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

Are Glow-in-the-Dark watches a radiation hazard? Should you store your best timepiece in a lead lined box?
14 September 2008


A Rolex submariner watch



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 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.


Glows in the dark, but no white paint. Is it safe? 765111 on back of watch..

Is an old glow in the dark Baby Ben dangerous ?
Found it in the back of a drawer.

I must admit that I am not familiar with a Baby Ben. What's one of those?

I was given a jar of old military watches by an old friend who’s husband had gone into a nursing home. My son is very interested in radiation and so was very keen to try his Geiger counter on them.
We opened the jar on our kitchen bench there was about 6 watches and one of them was disasembled. We then turned on the Geiger counter. It went crazy, getting up to 70 microsieverts per hour. Background radiation in our house is about 0.2 microsieverts. So we quickly put them back in the jar and wrapped them in a lead sheet.
Anyone have any advise. How dangerous was that exposure? I am not so worried about the gamma radiation as the risk of having inhaled a radium particle. Also, does anyone know if a glass jar full of radioactive watches that probably hadn’t been opened in decades, would have created enough radon gas to be seriously dangerous to us.
I went to the doctor but really, the doctor had no idea.

That dose is small compared with what you receive from the environment over the year. I wouldn't worry...

I am curious as to what is used in modern fabrics to make them glow in the dark. I recently read an article about Victorian match makers developing "Phossy Jaw" do to the exposure to phosphorus. Is phosphorus a component in modern fabric? Have contacted the manufacturer of fabric I just purchased but have not heard back yet. Don't want to make a quilt for baby before finding out if it's safe! Thanks

Materials that glow in the dark and are used in children's today, on clothes, stickers and fire exit signs are safe. They employ chemicals called phosphors that soak up energy from visible - and sometimes invisible forms of - light and then release it again, but more slowly and at a different wavelength (or colour). These materials do not use any ionising radiation and, as long as the chemical phosphors themselves are safe, which consumer forms are, they cannot harm you.

The ladies who put the glow stuff on the watch hands.Die at a very young age. Horrible so, I want to add. Don't think their boss gave them the heads up on that part of the job. Lol lol lol. Hope my boss doesn't share the same

I am.planning to purchase casio watch and i have heard that it glows.....any idea qhat substance does companies like casio use for glow factor

Tritium, as far as I know. I used to own one. It should even be marked 3H, the symbol for tritium.

HI. I just got a sports watch by PASNEW (Lapgo) that says it has an "electronic fluorescent lamp" in it. Furthermore, they don't want you to over-press the light button, as it will weaken over time. Question 1: Is this the same technology as in all "electroluminescent" products (ie. Casio) or is it different? I'm just confused about the word "fluorescent"; perhaps it's just a word tranlation choice... Got any thoughts? Also...why does the light "weaken over time"? Just curious. Thanks for any info!

The nomenclature is a bit confusing! The word "phosphorescent" used in these contexts often makes you think of the element "phosphorus", but its a misleading term. There is unlikely to be much phosphorus in a "phosphor"; these are coatings made from, usually, mixtures of chemicals that absorb and emit light at certain frequencies. Painted onto a screen or a lamp coating, they can generate certain ranges of colours; the phosphor on the inside of a strip light, for instane, converts short wavelength ultraviolet into visible "white" light. The word probably comes from the observation that elemental white phosphorus can spontaneously ignite and burn with a white glow.

Fluorescence is another way of making things glow. Jellyfish do it by pumping energy into a protein called GFP. Fireflies use an enzyme called luciferase and a substrate called luciferin to do it. Your watch does it by using electricity from an on-board battery to excite - most likely - a semiconductor, like an LED to produce light.

Good info. Okay next: do companies still use radium to effect a light reaction? I think the article said some are still do I know? I'd rather have my son wear ultra-safe materials on his wrist if possible. So what are the chances a company (Chinese in this case) still uses radium or any other element I'd rather avoid? Is there a way to find out? Thx!

Probably not. For a start, radium is universally acknowledged to be a risky substance to work with, even in places like China, where health and safety is often an after thought. There are strict controls on the production, movement and use of radioactive materials, so sourcing radium for an application like watch paint is likely to be preclusively tricky.

Tritium is used (brands like Rolex use it), because it's regarded as safer; the half life is - unfortunately - shorter, meaning that the watch face needs a refresh every decade or so...

The half life of tritium is 12 years, but that just means your beta radiation gets halved, and the intensity of the glow will be halved too. But that will not necessarily be weak enough to not see anything in dark. I have a watch with tritium markings for over 17 years, it is still gets lit enough to read in total darkness. Of course I don't exactly remember the initial brightness, but it is still quite readable.

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