Answer to Question #11972 Submitted to "Ask the Experts"

Category: Consumer Products — Watches, Clocks, and other Glow-in-the-Dark

The following question was answered by an expert in the appropriate field:


I have a keen interest in vintage wristwatches, especially military ones from World War II. Back then, the luminous paint applied to the hands and indices on the dials contained radium, which was obviously found to be harmful to the wearer later on and in part prompted adoption of tritium compounds in luminous paint.

My question is: What isotope of radium was typically used and what is its half-life? I see these watches in antique shops, jewelry stores, and online, and I wonder if the radium is still bombarding everything within line-of-sight with gamma radiation, or if it would have completely decayed by now, 70-plus years later.

If these watches are still active sources, that implies to me that they would be regulated by the Nuclear Regulatory Commission (NRC). Since I don't see any evidence of such regulation, I might conclude that they must not be a source of gamma radiation any longer. I just don't know; the majority of people wouldn't give it a thought, but as an engineer I can't dismiss the possibility so easily.

Should these old watches be handled in a special way, or should one just avoid them altogether? Can watches that do not have radium luminous paint but are stored in the same location as radium-containing watches become radioactive themselves; i.e., could the gamma rays ionize them?


You have raised thoughtful questions no doubt arising from your engineering interest in how things work. The bottom line for your concerns is that no special handling is required for radium-dial watches, except that you should not open them or attempt to remove the radium paint, which could result in radium ingestion. You may be interested in the response to a similar question, Question 8650.

I would like to respond to your questions by providing some historical information on how radium came to be used for illumination. See the Oak Ridge Associated Universities website for a good discussion on such uses of radium.

Of course it all goes back to the discovery of radium by Marie and Pierre Curie in 1898 and the isolation of a physical quantity of radium in the early 1900s. The Curies observed that purified radium will glow blue. Pierre made national news about 1903 when he invited dinner guests to the front steps of his house in Paris one night and proceeded to remove a vial of radium from the vest pocket of his jacket. The vial glowed brightly in the dark without any outside support, such as electricity. This was an astounding observation and showed the world that radium could be a useful source of energy. This eventually led to an industry for extracting radium from uranium ore and radium became widely used in World War I for illuminating instruments in military aircraft, so pilots could read their instruments at night. After the war, the radium industry expanded further for illumination of pocket watches, wristwatches, clocks, and for other uses of radium in medical applications.

The isotope of radium used for illumination is radium-226 (226Ra). This isotope is primarily an alpha-particle emitter with a half-life of 1,600 years. (Radium also emits a very weak gamma ray.) The alpha particles from radium will interact with crystals of zinc sulfide (ZnS) to produce a greenish glow. This is the basis of radium paint for illumination.

The radium, after emitting an alpha particle, is transformed into radon, which further decays to form a series of additional short-lived radionuclides that emit alpha, beta, and fairly strong gamma rays. The gamma rays from the radon decay products lead-214 (214Pb) and bismuth-214 (214Bi) are the source of potential radiation exposure to someone wearing a radium wristwatch. The alpha and beta radiation are blocked inside the watch.

The discussion in the link above shows that typical radiation doses from radium in watches could reach a few tens of microsieverts per year (µSv y-1). The average annual dose from naturally occurring radiation in the United States is about 3,000 µSv y-1. Thus, the risk to the wearer of a radium-illuminated watch due to gamma radiation is relatively low. So, why the concern for radium?

The main concern with radium has to do with ingestion. When radium is ingested, some of it is deposited in our bones because radium is similar in chemistry to calcium. Once radium is deposited in our bones in place of calcium, it will basically stay there forever due to the long half-life of radium. The alpha particles from radium could cause damage to bones, as was noted for radium dial painters who ingested large amounts of radium. (These painters are known as "The Radium Girls" and there are several books out about them.) The use of radium for illumination was largely discontinued by the late 1960s, mainly because of concerns for ingestion of radium, not because of concerns for gamma-ray exposures. Also, other sources of illumination became available, such as tritium.

Historically, radium as a naturally occurring radioactive material was not regulated by the NRC, which primarily regulates byproducts of nuclear reactors. The regulation of naturally occurring radioactive materials (NORM) was done by individual states until 2009 when the NRC began to regulate discrete sources of radium. States still regulate radium contamination of soils or similar materials.

From this discussion, you may conclude that gamma rays from a radium dial watch are not likely to be a great source of exposure. The main concern would be from removal of the radium paint and ingesting it. Also, the gamma radiation from radium will not cause any nearby materials to become radioactive. Radioactivity is a property of atoms where the nucleus has an excess or deficiency of neutrons to balance the repulsive forces of protons. When the number of neutrons and protons in a nucleus is out of balance, the nucleus will emit energy (called radiation) to become more balanced. Since gamma rays give up their energy by collision with electrons, the nucleus of atoms is unaffected by gamma rays. Therefore bombarding materials with gamma rays (or x rays) does not cause the material to become radioactive.

Ray Johnson, MS, PSE, PE, FHPS, DAAHP, CHP

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