Tritium Lights are illuminated with light sources that consist of glass tubes, that have been internally coated with phosphor and filled with tritium gas. Tritium gas (³H) is an isotope of hydrogen that emits low energy radiation in the form of beta rays, or electrons. These electrons stimulate the phosphor coating causing the tubes to continuously emit light. The mechanism is exactly the same as an electrical fluorescent lamp with the only exception being that the electrons are provided by the tritium gas rather than by an electric current. In this way Tritium Lights require no external energy source and, as such, are the most energy efficient lighting products available today. They are also referred to as radio-luminescent light sources, or wrongly, also called betalights.

Tritium gas (Hydrogen-3) is an isotope of hydrogen. It can be man-made, but is also a naturally occurring substance. It is a colorless and odorless, and much lighter than air. It is present in air and water all over the earth and is regularly ingested and breathed by everyone. All humans contain trace amounts of tritium and several other naturally occurring radioisotopes.

Exit Sign vs. Everyday Radiation
In the Code of Federal Regulations, Title 10, Part 20, the U.S. government has set the acceptable limit for radiation exposure from man-made sources at 300 millirem (= 3 mSv) per year for the general public. Therefore, as shown in this graph, even in the unlikely event that an Exit Sign is broken, there is no significant health risk.
The chart (opens in a new window) depicts some of the everyday radiation that we are exposed to in the course of our normal existence.