Tritium is a radioactive isotope of hydrogen that has two neutrons in the nucleus and one proton. It is produced naturally in the upper atmosphere when cosmic rays strike nitrogen molecules in the air, and it is also produced during nuclear weapons explosions and as a byproduct in nuclear reactors. Tritium is unstable and radioactive, and it decays by emitting beta radiation. The physical half-life of tritium is 12.3 years, meaning that it takes just over 12 years for tritium to decay to half of its original amount. Tritium is used in a variety of applications, including:

• Radioluminescent lights: Tritium can be combined with phosphor to create glow-in-the-dark lighting such as exit signs, emergency lighting in buildings, and airport runway lights.

• Medical and scientific research: Tritium is used as a tracer in biomedical research to study and diagnose heart disease, cancer, and AIDS.

• Nuclear fusion fuel: Tritium is used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and in hydrogen bombs.

Tritium is a relatively weak source of beta radiation, which means that it cannot penetrate the skin. However, it can pose a health risk if taken directly into the body in extremely large quantities. The most common form of tritium is tritiated water, which is formed when a tritium atom replaces a hydrogen atom in water (H2O) to form HTO. Tritiated water has the same chemical properties as water and is odorless and colorless.