Tritium is a radioactive isotope of hydrogen. It has a half-life of 12.32 years and decays into helium-3 by beta decay. Tritium is produced in nuclear reactors, and also occurs naturally in the atmosphere as a result of the atmospheric nuclear weapons tests conducted in the 1950s and 1960s.
Tritium is used in thermonuclear weapons, as a tracer in isotope geochemistry, and as a radioluminescent agent in watches, gun sights, exit signs, etc. It is also used as a marker for water purity (tritiated water), and to label organic compounds (tritiated methane).
Tritium poses no health hazard outside of its radioactivity; it is not bioaccumulative and does not pose a radiation dose threat like other radioactive materials such as plutonium or uranium. The main risk associated with tritium exposure is internal irradiation from inhaling or ingesting tritium-containing material. Internal irradiation can damage DNA and cause cancer.