For anyone curious about Tritium, here's some text from an article with respect to watches:
Radiation Dose
A wearer of a radium-dialed watch will receive a gonadal dose of not more than 4 mrad/yr. This is to be compared to the natural radiation dose of 100-300 mrad/yr. A NY-LA flight will give a dose of about 1 mrad. The wearer of a metal-backed Tritium dialed watch receives a much lower dose: about 0.03 mrad/yr. The dose from GTLS watches should be significantly lower than 0.03 mrad/yr because the Tritium is sealed in glass capsules. The rad has dimensions of energy absorbed per unit mass, and is equal to 100 ergs/gram. The S. I unit is the Gray (Gy) and is equal to 1 Joule/kilogram. 1 rad = 1 cGy (centiGray).
See "Medical Effects of Ionizing Radiation", 2nd ed., Fred Mettler and Arthur Upton, W. B. Saunders, Philadelphia, 1995, chpt. 2, "Sources of Radiation Exposure", p. 42.
Tritium is a pure beta emitter, with a beta range of 6 mm in air, 0.0052 mm in water. Thus, the beta particles cannot even penetrate the dead layer of the skin, which is 0.07 mm thick. The danger from Tritium comes from ingestion or absorption of Tritium through the skin. The beta particles from a Tritium decay within the body can in fact damage cells.
Totally plastic watches ( i. e. with plastic backs = Swatch) containing Tritium paint can allow some Tritium gas to diffuse through the back of the watch and thence through the wearer's skin. The concentration of Tritium in the urine for these wearers is ten times higher than that of persons wearing Tritium watches with metal backs. This amount is EXTREMELY, EXTREMELY low. This gives an annual dose of 0.4 to 2 mrads, less than that of a Radium-dialed watch.
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