Last night as I was putting my keys in my pocket, I was stricken with fear--is this glow-in-the-dark key ring doing harm by its proximity to several vital organs? As a child I demanded that disgusting varieties of cereal be purchased so that I could obtain a skull ring that, when taken into a closet, glowed an eerie green. Was the cereal contaminated? What is that stuff, and how does it work? Should I change to a rabbit's foot to hold my keys?
The rabbit has more to worry about than you do. 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 visible light, ultraviolet light, and X-rays and for the most part return that energy in kind. All in all, they’re not nearly as 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 (I hope you remember your high school physics). 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.
As far as the phosphorescent trinkets in your cereal are concerned, the FDA has found no reason to classify them as anything different from the other giveaways that are packaged with food. Since they come in contact with the food surface, and generally have not been produced under health guidelines, the gimmicks must be packaged in an FDA-approved plastic or film. The problem is sanitation, not fallout.
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 two 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. Still, radiation is a funny thing (ain’t it?), and the scientific community is a long way from understanding all its effects. There’s always the outside chance that your children will be born with softly glowing letters spelling "TIMEX" across their stomachs.
Send questions to Cecil via firstname.lastname@example.org.