In 1898, Marie and Pierre Curie announced their discovery of a new element that they named radium.
Marie, a two-time Nobel laureate, went on to study and characterize this highly radioactive substance. Its various salts were all self-luminous and shone in the dark like tiny electric light bulbs. Close contact with radium could result in serious skin burns. Its presence was found to sterilize plant seeds, heal some cancers and kill microorganisms.
Because of its success as a cancer treatment, it somehow became an all-powerful health tonic. It became an additive in a number of everyday products from toothpaste to cosmetics and even food and drink. It was much later that it became apparent that radium was poisonous and anything but healthy, unless properly used.
In the 1920s, several U.S. companies started producing "glow in the dark" watch dials as well as other types of dials where luminescence was useful. The dials were painted with zinc sulfide mixed with radium 226 salts. The people employed to paint the dials were mostly young women working in unventilated rooms and wearing smocks they laundered at home, thus contaminating their living area.
Initially, the women did not know the risks of radium to their health. They even enjoyed painting it onto their fingernails, jewelry and clothing in order to have them glow in the dark when going out on dates.
To paint the tiny watch dials, the workers would "point" their brushes in their mouths by licking and shaping the bristles, thus ingesting some of the radium. Over time, this practice caused serious and ugly jaw bone degeneration as well as malignancy and other dental problems. This exposure to radium later led to over 30 deaths among radium dial painters.
Radioactive dials found wide use in military aircraft and equipment in World War II. Radium watches were manufactured into the 1950s.
Today, luminous dials in all kinds of instruments have been largely replaced by safer phosphorescent light sources.
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