Answer to Question #8054 Submitted to "Ask the Experts"

My question is about possible contamination from watches with radioactive paint. I purchased a vintage lady's wristwatch from a thrift store a few months back and did not think it had any radioactive material in it because the small hands were dark brown and very thin, plus it looked like a dress watch. I put it on top of a wooden bookcase for storage. I decided to test it with my radiation detector and I was surprised to find it was radioactive. It registered 6.25 µSv h^-1 from about three-fourths of an inch away from the Geiger-Mueller tube on the glass side of the watch. Would this watch leave any contamination on top of the bookshelf or elsewhere? My guess is that the stem seal is not the greatest seal on the watch because of its age, and air probably moves in and out of the watch. I have heard about byproducts that are created by radium decay that can be a hazard as well; would these be a problem in this case? Would I need to clean the top of the bookcase any special way? My wife keeps her cookbooks on it as well. I also have some older Seiko watches and I understand promethium was used in the paint on them. I have stored a few of these in one of my dresser drawers. Do I need to be concerned about them contaminating my drawer and other items in it with decay products? At times I replace batteries in some of the Seikos with promethium and a few with tritium paint and have to open them up to do so. Would the battery and internal parts of the watch be contaminated with decay products? Should I be concerned with getting contamination on my hands and tools? Thanks for your time and answers for us worrywarts out here. Maybe I need to take up a new hobby?

First—all radioactive materials are now regulated by the Nuclear Regulatory Commission (NRC), and depending on the quantity and type of radiation, there are different levels of regulatory control. These range from nuclear reactors to the trivial amounts in some consumer products. The amount of radioactivity on these products is very small, negligible, and would not constitute a concern for your health.

Let's assume your 6.25 µSv level at three-fourths-inch distance is correct. Remember, this is from the surface of the watch; the backplate of the watch is thicker and, rather than glass, consists of metal, so any radiation coming from the backside would be much less, if even measurable. But let’s consider the front side. At a distance of one yard, 36 inches, from the surface of this watch this exposure level would be reduced to 1/2,304 = (0.75 inches/36 inches) squared, or 0.0027 µSv per hour. This would translate into an exposure (air kerma) of 2.37 mrad per year. We receive about 300 mrad yearly from all sources of natural background radiation, so this is less than 1 percent of natural background.

But, let’s be a devil's advocate and assume the front of the watch is in constant contact with your skin all year long; this would translate into a 5.5 rad surface dose to the skin. Occupational limits for the skin, different from the entire body, are 50 rads. No matter how you look at this, these are very low, and safe, levels.

Regarding contamination—as long as the watch is intact, there is no reason to believe there is any contamination. This is not a powder, but a painted surface. Unless you rub it and try to remove this material, it will probably remain where it is. However, since you have a detector, I would use some tissue paper and rub the watch surface and see if you can measure any radiation on the tissue, away from the watch. If you detect any radiation from the tissue, then it is highly probable that the radioactive material on the watch is somehow flaking and coming off the watch. You may want to contact your state radiation control agency, or the radiation safety office of your local hospital, and ask if you could have these items checked out with a radiation detector. If nothing else, this would give you some peace of mind.

If the radioactive material is radium, it will slowly decay to form radon gas, with other radiaoctive decay products, but the amount of such gas again would be very, very low. I won't even attempt to do the calculation, but any radon emanating from radium on watch dials would be very little.

These are all very low and safe levels. Remember—we are bathed in a constant environment of radiation from the universe and sun, from rocks and soil, and from within our own body such as radioactive carbon in our bones.

You may also search our own Health Physics Society site which has some useful questions and answers: http://hps.org/publicinformation/asktheexperts.cfm; simply search on "watches."

I hope this helps.

Orhan Suleiman
Topic Editor
Health Physics Society