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?
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.
The 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:
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.
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
That's good information from thedoc.
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).