Self-luminous 'EXIT' signs use radioactive tritium gas to glow for up to 20 years without electricity.

Tritium (H-3) is a radioactive isotope of hydrogen with a nucleus containing one proton and two neutrons. It has a half-life of approximately 12.3 years, which is why these signs are typically rated for a service life of 10 to 20 years. The technology is based on the principle of radioluminescence, specifically using beta particles emitted during radioactive decay.Inside the sign, tritium gas is hermetically sealed within borosilicate glass tubes coated internally with a phosphor material like zinc sulfide. As the tritium undergoes beta decay, it emits low-energy electrons. These electrons strike the phosphor coating, exciting its electrons to a higher energy state. When the phosphor electrons return to their ground state, they release energy in the form of visible light.The Nuclear Regulatory Commission (NRC) regulates these devices because they contain radioactive material. However, they are considered safe for the public because the beta particles emitted by tritium are very weak. They cannot even penetrate a single sheet of paper or human skin. The glass tubes provide an effective barrier that contains the radiation entirely within the device.This technology is highly reliable for emergency safety because it functions independently of the building's electrical grid. In the event of a fire or total power failure, the signs remain illuminated without the need for backup generators or batteries. This makes them ideal for remote areas or high-security facilities where maintenance must be kept to a minimum.