Answer to Question #12289 Submitted to "Ask the Experts"

Category: Consumer Products — Watches, Clocks, and other Glow-in-the-Dark

The following question was answered by an expert in the appropriate field:

Q

I am an avid watch enthusiast and often restore older wristwatches. This process usually requires completely disassembling the watch. My current project involves a watch that was manufactured in 1992 and likely used promethium-147 (147Pm) to create luminous hour indicators and hands. Will handling the dial and hands pose a health threat to me? Can I potentially contaminate my house, thereby putting my family at risk?

A

First, it is highly unlikely you are putting your family at risk. Although it is difficult to give you an answer that is absolutely correct without actual measurements of the amount of radioactive material that may be on the watch, it's not even certain that the watch is radioactive. The material on the watch dial and hands may not be 147Pm; rather, it may be a fluorescent or phosphorescent chemical.

But if it is 147Pm, the beta particles will not pose a hazard unless ingested in large quantities. This is a highly unlikely scenario—even if some of it rubbed onto your hands, and you touched your mouth.

Furthermore, if it is 147Pm, it has decayed away to a negligible amount. This is an old watch, and watchmakers quit using radioactive materials many years ago, replacing radioactive displays with fluorescent or phosphorescent chemicals or electronic displays. Prometheum-147 has a half-life of 2.6 years, and after 10 half-lives (26 years) the amount of radioactivity will be reduced to 1/14,000th of its original activity. That's clearly a small and even negligible amount of radioactivity.

So, in conclusion, handling the dial with your hands should not pose a health threat to you or potentially contaminate your house. Your family would not be at risk.

Orhan Suleiman, MS, PhD, FAPM, FHPS

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 13 December 2017. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.