Stars glowing in the dark seem to be a symbol of childhood. Perhaps you attached them to your bedroom ceiling in fifth grade, then came home decades later to find them still glowing faithfully every evening. There are now glow-in-the-dark sheets, glow-in-the-dark pajamas, glow-in-the-dark paint, and more.
But what exactly glows inside these objects? And is the eerie glow that makes them so attractive really safe?
Where does the glow come from?
A number of minerals become phosphorescent naturally; put them in the dark after they have been lit for a while and they will light up. Copper-treated zinc sulfide is one of the substances commonly used in glow-in-the-dark toys, says Dean Campbell, a chemistry professor at Bradley University in Peoria, Illinois. Light hitting it causes electrons to break off and go on their way. Along the way, they are temporarily trapped by copper spilled through the matrix.
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Then, “when [an electron] returns home, it emits a glow,” explains Campbell. When the energy added to a substance by light dissipates, the material becomes dark again. All day long, the electrons in the stars on the ceiling perform this dance, although we only see it at nightfall (or when all the doors are closed and the curtains are drawn). Another commonly used substance in glowing products is strontium aluminate, treated with the rare earth metal europium.
However, this effect does not last long once the lights go out.
Why glowing things can bother us
In 1902, engineer William J. Hammer realized that if phosphorescent minerals were mixed with something that would energize them indefinitely, they could also glow indefinitely. Radium was one such long-lasting energy source. By mixing radium and zinc sulfide with varnish, a beautiful, permanently glowing paint was obtained.
Hammer experimented with drawing a variety of things with it. including light switches, toys and buttons. Eventually, they began to paint numbers on dials to make them glow in the dark. Beginning in 1917, the Radium Dial Company in Ottawa, Illinois, hired young women to draw numbersand, making sure that the paint was harmless, they licked the brushes to bring them to fineness. There were also similar factories in New Jersey and Connecticut.
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Ingesting radioactive radium and getting it on their lips and teeth meant that their jaws and faces soon began to fall apart, and many died horribly from radiation-induced cancer. According to Campbell, there is now a memorial to these “radium girls” in the city of Ottawa.
“It’s a bronze statue of a young girl,” he says. “She stands on the dial, holding a withered flower in one hand and brushes in the other.” Such paints are now no longer used in watches.
Thus, glow-in-the-dark materials have not always been safe, which may be why people are concerned about modern glowing objects.
Just don't eat it
Despite their spectral glow, phosphorescent substances used today are no more dangerous than many other substances used in everyday life. However, it is not recommended to, say, lick stars or consume paint, like most non-food items.
It's best to simply attach them to the ceiling.
