What Makes Stuff Glow in the Dark? And Is It Safe?

Glow-in-the-dark stars feel like a staple of childhood. You may have stuck them to your bedroom ceiling in fifth grade and then discovered them, on a visit home decades later, still lighting up faithfully every evening. There are now glow-in-the-dark sheets, glow-in-the-dark pajamas, glow-in-the-dark paint, and so much more.

But what exactly is glowing inside of these objects? And is the eerie glow that makes them so alluring really safe?

Where the glow comes from

A number of minerals phosphoresce naturally; put them in the dark after they have been illuminated for a while, and they will glow. Zinc sulphide treated with copper is one substance commonly used in glow-in-the-dark toys, says Dean Campbell, a professor of chemistry at Bradley University in Peoria, Ill. Light striking it causes electrons to pop off and go for a wander. Along the way, they get trapped for a while by the copper sprinkled through the matrix.

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Then, “when [an electron] returns home, it gives off a glow,” explains Campbell. When the energy added to the substance by the light has been dispersed, the material is dark again. All day long, electrons in the stars on the ceiling are performing this dance, although it’s only visible to us when night falls (or when all the doors are shut and the curtains are closed). Another commonly used substance in the glowing goods is strontium aluminate treated with the rare-earth metal europium.

This effect lasts only a little while, though, once the light is gone.  

Why things that glow can make us uneasy

In 1902, the engineer William J. Hammer realized that if phosphorescent minerals were mixed with something that would keep feeding them energy indefinitely, they could also glow indefinitely. Radium was one such long-lasting energy source. Mixing radium and zinc sulfide with varnish produced a beautiful, constantly glowing paint.

Hammer experimented with painting all kinds of things with it, including light switches, toys, and push pins. The paint was eventually used to paint the numbers on watch faces, so they would glow in the dark. Starting in 1917, the Radium Dial Company, in Ottawa, Ill., employed young women to paint the numbers, and, having been assured the paint was harmless, they licked the brushes to bring them to a fine point. There were similar factories in New Jersey and Connecticut as well.

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Ingesting the radioactive radium and getting it on their lips and teeth meant that their jaws and faces soon began to fall apart, and many died gruesome deaths of radiation-induced cancer. A memorial now stands to these “radium girls” in the town of Ottawa, Campbell says.

“It’s a bronze statue of a young girl,” he says. “She’s standing on a clock face, and she’s holding a wilted flower in one hand and paint brushes in the other hand.” Such paints are now no longer used in watches.

So glow-in-the-dark materials have not always been safe, which may be behind people’s worries about modern glowing objects.

Just don’t eat it

Despite their spectral glow, the phosphorescent substances used today are not more dangerous than many other substances used in daily life. That said, it’s not recommended that you, say, lick your stars or consume the paint, any more than you’d eat most non-food objects.