsbach (1858-1929). Gaslights using the thorium mantle (typically a thumb-size cloth bag coated with a thorium compound) offered a big advantage: because the thorium could incandesce at extremely high temperatures without melting, they were far brighter than ordinary lamps. Soon they were being used for household illumination in gaslit cities everywhere. Gas company moguls told themselves: This will enable us to kick the electric light's butt.

OK, doomed effort. But the thorium mantle did enable the American gas lighting industry to survive a surprisingly long time. At its peak, it sold 40 million mantles annually. Mantles were manufactured in quantity here as late as the 1940s — later in other countries. Some streets in London (and at least a few in the U.S.) are lit with gas even now.

In retrospect we'd have been luckier if thorium mantles hadn't worked quite so well — mantle production has left behind a formidable environmental mess. Radioactive leftovers from the manufacturing process contaminate soil near former mantle factories to this day. Were it not for the federal Superfund cleanup program, they'd be doing so for quite a few more days — the most common isotope of thorium has a half-life of 14 billion years.

Given such problems, one might think that what remains of the gas lighting industry — the camping lantern industry, more or less — would have switched to an alternative technology, or else decided that the lights over our nation's picnic tables didn't need to be quite so bright. Some companies did, but not all of them. Coleman, the best-known U.S. manufacturer, phased out thorium gas mantles in the early 90s. (Today its mantles are made with nonradioactive yttrium.) Other firms still sell thorium mantles but have moved their factories to developing countries.

The factory workers arguably have a lot more to worry about than you, your wife, or the Girl Scouts. Thorium's radioactivity is mostly emitted as alpha particles, which only travel a couple centimeters in air and can be stopped by a sheet of ordinary paper. A study conducted at Oak Ridge National Laboratory in 1981 concluded that using a Coleman-type lantern with thorium mantles every other weekend for a year would expose you to a radiation dose of 0.3 to 0.6 millirems, far less than the normal background dose of a few hundred millirems per year.

You don't want to carry thorium mantles in your pocket, though, or inhale or ingest the dust from a burnt mantle. With thorium atoms parked in your lungs or liver, your exposure would shoot up to 200 millirems per year (and stay there — remember that half-life). That strikes me as a nonnegligible danger, but even the higher dose is modest, and most say the risk is manageable.

In the meantime, look at the bright side. Your wife can entertain the Girl Scouts with numerous educational demonstrations about the wonders of the atom. For example, you can build a kitchen-table cloud chamber using dry ice and alcohol — drop in a thorium mantle and you'll be able to see the condensation trails left by the radioactive particles.

If the kids are really ambitious, and I wouldn't put it past some of the little Madame Curies I know, they can build their own model breeder reactor. In 1994 17-year-old David Hahn did just that in his mom's potting shed near Detroit as an outgrowth of work on a Boy Scout merit badge. He used tinfoil, duct tape, uranium powder from ore, radium from old luminous clock dials, americium from smoke detectors, and thorium ash from thousands of mantles. Even after the thing was disassembled, local radiation levels were 1,000 times background. One appreciates enterprise, but cheezit, kid, couldn't you stick to helping old ladies cross the street?

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