Science Questions

Are glow-in-the-dark watches a radiation hazard?

Sun, 14th Sep 2008

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Question

Sandra, Melbourne asked:

I was just wondering, is there any radiation that would come from a glow-in-the-dark watch that would be harmful to the wearer?

Answer

Philip Clark, University of Edinburgh, Experimental Particle Physics Group:

It depends very much on the type of dial that youíre considering.  By far the most common watch that you come across thatís glow in the dark is called a phosphorescent watch.  Essentially the watch is coated in a paint which absorbs light and then re-emits it.  These watches are completely harmless. 

Rolex submarinerThe second type of watch is called a tritium watch.  The modern way to do this is you have the same phosphorescent paint but this time itís mixed with small tubes filled with tritium.  Tritium is radioactive and emits beta particles.  These have the same effect of exciting the phosphorescent paint.  This time tritium has got a half-life of 12 years.  The beta particles that are emitted are not very energetic so if anything they couldnít even penetrate the outermost skin layer. 

The third watch Iíd like to mention is a radium watch.  They have very much the same design but this time instead of tritium theyíre mixed with radium.  The half-life is sixteen hundred years.  However, they may not seem to be as radioactive because the phosphor in the paint gets eaten up by the radium.  Iíve got a small demonstration here so Iíve got an old watch that Iím going to hold a Geiger counter to.  If I turn the Geiger counter on youíll hear it clicking:

<click>

Thatís when I hold it slightly close to the watch.  If I hold it really close to the watch:

<much more rapid series of clicks>

And the take it away from the watch then the background count Ė you can hear the occasional count just now is much lower than if you hold it closer to the watch.  These watches are extremely radioactive.  However theyíre still not too harmful unless you were to break the watch and inhale it or somehow ingest the watch.

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I assume the question relates to those watches that have hands which absorb and re-emit light, rather than the electro luminescent kind. Early examples of these used radioactive materials, I believe it was radium, which was harmful. However, there is no harm from the materials used in watches today. rhade, Fri, 12th Sep 2008



I haven't heard the podcast yet, but if it's an old watch more than 40 years old then it might contain a radioactive isotope. A radioactive glow-in-the-dark would continue glowing even if left in the dark for many hours.

Modern non-radioactive glow-in-the-dark materials are "charged up" by visible and ultraviolet light, but their glow becomes weaker over many minutes so that once they've been in the dark for half an hour barely glow at all. techmind, Mon, 15th Sep 2008

That's good information from thedoc.

I know that anecdotes may have limited value in assessing risk, but this experience sticks out in my mind. An older man was showing me his older watch, and we got to talking about how it glowed in the dark, and then he removed the watch and showed be a lump on his wrist directly under the watch. He seemed to think the radiation from the watch might have caused the lump which I'm guessing was sort of a benign cyst which he had for some time, I don't think he even told his doctor about it. Nor did he seem to feel the watch threatened him (at least, not his life), because in spite of his suspicion about the lump, he continued to wear the watch.

I'm pretty sure the watch was the radium type thedoc mentioned. Ever since I saw that guys lump in his wrist, I've never wanted to wear a watch with radium in it I don't know how rational that is based on one anecdote, but knowing what I do about radiation, it certainly seems plausible to me that the lump might have resulted from radiation from the watch. Arbitrageur, Mon, 20th Jun 2011

As a boy 75 years ago I scraped some of the Radium paint from an watch dial and I have often wondered if I ingested any, if I did it has not as yet had any life shortening effect. syhprum, Tue, 21st Jun 2011

I wear a watch with tritium tubes - and after 40 years it is still bright enough to read in the pitch dark (although maybe the previous owner had them replaced). 

The watch picture above has luminescent paint - either Luminova or Super-Luminova;  in the early 60s Rolex changed from Radium to Tritium, but dials with Tritium are marked with a T either side of the SWISS / SWISS MADE underneath the 6. imatfaal, Tue, 21st Jun 2011

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