Radium dial

• Undark produced by the United States Radium Corporation
• Luna produced by the Radium Dial Company
• Marvelite produced by the Cold Light Manufacturing Company (a subsidiary of the Radium Company of Colorado)

Radium was discovered by Marie and Piere Curie in 1898[1] and was soon combined with paint to make luminescent paint, which was applied to clocks, airplane instruments, and the like, to be able to read them in the dark. [2]

In 1914, Dr. Sabin Arnold von Sochocky and Dr. George S. Willis founded the Radium Luminous Material Corporation. The company made luminescent paint. The company later changed its name to the United States Radium Corporation.[3]

Radium dials were typically painted by young women, who used to 'point' their brushes by licking and shaping the bristles prior to painting the fine lines and numbers on the dials. This practice resulted in the ingestion of radium, which caused serious jaw-bone degeneration and malignancy and other dental diseases reminiscent of phossy jaw. The disease, radium-induced osteonecrosis, was recognized as an occupational disease in 1925 after a group of radium painters, known as the Radium Girls, from the United States Radium Corporation sued. By 1930, all dial painters stopped pointing their brushes by mouth. Stopping this practice drastically reduced the amount of radium ingested and therefore, the incidence of malignancy, to zero by 1950 among the workers who were studied.

"Luminous Processes employees interviewed by a journalist in 1978 had been left ignorant of radium's dangers. They were told that eliminating lippointing had ended earlier problems. They worked in unvented rooms, they wore smocks that they laundered at home. Geiger counters could pick up readings from pants returned from a dry cleaner and from clothes stored away in a cedar chest."[4]

Although old radium dials may no longer produce light, this is frequently due to the breakdown of the crystal structure of the luminous zinc sulfide rather than the decay of the radium. The radium isotope (Ra 226) used has a half-life of about 1,600 years,[5] so even very old radium dials remain essentially just as radioactive as when originally painted ~100 years ago, whether or not they still are luminous. Care should be taken in handling these materials, especially where the paint is damaged, to prevent the inhalation or ingestion of flakes or dust which may contain still radioactive materials.

Radium paint can be ingested by inhaling flaking paint particles. The alpha particles emitted by the radium, which is taken up in bone, will kill off surrounding bone tissue, resulting in a condition loosely referred to as radium jaw. Inhaled or ingested particles may deposit a high local dose with a risk of lung or gastrointestinal cancer due to the radiation dose. The risks are hard to quantify due to the variable levels of radium in the paint and the quantity ingested or inhaled.

The most common isotope of radium is ²²⁶Ra, which most commonly emits alpha particles.[5] Alpha particles are shielded by most thin materials, including paper. ²²⁶Ra emits only alpha particles 97 percent of the time, however for the remaining 3 percent of decays it emits 186 keV gamma rays. Therefore, the radiation hazards from radium paint may not be entirely shielded by the overcoat varnish, case, crystal, or container.

In addition to the danger of gamma rays from radium, ²²⁶Ra is rarely present without traces of other transuranic elements or their decay products, which may emit beta or gamma radiation. Since the transuranic elements have very different nuclear properties, their contributions to watch dial gamma and beta radiation is difficult to estimate: Some relatively rare trace elements can produce prodigious amounts of penetrating radiation, and other, more common trace elements can produce only the less harmful alpha radiation (which is only harmful if the radiator is ingested).

Radium dials held near the face have been shown to produce radiation doses in excess of 10 µSv / hour. After about 20 minutes this delivers the equivalent of one whole day's worth of normal background radiation. This rate probably only represents the dose rate from gamma emission, as the alpha emission will be stopped by the lacquer, or crystal, or case; hence, the dose rate following ingestion or inhalation of the dust could be much higher.

• Radioactive phosphors

• Radium Girls: Women and Industrial health reform, 1910–1935, ISBN 978-0-8078-4640-7