Glow-in-the-dark stars are a cherished childhood memory for many. You might have affixed them to your bedroom ceiling in elementary school, only to find them still casting their reliable light decades later during a visit home. Today, the market offers a wide array of glowing products, including sheets, pajamas, and paints.

Yet, what precisely gives these items their luminescence? And is the captivating, ethereal glow they emit truly harmless?

The Source of the Illumination

Certain minerals exhibit natural phosphorescence; when placed in darkness after exposure to light, they emit a glow. Dean Campbell, a chemistry professor at Bradley University in Peoria, Ill., notes that copper-treated zinc sulfide is a common material in glow-in-the-dark toys. When light strikes this substance, electrons detach and move freely. These electrons are then temporarily trapped by the copper particles embedded within the material.

Campbell elucidates that “when [an electron] returns home, it gives off a glow.” Once the energy supplied by the light has dissipated, the material ceases to glow. Throughout the day, electrons within ceiling stars perform this cycle, though we only perceive their illumination at night (or when all lights are off and curtains drawn). Strontium aluminate, activated with the rare-earth metal europium, represents another widely used component in these luminous goods.

However, this luminous effect typically fades relatively quickly once the light source is removed.

Why Luminous Items Can Cause Concern

In 1902, engineer William J. Hammer discovered that combining phosphorescent minerals with a substance capable of continuously supplying energy would enable them to glow indefinitely. Radium served as such a persistent energy source. A vibrant, perpetually glowing paint was created by blending radium and zinc sulfide with varnish.

Hammer experimented by applying this paint to various items. It eventually found use in painting watch faces, allowing them to glow in low light conditions. Beginning in 1917, the Radium Dial Company in Ottawa, Ill., employed workers who, believing the paint to be benign, moistened their brushes with their lips to achieve a fine point. Similar manufacturing facilities also existed in New Jersey and Connecticut.

The ingestion of radioactive radium and its contact with their lips and teeth led to severe degradation of their jaws and faces. Many of these “radium girls” suffered horrific deaths from radiation-induced cancer. A memorial commemorating these women now stands in Ottawa, according to Campbell.

He describes it as “a bronze statue of a young girl.” “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 radium-based paints are no longer used in timepieces.

Therefore, glow-in-the-dark materials haven’t always been safe, which likely fuels contemporary anxieties regarding modern luminous products.

Simply Avoid Ingestion

Despite their mysterious radiance, the phosphorescent substances utilized today pose no greater risk than numerous other everyday materials. Nonetheless, it is advisable not to, for instance, lick your glow-in-the-dark stars or consume the paint, just as one would avoid eating most non-food items.

The best practice is simply to affix them to the ceiling.