Tritium Exit Signs

Tritium gas (chemical symbol H-3) is an isotope of hydrogen gas, colorless, odorless, and lighter than air. Most physical characteristics are the same as those of hydrogen. Still, the atoms in Tritium are slightly different, causing them to emit low-energy radiation in the form of beta particles, or electrons. Tritium is considered “user-friendly” because it has the lowest energy rating of any radioisotope.

In the 1970s, Tritium began to be used in exit signs as a safer alternative to radium. Radium had previously been used for luminescence in exit signs, but was phased out due to its higher radioactivity and safety concerns, with Tritium becoming the preferred option. Radium had been the preferred self-luminous material for most of the 20^th century until the early 1970s. In the early 1900s, radium was believed to have health benefits and was added to many consumer products, including food, hair products, and toothpaste. More notably, radium was used in watches, clocks, and other components requiring luminescence. The tragic story of the “Radium Girls” revealed the health dangers of radium exposure.

Tritium is used in exit signs because it enables the signs to remain illuminated continuously, even during power outages or emergencies, without the need for electricity or batteries. Tritium is a radioactive isotope of hydrogen that emits low-energy beta particles.

Because tritium exit signs do not rely on any external power source, they will continue to glow during power failures, fires, or any situation where electrical systems might fail. This self-lumination ensures that exit paths remain marked for safe evacuation. Tritium exit signs require no wiring, batteries, or ongoing maintenance, making them especially useful in locations where electrical access is limited or maintenance is difficult.

The visual appearance of a Tritium exit sign includes the legend (the word “EXIT”), which glows green because the Tritium’s light activates a green phosphor coating. The background is non-illuminated (opaque) and can be green, red, black, or white.

When an area is well-lit (more than 30 lux, typical indoor lighting), the tritium illumination is faint compared to the ambient light, so the letters might not appear glowing. However, the color and contrast between the legend and background still make the sign easily readable as required by life-safety codes.

The electrons (beta particles) from Tritium energize a phosphor compound, which is coated on the inside of the glass tubes. Phosphor has the property of emitting light when stimulated by electrons, and is used in television tubes and fluorescent lights. The surface of the tritium tube emits light continuously when the phosphor layer is energized. This reaction is also called radioluminescence. The mechanism in this process is exactly the same as in an electrical fluorescent lamp, except that electrons are provided by Tritium instead of by electrical current. The energy of tritium electrons is much less than that generated in electrical lamps, so you can’t see tritium tubes glow except in dim light or darkness.

Tritium exit signs typically remain bright for more than 10 years. Over time, as the tritium decays, the light output gradually diminishes, and the sign will eventually need to be replaced.

Tritium gas, like other radioisotopes, decays at a known rate. This rate is usually specified by the “half-life,” which is the time taken to decay to half the initial energy. For Tritium, this is 12.3 years. National codes have established the minimum brightness required for tritium exit signs. Using the half-life calculation and the code requirements, the Tritium necessary in a new sign to produce light for the guaranteed period before the minimum brightness can be determined. Some exit signs include up to 20 years of guaranteed useful life before replacement is needed.

No, because the product emits no radiation. It contains radioactive material, but the product is not “radioactive.” Tritium gas is hermetically sealed in the sign into Pyrex glass tubes, like a miniature fluorescent lamp. The glass completely stops all radiation. Tritium beta emissions are so weak that even in free air, the particle can only travel less than 1/4 inch. A sheet of tissue paper will stop it.

If Tritium is released from broken tubes, the sign could emit very little radiation. Government regulations require that these signs pass drop tests from 8 feet to concrete. These signs are very tough, and breaking tubes in an installed sign is difficult.

Released tritium gas will behave exactly like hydrogen-it’s very light and dilutes rapidly in air, which reduces the amount anyone can breathe. However, if a person is standing inches from the sign, breathing hard for at least 5 minutes during and after a breakage, and absorbing all the Tritium. Under these extremely conservative and unrealistic conditions, a person might get a radiation dose about the same as a tooth x-ray. This dosage level is not hazardous.

Manufacture of Gaseous Tritium Light Sources and Devices (GTLS) is regulated under Title 10 of the U.S. Nuclear Regulatory Commission Code of Federal Regulations or similar regulations under the jurisdiction of Agreement States (which are subject to inspection by the USNRC). These regulations are voluminous and contain all necessary technical specifications to operate factories and products safely. A copy of a portion of these regulations is included with every self-luminous exit sign shipped in the U.S.A. Sign Brightness & Luminous Life. The minimum brightness for self-luminous exit signs is defined in National Fire Protection Association Life Safety Code 101 (1997). Thus, the amount by which the initial brightness exceeds this figure will determine the useful life before replacement is required.

Such testing is exempted explicitly under 10CFR Part 31.5 (c) (2) (ii) for devices containing Tritium. However, testing is required for fire safety reasons, just as for electrical signs. Inspectors like that each tritium sign carries an easily read “replacement date” so they and the user know exactly when replacement is required, just as for fire extinguishers.

The product is licensed for use in all buildings used by the public, including schools. The USNRC approved an exhaustive analysis of potential hazards and a detailed examination of the design of all products, including prototype testing representing the most severe conditions of use.

During its working life, as marked on the label for each product, if a broken or defective sign, i.e., one in which the light sources appear unlit or unevenly illuminated in darkness, is detected, the manufacturer should be contacted to arrange for replacement and return of the defective sign.

There are several points of emphasis concerning the visual characteristics, safety considerations, and performance behavior of Tritium self-luminous exit signs. These include how they appear and function under different lighting conditions, and how they must be handled at end-of-life because they contain radioactive material.

As mentioned, Tritium exit signs use tritium gas (H-3), a radioactive isotope of hydrogen, sealed in glass tubes coated with phosphor. Because Tritium is regulated by the U.S. Nuclear Regulatory Commission (NRC), these signs cannot be thrown away in regular waste. They must be handled, transported, and disposed of through licensed channels that specialize in radioactive materials recovery or recycling.

Self-luminous exit signs will not cause or contribute to igniting any hazardous or explosive atmospheres. A self-luminous exit sign’s illumination is not dependent upon the use of any electrical power, either internal or external. Since the sign does not require electrical power, the potential for arcing at any contact point is eliminated. Also, as there is no requirement for a power source, no elements or filaments can glow or become hot, which may cause ignition.

Tritium exit signs are a safe and effective alternative when electrical power is not readily available or when green building standards are desired. Many tritium exit signs are recyclable at the end of life and may come in slim, recessed, or vandal-proof housings. Download the Evenlite tritium exit sign PDF here.