Glow in the Dark Danger

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

It depends very much on the type of dial that you're considering. By far the most common watch that you come across that's 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 it's 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 couldn't even penetrate the outermost skin layer.

The third watch I'd like to mention is a radium watch. They have very much the same design but this time instead of tritium they're 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. I've got a small demonstration here so I've got an old watch that I'm going to hold a Geiger counter to.

If I turn the Geiger counter on you'll hear it clicking: That's when I hold it slightly close to the watch. If I hold it really close to the watch and the take it away from the watch, then the background count - 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 they're still not too harmful unless you were to break the watch and inhale it or somehow ingest the watch.