In the past Tritium was used in watch dial lume. Even though it is mildly radioactive, it was deemed safe because its radioactivity doesn't penetrate the skin. However, we have other tissues in our body other than skin, and I'm particularly interested in its effect on eye tissue. Does Tritium radioactivity penetrate the eyes, or not? Also, perhaps it doesn't even get past the watch glass, and the whole query is moot?

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Tritium decays by emission of low-energy (<19 keV) electrons (and anti-neutrinos that are irrelevant here) [1]. Such particles have a very short range in matter, of the order of a few cm in air, and less than a micrometer in water [1]. These electrons are most likely stopped by whatever material houses the tritium. For instance in a watch, the light comes from the interactions of the electrons with a phosphor mixed with tritium in a paint used on the watch dial [2], so there is little risk if the watch is intact. Inhaling or ingesting tritium is, however, a health risk, as is diffusion of tritium through the skin after contact [1].

[1] Myers and Johnson 1991, Toxicity and Dosimetry of Tritium: A Review https://inis.iaea.org/collection/NCLCollectionStore/_Public/23/082/23082972.pdf?r=1.

[2] Frame, Radioluminescent Paint https://www.orau.org/ptp/collection/radioluminescent/radioluminescentinfo.htm


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