hour per curie. The dose rate measured from 10 mm away will be two orders of magnitude lower than the measured surface dose rate. Given that the half-value thickness of 10 keV photon radiation in water is about 1.4 mm, the attenuation provided by tissue overlaying blood-forming organs is considerable. The primary danger from tritium arises if it is inhaled, ingested, injected, or absorbed into the body. This results in the absorption of the emitted radiation in a relatively small region of the body, again due to the low penetration depth. The biological half-life of tritium – the time it takes for half of an ingested dose to be expelled from the body – is low, at only 12 days. Tritium excretion can be accelerated further by increasing water intake to 3–4 liters/day. Direct, short-term exposure to small amounts of tritium is mostly harmless. If a tritium tube breaks, one should leave the area and allow the gas to diffuse into the air. Note that it is lighter than air and will float upwards like helium. Tritium exists naturally in the environment, but in very small quantities. Legislation Because tritium is used in boosted fission weapons and thermonuclear weapons (though in quantities several thousand times larger than that in a keychain), consumer and safety devices containing tritium for use in the United States are subject to certain possession, resale, disposal, and use restrictions. In the US, devices such as self-luminous exit signs, gauges, wristwatches, etc. that contain small amounts of tritium are under the jurisdiction of the Nuclear Regulatory Commission, and are subject to possession, distribution, and import and export regulations found in 10 CFR Parts, 30, 32, and 110. They are also subject to regulations for possession, use, and disposal in certain states. Luminous products containing more tritium than needed for a wristwatch are not widely available at retail outlets in the United States. They are readily sold and used in the UK and US. They are regulated in England and Wales by environmental health departments of local councils. In Australia products containing tritium are licence exempt if they contain less than tritium and have a total activity of less than , except for in safety devices where the limit is total activity. See also List of light sources Radium Girls References External links Cleanup of a broken tritium sign Radioluminescent items Luminor 2020 – Debunking Panerai's fictional history