Answer to Question #1673 Submitted to "Ask the Experts"

Q

Is there a significant risk associated with radium from a damaged wristwatch? As a child, I had an old radioluminous watch which, at one time, was missing its crystal, but this was later replaced (~15 to 20 years ago). I didn't become aware it was radioactive until several years later. I believe the watch to have been circa the late '40s. As I remember, the numerals and hands were pretty much intact, but I haven't seen it in about five years since I gave it away. The recipient apparently measured its activity in a 4 pi detector and had a very rough recollection of perhaps 650,000 counts/minute, which I calculate to be ~300 nCi. I've done a little research, and I understand the vast majority of such watches had <500 nCi in them. I tried to track down the watch a couple of years ago, but didn't have any success. Recently I've become concerned about the radium that may have flaked off while the crystal was missing. In particular, the case the watch was in may have been used briefly before the crystal was replaced. I'm writing to ask if the amount of radium that may have flaked off is sufficiently dangerous to require testing (and should I make an even more concerted effort to locate the watch and case)?

A

There is no significant risk from the damaged wristwatch. While it is possible that some radium was accidentally ingested due to contact with the watch face, the quantity would be exceedingly small. Studies of the radium-dial workers have not observed excess bone cancer (radium is a bone-seeker) in the low-dose region that we are talking about. Indeed, it is often assumed that there is an effective threshold for exposures from ingested radium. This is a type of exception to the working assumption used in the field of radiation protection that an excess risk is possible at even the smallest of doses. The National Academy of Sciences Report BEIR IV (page 12) has this to say: "below an average skeletal dose of about 0.8 Gy [80 rad] the chance of developing bone cancer from radium-226 and radium-228 during a normal lifetime is extremely small - possibly zero." In the BEIR IV report refer to Chapter 4 for the discussion pertaining to radium. Let me say a little about the activity on the watch and its measurement. A typical watch might have contained approximately 0.1 microcuries (100 nCi) of ²²⁶Ra. This equates to 220,000 disintegrations per minute of ²²⁶Ra. However, in addition to ²²⁶Ra, there would also be a similar activity of each of the following short-lived decay products of ²²⁶Ra: ²²²Rn, ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po (even though ²²²Rn is a noble gas, much of it will be trapped in the paint). There will also be a smaller activity of ²¹⁰Pb, ²¹⁰Bi, and ²¹⁰Po—the exact amounts of these would depend on the age of the radium paint. In other words, most of the counts from your watch would have been from the decay products of ²²⁶Ra. If you do not factor in their contribution, you end up overestimating the ²²⁶Ra concentration. I say all this because you appear to be concerned that your watch was unusually "hot." Finally, and this is an exceedingly picky point, it doesn't make sense for the measurement you mention to have been performed with a 4 pi detector because the measurable emissions would only be coming off the watch face, not through the back. Paul Frame, CHP, PhD