Radiation from old watch dials: How dangerous?
Posted: February 15th, 2019, 10:00 am
This was originally posted by oyang on HF, on Fri Feb 06, 2015:
Since I have a gen RP dial and am planning on putting it in a watch to wear, I decided to do a little research on the radiation risk. I have a background in medical research, using some radioactive reagents in my work, so I am somewhat familiar with radiation risks already. I figured I'd share what I found out. Please note these are just my opinions, based on information I've gotten on the internet, so I could be off base. If I had an attorney, he/she would tell me to add a disclaimer that you should assess the risk yourself or consult an expert, because I'm just an educated amateur.
First of all, the names "Luminor" and "Radiomir" used to refer to the type of radioactive material used for glow; these terms have lost their original meaning in the modern Panerai company. Luminor was the later type, using tritium. This is the easy one to assess: tritium is a low energy beta emitter. Beta radiation is low energy and easily blocked even by just a layer of plexiglass and certainly by the movement and caseback of a steel watch. Also, the half life to decay is only 12 years, and so a decades old watch won't even be radioactive anymore. Bottom line: Luminor watches should pose no risk to wear as much as you want.
Radiomir came first, referring to radium-based paint. In the 3646 sandwich dial, this paint was slathered thickly onto the back of the plexy insert, using much more than a regular painted dial. From what I've read, the most was used in the 6154, more than the 3646. At any rate, the Radiomir was based on radium, which is a high energy gamma emitter. This type of radiation is highly penetrating, typically requiring thick lead shielding to stop.
Generally Radiomir watches will no longer have much glow if any, but that isn't because the radiation is gone; the radium isotope utilized has a half life of over 1000 years. The glow is gone because the component in the paint that converts radiation into light is degraded.
I found a picture of someone testing a 3646 with a geiger counter. From the face side, it was emitting about 38µSV (micro-Sieverts)/hour, and on the back (shielded by the movement and caseback) less, 13.7µSV/hour. OK, so what does that mean, and how dangerous is it? A Sievert is a measurement of the biological impact of radiation, based on the type and amount of emitted radiation energy. The earth is bombarded with radiation from the cosmos/sun. It's estimated that a person is hit with about 600µSV per year. So, wearing a watch putting out 13.7µSV/hr means you are getting a year's worth of cosmic radiation focused on your wrist in under 5 hours. Is that bad? Well, when you are in an airplane high in our atmosphere, you are exposed to higher radiation than down on solid ground, about 8µSV/hour, more than half the amount from wearing the watch, although it is distributed over your entire body and not focused on your wrist.
We know most about what happens with high dose radiation exposure in a brief period, based on various accidents and gruesome experiments. With huge doses in a short amount of time (e.g. Hiroshima, Chernobyl), it's simple: you have tissue damage in areas where cells are dividing rapidly (bone marrow, intestines, skin). This starts at doses of 1 SV, (1 million µSV) and becomes fatal around 5 SV. Over prolonged amounts of time, the body can withstand more radiation because it has time to repair ongoing damage, rather than taking a huge hit all at once. Of course wearing a 3646 is nowhere close to these doses; you'd have to wear it continuously for 70,000 hours, or about 8 years, to get a total exposure of 1 SV.
So the problem with chronic radiation is risk of cancer, because it damages DNA, causing mutations that can affect genes that regulate cell growth. What is the cancer risk? The data are less clear here, because animals have different metabolism and cancer risk than humans, cancer often occurs late after exposure and is affected by other risk factors, and experimentation on humans has been limited thankfully. I found two estimates of cancer risk from short term exposure to radiation. One was that an acute exposure to 100mSV (which is 0.1 SV or 100,000 µSV) causes an 0.79% risk of developing cancer sometime in the future. Another was that an acute exposure to 1 SV (a million µSV) confers a 5.5% risk of eventual cancer. I'm not sure how those estimates were made, and how many years into the future they mean, but it gives you a rough idea nevertheless. Another reference point is that a typical chest X ray, considered extremely safe, is an exposure of 100 µSV. So, wearing the 3646 for 7 hours is about the same as the X ray, focused on your wrist instead of your whole chest. The wrist isn't a very vulnerable area; rapidly dividing cells in the bone marrow and gastrointestinal tract are the most dangerously vulnerable areas. In an adult, the long bones have very little active marrow (most of it is in the flat bones, like the sternum, vertebrae, pelvis, skull). Bone and muscle cancers are very rare because those tissues tend not to have dividing cells. Skin cancer would be a risk probably, but even then a pretty small one and typically treatable except for advanced melanoma.
What's my conclusion? In the grand scheme of things, wearing a 3646 for a day is minimally risky, probably similar to getting an X ray of your wrist. The chance that anything bad would happen is really tiny; you're way way more likely to die in a car accident, get hit crossing the street, or die of flu. I'd probably not wear it every day for years and multiply that risk by several hundred-fold. I've never heard of any Panerai-wearing frogmen getting cancer from their exposure. The "radium girls" (look it up on Wikipedia) did get sick and die from radium exposure, but they were painting watch dials all day and licking the brushes.
If and when I install my dial into a 3646-style case (hopefully a gen I'm trying to track down), I'll check how much radiation escapes through the back, using a geiger counter at work. Actually, I did check my naked dial but I forget the radiation level it put out. I'll check it again and post here later. I can probably temporarily put it in my 3646 homage case with my 618 movement and get an estimate. If the radiation is similar to the 13.7 µSV/hr mentioned above, I might try to design a lead shield to put inside the caseback if there is enough room, and if that reduces it enough I'd feel comfortable wearing it all the time. We shall see! It's a good problem to have.
This is later reply by oyang:
As another point of reference, I measured a genuine Kieffer radium dial (actually sold here on HF a couple of years ago). Unlike the sandwich dial, the Kieffer has painted numbers and therefore less radioactive material. Putting the geiger counter probe directly against the dial, it was putting out about 17µSV/hr. Separated by a plexi, the count dropped to about 7µSV/hr, which is due to the distance and not the plexi (radiation is diluted in direct proportion to the square of the distance from the source). Unfortunately I did not measure the radiation through the dial mounted on a movement and through a caseback, but I'd imagine a lot of the radiation would be blocked, dropping significantly below 7µSV/hr. I'd feel comfortable wearing that fairly frequently, given the numbers I mentioned above. A single shot of 100 mSV increases cancer risk by 0.8%; this is 1.6 years of continuous wear to reach that dose, and the fact that it is spread out over 1.6 years instead of given as one exposure probably dilutes the risk to nothing.
Since I have a gen RP dial and am planning on putting it in a watch to wear, I decided to do a little research on the radiation risk. I have a background in medical research, using some radioactive reagents in my work, so I am somewhat familiar with radiation risks already. I figured I'd share what I found out. Please note these are just my opinions, based on information I've gotten on the internet, so I could be off base. If I had an attorney, he/she would tell me to add a disclaimer that you should assess the risk yourself or consult an expert, because I'm just an educated amateur.
First of all, the names "Luminor" and "Radiomir" used to refer to the type of radioactive material used for glow; these terms have lost their original meaning in the modern Panerai company. Luminor was the later type, using tritium. This is the easy one to assess: tritium is a low energy beta emitter. Beta radiation is low energy and easily blocked even by just a layer of plexiglass and certainly by the movement and caseback of a steel watch. Also, the half life to decay is only 12 years, and so a decades old watch won't even be radioactive anymore. Bottom line: Luminor watches should pose no risk to wear as much as you want.
Radiomir came first, referring to radium-based paint. In the 3646 sandwich dial, this paint was slathered thickly onto the back of the plexy insert, using much more than a regular painted dial. From what I've read, the most was used in the 6154, more than the 3646. At any rate, the Radiomir was based on radium, which is a high energy gamma emitter. This type of radiation is highly penetrating, typically requiring thick lead shielding to stop.
Generally Radiomir watches will no longer have much glow if any, but that isn't because the radiation is gone; the radium isotope utilized has a half life of over 1000 years. The glow is gone because the component in the paint that converts radiation into light is degraded.
I found a picture of someone testing a 3646 with a geiger counter. From the face side, it was emitting about 38µSV (micro-Sieverts)/hour, and on the back (shielded by the movement and caseback) less, 13.7µSV/hour. OK, so what does that mean, and how dangerous is it? A Sievert is a measurement of the biological impact of radiation, based on the type and amount of emitted radiation energy. The earth is bombarded with radiation from the cosmos/sun. It's estimated that a person is hit with about 600µSV per year. So, wearing a watch putting out 13.7µSV/hr means you are getting a year's worth of cosmic radiation focused on your wrist in under 5 hours. Is that bad? Well, when you are in an airplane high in our atmosphere, you are exposed to higher radiation than down on solid ground, about 8µSV/hour, more than half the amount from wearing the watch, although it is distributed over your entire body and not focused on your wrist.
We know most about what happens with high dose radiation exposure in a brief period, based on various accidents and gruesome experiments. With huge doses in a short amount of time (e.g. Hiroshima, Chernobyl), it's simple: you have tissue damage in areas where cells are dividing rapidly (bone marrow, intestines, skin). This starts at doses of 1 SV, (1 million µSV) and becomes fatal around 5 SV. Over prolonged amounts of time, the body can withstand more radiation because it has time to repair ongoing damage, rather than taking a huge hit all at once. Of course wearing a 3646 is nowhere close to these doses; you'd have to wear it continuously for 70,000 hours, or about 8 years, to get a total exposure of 1 SV.
So the problem with chronic radiation is risk of cancer, because it damages DNA, causing mutations that can affect genes that regulate cell growth. What is the cancer risk? The data are less clear here, because animals have different metabolism and cancer risk than humans, cancer often occurs late after exposure and is affected by other risk factors, and experimentation on humans has been limited thankfully. I found two estimates of cancer risk from short term exposure to radiation. One was that an acute exposure to 100mSV (which is 0.1 SV or 100,000 µSV) causes an 0.79% risk of developing cancer sometime in the future. Another was that an acute exposure to 1 SV (a million µSV) confers a 5.5% risk of eventual cancer. I'm not sure how those estimates were made, and how many years into the future they mean, but it gives you a rough idea nevertheless. Another reference point is that a typical chest X ray, considered extremely safe, is an exposure of 100 µSV. So, wearing the 3646 for 7 hours is about the same as the X ray, focused on your wrist instead of your whole chest. The wrist isn't a very vulnerable area; rapidly dividing cells in the bone marrow and gastrointestinal tract are the most dangerously vulnerable areas. In an adult, the long bones have very little active marrow (most of it is in the flat bones, like the sternum, vertebrae, pelvis, skull). Bone and muscle cancers are very rare because those tissues tend not to have dividing cells. Skin cancer would be a risk probably, but even then a pretty small one and typically treatable except for advanced melanoma.
What's my conclusion? In the grand scheme of things, wearing a 3646 for a day is minimally risky, probably similar to getting an X ray of your wrist. The chance that anything bad would happen is really tiny; you're way way more likely to die in a car accident, get hit crossing the street, or die of flu. I'd probably not wear it every day for years and multiply that risk by several hundred-fold. I've never heard of any Panerai-wearing frogmen getting cancer from their exposure. The "radium girls" (look it up on Wikipedia) did get sick and die from radium exposure, but they were painting watch dials all day and licking the brushes.
If and when I install my dial into a 3646-style case (hopefully a gen I'm trying to track down), I'll check how much radiation escapes through the back, using a geiger counter at work. Actually, I did check my naked dial but I forget the radiation level it put out. I'll check it again and post here later. I can probably temporarily put it in my 3646 homage case with my 618 movement and get an estimate. If the radiation is similar to the 13.7 µSV/hr mentioned above, I might try to design a lead shield to put inside the caseback if there is enough room, and if that reduces it enough I'd feel comfortable wearing it all the time. We shall see! It's a good problem to have.
This is later reply by oyang:
As another point of reference, I measured a genuine Kieffer radium dial (actually sold here on HF a couple of years ago). Unlike the sandwich dial, the Kieffer has painted numbers and therefore less radioactive material. Putting the geiger counter probe directly against the dial, it was putting out about 17µSV/hr. Separated by a plexi, the count dropped to about 7µSV/hr, which is due to the distance and not the plexi (radiation is diluted in direct proportion to the square of the distance from the source). Unfortunately I did not measure the radiation through the dial mounted on a movement and through a caseback, but I'd imagine a lot of the radiation would be blocked, dropping significantly below 7µSV/hr. I'd feel comfortable wearing that fairly frequently, given the numbers I mentioned above. A single shot of 100 mSV increases cancer risk by 0.8%; this is 1.6 years of continuous wear to reach that dose, and the fact that it is spread out over 1.6 years instead of given as one exposure probably dilutes the risk to nothing.
