You may recall the story of 17-year-old Eagle Scout David Hahn, who, in 1995, almost managed to build a nuclear reactor in the backyard shed of his mother's Michigan home. The story made headlines when local authorities got wind of the operation, triggering a federal response. The EPA was soon called in to clean up the home's backyard as a Superfund site. Turns out that among the many places from which Hahn had gathered his radioactive material was household clocks.
For decades, watch and clock makers used paint with radium, a highly radioactive substance, to create luminous, glow-in-the-dark dials and hands for clocks and watches. The common practice of coating watch dials and clock faces with radium, which has a half-life of 1,600 years, ended in the 1970s. These days, most glowing watches use a radioactive isotope of hydrogen called tritium or the radioactive element promethium, which have a half-life of roughly 12 and 3 years, respectively.