In the early part of the 20th century, watches were produced with hand-painted dials that glowed in the dark. The paint contained ZnS phosphor and radium, which decayed to produce alpha particles that excited the ZnS to glow when struck by a charged particle. The people (mostly women) who painted the dials often put a fine point on their paintbrushes by rolling the brush on their tongues. Many workers in this industry subsequently died of bone cancer and leukemia when their bone marrow was destroyed. Imagine that you are writing a letter from the future to a family member working in the early-20th-century watch industry and explain to your relative what the hazards are and what safety equipment your relative should use.