In the 1920s,
several US watch companies produced "glow in the dark" dials painted with radio-luminescent
paint, composed of zinc
mixed with radioactive radium (226Ra) salts. The employees hired to paint the
dials were mostly young women. The paint was applied to the
numerals with a small brush: as the brush became flattened, the
women "pointed" the
brushes on their tongues between applications, and thereby
ingested a small quantity of radium each time.
Radium is an alpha-particle emitter that is chemically
similar to calcium, and
is therefore a 'bone seeker'.
Once ingested, 226Ra
accumulates in the long bones, irradiates osteoblasts and
other nearby cells in the bone marrow with high-energy,
short-distance alpha radiation, and produces bone
cancer and other genetic damage. The graph shows that the
incidence of bone cancer increases with an increasing "body burden" of 226Ra. [For more information, see "The
found wide use in military aircraft in World War II, and radium
watches were manufactured into the 1950s [right]. Modern photo-luminescent
watches are light-activated, and do not use radioactive material.