In Cecil Adams' column "The Straight Dope," which has appeared in alternative newspapers since 1973, he answered the following letter:
"What is the stuff that goes into glow-in-the-dark products? How does it work? Is it harmful?"
Adams replied: "The phosphorescent pigments used in making glow-in-the-dark items do emit X-rays, but at such a low level that they're barely detectable. Phosphors absorb energy from visible543975783as harmful as that flood of electromagnetic radiation we call sunlight.
"The phosphor generally used in pigment is calcium sulfide, 'activated' by bismuth, with additional traces of copper, silver or lead. When light energy strikes the phosphor's atoms, some electrons are kicked up into high-energy orbits. Later, these electrons fall back down into their regular orbits, releasing some electromagnetic radiation in the process, mainly in the form of visible light. Hence, phosphors work as a sort of energy storage system, gathering up light and slowly releasing it as the stimulated electrons return to normal.
"Some phosphors, used mainly in watch dials, are genuinely radioactive. Instead of being activated by light, zinc sulfide crystals are stimulated by alpha rays, supplied by trace amounts of radium--no more than 2 parts per million--mixed into the pigment compounds. Again, the best information available at the moment finds no real threat in this low-level radiation, and the amount of radium used is strictly controlled by no less an authority than the Nuclear Regulatory Commission."167772161