Radioactive Luminous PaintCopyright © David Boettcher 2005 - 2023 all rights reserved.
During World War One it was quickly realised that a watch with luminous hands and numbers was essential for an officer in the trenches, it allowed the time to be checked in low light or in the dark without using a light that could have attracted a shot from an enemy sniper. This is discussed in more detail on my page about Trench Watches. Ever since military, and many civilian watches, have had luminous dials.
There are broadly two types of luminous paint that a watch collector might encounter, modern luminous substances that are quite safe, or earlier paints based on radioactive materials such as radium that need to be treated with caution. These are called radioluminescent paints because their luminosity comes from radioactivity stimulating a fluorescent material to emit light.
Note that old radium paint no longer glows in the dark because the radiation burns out the fluorescent material over a short time, about 5 years, but the radium has a radioactive half life of about 1,600 years and therefore remains very nearly as radioactive as when the paint was new.
If you “consult” internet watch forums you will find that some some people get very exercised about the dangers of radium paint, while others don't know what the panic is all about. I even read a comment that the radiation from radium luminous paint is less dangerous than a mobile phone. It is safe to say that those who don't know what the panic is all about simply don't understand the subject and should be ignored. Radium based luminous paint was banned in the 1960s on health grounds, when attitudes to risk were less precautionary than they are now, so to say that radium paint is less dangerous than mobile phones, which clearly haven't been banned, is plain wrong and highly irresponsible.
Caution is rarely a bad thing, but it doesn't get you much kudos on internet forums. However, here I can take a more measured view of the dangers without being flamed. My view is that it is not necessary to avoid watches with radium paint altogether, but it is necessary to understand it and the risks it can pose, particularly if you are going to collect or work on old watches that may have this type of paint.
There are several sources of hazards from radioluminescent paint, all of which are due to radiation given off by radioactive decay of elements within the paint and the subsequent decay of the resulting elements. The risk from these hazards is relatively small, but you should be aware of them. These can be broadly summed up as follows, but for the full detail please read the rest of this page.
- Radium and other materials used to make radioluminescent paint decay by emitting alpha and gamma particles as radiation. The alpha particles are stopped by the case or the crystal, but the gamma radiation easily passes through the case.
- Wearing a watch with radioactive luminous paint is not particularly dangerous. However, the wearer and those around will be exposed to some extra gamma radiation of varying amounts. Note that very old watches from around the World War One era have relatively large amounts of radioactive material in their paint and give off much stronger radiation than later luminous watches.
- Radiation damage over the years makes radioluminescent paint very brittle and it easily breaks into flakes and dust. If the watch is opened and dust or other decay products are inhaled and enter the lungs, alpha radiation can cause lung cancer. Degraded paint is more of a problem with very early luminous watches due to their age and the amount of radioactive material used in the paint.
- Radium luminous paint gives off radon gas which also emits alpha particles when it decays and which, if inhaled, can cause lung cancer, so watches with radioluminescent paint should be kept in a well ventilated space or a sealed container.
Radium atoms decay into Radon, which is a gas at room temperature, by emitting alpha and gamma particles as radiation. Radon itself decays by splitting into lighter atoms which are also radioactive, emitting more radiation in the process. Gamma radiation will pass through a wristwatch case and irradiate the wrist, or the eye during examination of a watch. The wrist is relatively insensitive to radiation, but do not spend too long closely examining the dials of radioluminescent watches. Alpha radiation cannot penetrate a watch case but radon gas escaping from the watch case, or paint dust or flakes, can be breathed in and enter the lungs. The concern then is that they emit alpha radiation which can be damaging, carcinogenic, to the delicate lung tissue. The risk from a single watch to an individual collector from these hazards is relatively small.
Once the dangers of radiation and radioactive paint were appreciated in the 1920s, the amount of radioactive material in luminous paint was reduced, and watches from the 1950s and 1960s show only relatively low levels of radioactivity. Early wristwatches from World War One era when luminous paint was first widely used, before the dangers of radium based luminous paint began to be understood in the 1920s, can have a surprisingly high level of radioactivity. The desire to achieve a good glow meant that radium paint had a high level of radioactive material and was applied with a heavy hand. Such watches need to be treated with extra caution.
None of the material on this page constitutes or should be taken as advice about handling watches or anything else with radium based paint. I relate what I have found out about the effects of radium based luminous paint and my own practice in dealing with old luminous paint on watches solely for the purpose of providing information and this is not to be construed as advice or recommendations. Old radium paint is fragile due to radiation damage over the years and it is prone to flaking and disintegrating into small particles and virtually invisible dust, all of which are radioactive, and therefore it needs to be approached and handled with great care. If you plan to work on anything bearing or containing radium paint at your own risk you should first read as much as you can about it and come to your own conclusions, and if in any doubt, leave well alone.
The amount of radioactive material in radioluminescent paint was gradually reduced as technology improved and the long term health effects of radiation were gradually taken more seriously. If your watch was made before 1970 and has luminous numerals and hands, then it is quite likely that the paint contains radium or other radioactive material. If it does, then that paint is still radioactive. By the 1950s radium paint was being phased out, although I have seen radium hands being offered for sale in a 1950 USA watch supplies catalogue and an advert for ladies' watches in 1956 that stated they had radium dials. Radium luminous paint continued to be used in some applications, primarily military, well into the 1960s.
Radium paint could also have been applied or touched up by a watch repairer. An article in the March/April 2012 NAWCC Bulletin by Gerhard Spory entitled "Memories of my Father" mentions that his father, working as a watch repairer in New York, kept a vial of luminous radium paste that he used to re-lume watch hands and dials. Spory told me that he saw this perhaps in the late 1940's and early 1950's. Watch repairers were often rather sanguine about the health dangers of materials they worked with - Spory mentions his father using potentially lethal potassium cyanide to clean watch parts, which was used to brighten gold plated movement parts.
The only sure way to tell whether what looks like old luminous paint (or any paint or other material) is radioactive is to test it with a radiation detector.
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Modern Luminous Paints
Modern luminous compounds fall into two categories; “Light charged” compounds which store energy from light and release it later, and “Safe” radioactive compounds. Light charged paints contain no radioactivity whatsoever
Light Charged Paints
Light charged or “afterglow” luminous paints charge up in sunlight, and in artificial light to a lesser degree. They work by absorbing energy from ultraviolet light and then releasing it slowly over time as visible light. Although the release of visible light is continuous and happens in daylight, it is much less visible than in the dark. These paints tend to initially glow in the dark very brightly, but the bright glow soon fades. Super-LumiNova is probably the best known of this group of compounds, which also includes Luminova and Lumibrite.
The characteristic fading of a light charged compound gives a way of checking whether an old watch or instrument has had its original radioactive paint replaced by a modern non-radioactive alternative; simply expose the item to sunlight and then take it in to a dark room. If it initially glows very brightly, but the brightness fades noticeably over a short period of time, you can be pretty sure that it is a modern light charged compound. You need at least some familiarity with a modern item that definitely has light charged paint because the zinc sulphide used as a phosphor in radioactive paint will also charge up in daylight, but it glows in the dark much less. If you are not very familiar with the characteristics of the two compounds then you really need to find a modern item to compare the time response of the glow with your test piece. The only way to be really sure whether radioactive material is present to use a radiation detector.
Safe radioactive luminous elements are very similar in principle to radium based paint, but use radioactive tritium contained in small glass containers or in specially prepared paint. Tritium is unstable and decays by emitting a low energy beta particle which is unable to penetrate the skin, but it can still be a radiation hazard if tritium gas or material containing tritium is inhaled or ingested.
Glass Containers: Tritium gas is contained within a hollow glass vessel rather like a very small neon or fluorescent tube. The vessel is coated on the inside with a phosphor which radiation from the tritium causes to glow. Tritium gives off beta radiation which cannot leave the glass container and is far less dangerous than the gamma radiation given off by radium. Watches with tritium luminous compounds glow all the time, like radium compounds, do not need charging up in sunlight, and are completely safe as long as the glass container is not broken. If you are unfortunate enough to break a container of tritium, just stand clear and let the tritium gas disperse into the atmosphere, opening widows and doors to speed the process. This is also quite safe and will not expose you to significant amounts of radiation. There is always a small amount of naturally occurring tritium in the atmosphere escaping from natural sources in the ground, which is where the tritium in your watch came from in the first place.
Tritium Paint: A self-luminous paint comprises a phosphor and a source of radioactive energy for exciting the phosphor to luminescence, usually held together by a binder like a clear varnish. Tritium is a gas, an isotope of hydrogen, which can't be mixed it into paint. To make tritium paint, the tritium has to be bound to or into something. Because tritium is chemically the same as hydrogen it will react with oxygen to form ‘super-heavy water’, T2O (tritium has an atomic mass of three, one proton and two neutrons; another isotope of hydrogen, deuterium, has an atomic mass of two, one proton and one neutrons, and makes ‘heavy water’ D2O). The super-heavy tritiated water is then made into polyvinyl acetate, which is used to make water-based paints and PVA adhesives. This is used as the binder with a phosphor to make self luminous tritium paint. I believe that tritium paints were discontinued in the early 1990s.
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Radium Luminous Paint
Some vintage watches, especially military watches, and also devices such as gun sights and aircraft cockpit instruments, had the numerals and hands made luminous or “glow in the dark” by painting them with radioluminescent paint, where the glow from the paint is caused by radiation from radium causing a phosphor to glow. This paint was not like the safe luminous compounds used in today's watches.
The luminous paint used on World War One (WW1) era trench watches was made by combining radium salts and a phosphor, both powders, in a binder of clear varnish. The phosphor was zinc sulphide doped with an activator, usually copper, which glowed brightly when hit by radiation from the radium. This paint shone brightly all the time, day and night, without needing exposure to sunlight, and continued to glow even when stored away from light. However, after about three or four years the zinc sulphide phosphor was burned out by the continual radiation bombardment and no longer glows in the dark - but the radium has hardly decayed at all and is still there, emitting radiation.
Article from November 1915
The article shown here "The Luminous Watch" was published in November 1915 during World War One. If a soldier already had a perfectly serviceable pocket watch or wristwatch that simply wasn't luminous, then this could be upgraded to a luminous watch by putting dots of radium paint on the dial next to the hour numerals, and changing the hands to the luminous radium type, as described in the article shown here from November 1915. A pocket watch could be upgraded for 10 shillings, a wristwatch for seven shillings and sixpence.
You will notice that the article says that the "best quality of luminous paint" will last about three years. If you find a watch with the original radium based paint like this you will discover that it no longer glows in the dark. This is because the fluorescent material has long since been worn out by the constant bombardment of radiation, but the radium, which has a half life of about 1,600 years, will still be very nearly as radioactive as when it was new and you need to be aware of this and take some basic safety precautions.
Luminous radium paint on dials was first used around 1910, before the dangers of radioactivity were fully understood. Although health problems in workers applying radium paint to watch and clock dials were noticed in the late 1920s, radium paint continued to be used. There wasn't a suitable safer substitute available, and military authorities continued to specify radium based paint for important applications such as compasses, instruments, gun sights and, of course, watches.
The amount of radioactive material in radioluminescent paint was gradually reduced as technology improved and the long term health effects of radiation were gradually taken more seriously. By the 1950s it was realised that the long term dangers to health were significant and that safer alternatives could be used and radium paint was phased out, although it continued to be used in some applications into the 1960s.
Air Publication 112G-0815-1 July 1966
Although the dangers of radium luminous paint had been identified by the 1950s and alternative luminous compounds were being used or developed, the use of radium based luminous paint didn't stop overnight, nor were all the items with paint taken out of service at that time. There was nothing else that gave such a bright and lasting glow in the dark effect, there isn't anything even today that compares to the brightness of luminous radium paint. Radioactive luminous paint continued to be used for some applications, and of course watches and other items such as alarm clocks and aircraft instruments manufactured in earlier times continued to be used. The picture here shows a warning in an Air Publication British military standard issued to the Navy and Royal Air Force in July 1966.
If your watch was made before 1960, and has thick paint, often even gungy looking in WW1 era watches, often yellowish brown on WW1 era and green on later watches, on the hands and numerals, and the hands and numerals are designed to carry a large area of paint rather than thin brush strokes, then the likelihood is that the original paint was made with radium, even if it no longer glows in the dark at all.
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Old Radium Paint
Borgel Watch with Radioactive Paint
The half-life of radium is about 1,600 years, so over the 100 or so years since a watch dial was painted with luminous paint, the radium activity will have decayed about 4%, about 1% for every 25 years. This means that paint that was made with radium 100 years ago will still be 96% as radioactive today as the day it was made. Radium and its decay products (such as radon gas) have the potential to cause various health risks, and therefore watches with radium paint should be handled carefully and in ways to minimize these risks. The danger is not so much from radiation received from the watch when wearing or being near to it, but more from inhalation or ingestion of paint particles or contaminated dust.
Why doesn't old radium paint glow in the dark? You can't see radiation, and radium paint only glowed because the radioactive radium was mixed with a fluorescent compound, usually based on zinc sulphide. Radiation from the radium would hit the zinc sulphide, which would emit light in response. Over time the fluorescence property of the zinc sulphide was worn out by the radiation, and that is why the paint no longer glows in the dark. The radium is still there, almost as active as the day the paint was mixed, but the fluorescent compound no longer gives off light when it is hit by radiation.
There is also another problem. The radium and zinc sulphide were powdered and mixed with a binder such as clear varnish to make the paint. Over the years the radiation breaks down the molecular bonds in the varnish and it becomes weak and brittle. This is why old radium paint has often flaked off the hands and numerals of a watch dial. The flakes of paint can be as fine as dust, which can obviously be easily distributed about by accident, and even accidentally breathed in. If it was ordinary paint this wouldn't be much of a problem, but each flake of radium paint carries an amount of radioactive radium with it.
Radium and its decay products, the things it breaks down into (and the things that those things also break down into) emit alpha, beta and gamma rays.
Alpha particles are composed of two neutrons and two protons, the same as the nucleus of a helium atom. This means that they are (relatively) large and slow moving, and have a positive charge. They can be stopped by a few inches of air or by the surface layers of your skin or, more importantly, the case or crystal of your watch. However, if flakes of radium paint are inhaled they can sit in the lungs, where the alpha radiation can hit the delicate (much more delicate than skin) internal tissue of the lungs and cause tumours. Radium behaves chemically like calcium and, if ingested, is deposited in bones mineral, where it remains for a very long time.
Beta particles are electrons which will also be stopped by the watch case or crystal. Gamma rays are electromagnetic radiation, like light or X-rays, but with higher energy. They will easily pass through the case of a watch.
There is no need to panic if you realise you have watch with radium paint. The watch that described below that was tested with a radiation detector is a 1918 World War 1 era watch, and whoever painted the luminous paint on the hands and dial seems to have applied a good thick coating, and the paint seems to have been particularly radioactive. I guess that at the time they just wanted a really good glow in the dark, and didn't appreciate the dangers. Later watches that I have tested since have much less paint on the hands and numerals, and are a lot less radioactive. I have a later watch from the 1920s, still in a Borgel case like the first one I tested, but the paint is a lot finer in application, rather than the big daubs on the first watch, and perhaps incorporates less radium - after all, it was an expensive substance. This second watch with the finer paint registers much lower on the radiation detector, and I wouldn't be anything like as concerned in wearing it.
Any watch with radium paint on the dial is reasonably safe to wear on a once-in-a-while basis, and perhaps more regularly than that if the paint has been lightly applied, unlike the watch pictured on this page. But to minimise risks it would still be sensible to not wear it all the time, and certainly don't sleep wearing it, or keep it on your bedside table, and be aware of the radon gas that will be leaking from the watch so keep it in a well ventilated area.
The much more significant danger occurs when opening or working on such a watch, and workers must be particularly careful not to breathe in any of the paint or its dust, or to spread bits of the paint around, contaminating the workspace.
Further information for persons working with watches that have this type of luminous paint was contained in an article in HSE Radiation Protection News Issue 21 May 2002 "New Controls Introduced On Timepieces Containing Radioactive Substances".
The article states "Up until now those in the retail and antique trade have been free to dispose of damaged clocks and watches luminised with radioactive material, that are beyond repair, in the dustbin with other general refuse. Today because of a change in legislation made necessary by the Basic Safety Standards Directive (96/29/EURATOM) the situation has changed ... and you may be required, in certain circumstances, to seek Environment Agency approval under the Radioactive Substances Act 1993 (RSA 93) before disposing of such timepieces. Those circumstances are:
- if the timepiece is not made to international standards (which govern the use of radioactive substances at the time of manufacture); or
- if more than 5 timepieces, which meet international standards, are to be disposed of."
When watches were made with radium paint during the first half of the twentieth century there were no international standards governing the use of radioactive substances, these all seem to be modern (post 1950s) standards governing the use of tritium and promethium compounds. This would mean that all watches with radium paint fall into the first of the two categories listed and would need approval for disposal.
The article goes on to say "A major implication of this change in the law is that those in the retail and antique trade will now need to know what radioactive substances are on their premises and in what quantity." Although it is a "major implication", since this newsletter was published in 2002 there appears to have been no attempt to make anyone aware of the legislation, or any action taken to enforce it. I would say that the overwhelming majority of collectors, and those in the retail and antique trade, are totally unaware of this legislation.
The article finishes with a final clause "Please note this legislation does not apply to private individuals with a related horological hobby/collection."
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Testing for Radioactivity and Radon
If you think your watch has luminous paint on the hands and numerals, the first easy test is to keep it in the dark overnight and examine it while still in the dark. Then expose it to sunlight and take it into a dark room. If it was dark after being kept in the dark overnight, and then charged up in the sunlight and glows brightly in the dark, it has a modern "light charged" luminous compound and is quite safe. Although zinc sulphide will charge up in sunlight and then glow in the dark, old radiation damaged zinc sulphide exhibits very little response to sunlight and its characteristics are very different to modern luminous paint. If the zinc sulphide doesn't glow in the dark due to radium radioactivity, but does glow when charged up by sunlight, then logic says there isn't any radium present.
However, if the paint remains dark at all times, then it may have radium paint. If it does glow faintly in the dark but doesn't charge up in sunlight, then it almost certainly has radium paint.
I bought a radiation detector so that I can test my own watches for radium based luminous paint. I am glad that I did - the first watch that I tried it on, the same black dial trench watch with plenty of luminous paint left on the dial, gave a strong reading as you can see below.
Having now examined and tested a few watches it appears that some of the early service watches from World War One era are very radioactive, much more so than ones made just a few years later. When it was first discovered radiation was not realised to be harmful, and the desire to get a good strong glow in the dark seems to have resulted in radium paint with a very high level of radium content which was applied with a heavy hand. Later watches from the era of the second world war seem to have paint with lower radium content and which was applied more sparingly.
The first picture shows the radiation detector reading 0.14 micro sievert per hour background level, which is pretty normal, and the alarm level set by the factory at 0.3 micro sievert per hour (µSv/hr). The manual says that levels above 0.2 µSv/hr above background radiation need investigating and either clean up or evacuation, so with background at 0.14 µSv/hr the alarm level of 0.3 µSv/hr is just below the level at which action is required.
The second picture shows the effect of putting the watch next to the detector: the level jumps to 1.92 micro sievert per hour and the audible alarm goes beserk. In the manual accompanying the detector, it says ". . . if a dose rate of more than 1.20 micro sievert per hour is displayed, it is necessary to leave the zone urgently . . ." Although this warning refers to radiation levels which affect a whole area, rather than from a point source such as a watch where the intensity of the radiation experienced can be diminished by simply moving away from the watch, it does show that the amount of radiation given off by the watch is more than 10 times the naturally occurring background radiation and should not be simply ignored.
Most watch repairers and many watch collectors are aware of the dangers of ionising radiation from radioluminescent paint. However, it has only recently (June 2018) been noted that radon gas given off as radium decays can also be a significant hazard. Because of this, any items that are likely to give off radon gas should be kept in sealed containers and only taken out in places where there is plenty of ventilation.
It is also worth considering getting a radon detector to monitor the radon levels in your working and living spaces to make sure they are below the advised thresholds. Levels of airborne activity above 100 becquerel per cubic metre require action to reduce them. One becquerel is defined as one radioactive decay per second.
Smartphone Pocket Geiger
Pocket Geiger Smart Radiation Detectors
Click on the image or THIS LINK to go to the web site.
I bought my radiation detector some time ago and it was quite expensive. However, as a result of the nuclear accident at Fukushima, Japan, an organisation was set up to develop inexpensive radiation detectors for everyone. The resulting product is the Pocket Geiger Smart Radiation Detector, an accessory which is available for both IOS (iPhone, iPad etc.) and Android devices that works with a free downloadable app to display radiation readings on your smart device.
The Pocket Geiger is available in several different models and prices from around £40 from Radiation Watch UK. I haven't tried one of these myself yet so this is not a personal endorsement, but it looks like a handy and inexpensive piece of kit for anyone handling old watches with potentially radioactive paint, or even for anyone who is just interested to know how much radiation is in their surroundings. If you get one of these, do let me know how you get on with it.
Detect Alpha, Beta or Gamma?
Radiation comes in alpha, beta and gamma flavours. Most radiation detectors measure just gamma radiation. I am sometimes asked whether a detector that measures beta and alpha radiation would be better.
I use a gamma radiation detector simply to tell me if there is radioactive paint present or not. If it is, then I know what it was made from and I don't care to investigate in more depth.
Some models of radiation detector can also measure beta radiation. If you have a surplus of money and are interested in beta radiation, then by all means go for one of these. If you are just interested in detecting radioactive paint on watches it will only be your wallet that notices the difference between the two models.
Alpha radiation is easily stopped, which makes detection difficult. Alpha detectors have a very thin mica window over the sensor which only attenuates the radiation slightly. They are expensive and very fragile; the mica window breaks very easily, and then you just have a really unusual paperweight. In my view, measuring alpha is an unnecessary and expensive exercise for watch collectors.
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Another Radioactive Watch
I didn't measure the radiation level of the watch above with the dial out of the case. However, here is another which I did test.
When it arrived it was in a zip lock bag, and the crystal was missing. I could see that the paint was the original radium paint so I decided to see what I was dealing with before I took it out of the bag. As you can see, I got a reading of over 5 µSv/hr. This rather surprised me because it was a lot higher than I had seen with the watch I described above, and this one seemed to have less paint remaining than that one. The higher reading may have been due to beta radiation that would have been stopped by the crystal but which could pass through the zip lock bag. I decided to remove the hands and dial, working on disposable paper towels and wearing a dust mask.
The next picture shows the dial and hands next to the radiation meter and, as you can see, it has gone off scale, so the radiation from the dial and hands is over 9.99 µSv/hr. I don't know how much over, because this is the maximum reading of the meter. As you can also see from this picture, there isn't much of the original radium paint left, just bits on the numbers from 12 to 4 and a bit on the 10 and the hour hand. That this small amount of paint produced so high a reading surprised me.
I then stripped the radium paint off the dial and hands and took some more readings. The dial then gave a reading of 1.47 µSv/hr so I guess I didn't quite get rid off all the paint.
I tested the movement after I had removed the dial, and also the case, before I did anything to them. I got a reading of 0.64 µSv/hr off the movement and 0.43 µSv/hr off the case.
Why were the movement and case showing a reading after I had removed the dial? One of the decay products of radium is radon gas, which can obviously move around inside the watch and, after a few days, itself decays into solid materials. These are also radioactive and settle as dust on the watch movement and case. Obviously this dust is quite mobile so you need to be careful not to spread it around and contaminate your workspace. Another explanation is that the interaction of the gamma radiation with the jewels in the watch movement can result in the emission of neutrons that can activate metal parts.
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In the 1970s and 1980s when I was working on nuclear power stations construction sites I was a "Classified Radiation Worker" qualified for "radiation and hot temperature conditions", which meant that I was "allowed" to receive as part of my work an annual radiation dose of up to 5 rem, 20 times what was allowed for the general public to be exposed to. (In fact, I probably got more dose from the annual chest x-ray I had as part of my radiation worker's medical tests than I did in work.) I am used to talking in rems so the modern Sievert units leave me a bit in the dark, but I decided I needed to get up to speed and get familiar with the new units that my radiation detector uses, so here goes:
The impact of ionising radiation is measured as energy absorbed in body tissue in units of "gray". Exposure to different types of radiation does not produce equal biological effects, so when discussing radiation effects an equalising unit called a rem (Roentgen equivalent man), or now a Sievert (Sv) is used. Regardless of the type of radiation, one rem or sievert of radiation received produces the same biological effect. One sievert equals 100 rem, so my allowed exposure that used to be 5 rem per year would now be called 50 millisieverts per year (mSv/yr) (although I notice they have now reduced the allowed exposure for nuclear workers to 20 mSv/yr, or 2 rem/yr.)
In the UK, the legal limit for radiation exposure for members of the public from artificial sources such as nuclear plants is 1 mSv a year. The "background radiation", the natural radiation that we are all exposed to is reckoned to be about 2.7 mSv per year, some of which comes from natural sources in the ground and in rocks, and some from cosmic radiation. Flying exposes you to more cosmic radiation and an airline crew flying the New York to Tokyo polar route are estimated to receive an annual exposure of 9 mSv per year because of this.
A steady dose rate that gave an exposure of 1 mSv/yr would be about 0.1 micro sievert per hour (µSv/hr), 10 mSv/yr would be about 1 µSv/hr. The instruction manual which came with my radiation detector says that "defensive measures" (it's translated directly from Russian) should be employed if the measured radiation level in a room exceeds the external background level by 0.2 µSv/hr or more, which would correspond to 1.75 mSv/yr. But this would be all around you giving you a whole body does, not like the dose from the point source of a watch.
The background reading I measured away from the watch above of 0.14 µSv/hr would be 1.22 mSv/yr, which is below the usually quoted "typical" background radiation. The elevated level next to the watch of 1.92 micro sievert per hour works out at 16.8 mSv/yr, or about 10 times background radiation. This will all be gamma radiation escaping through the case or crystal of the watch. So this is the level of radiation someone wearing the watch would be exposed to. Obviously it's not "whole body" radiation, which the limits I discussed above are based on, but it is higher than I would want to be continuously exposed to.
I discussed this whole matter with the NRPB. The view of the person that I spoke to was that the dose of radiation that would be received from working on a watch like this for a short time was low enough to not be a concern. A dose rate of say 10 µSv/hr would take 100 hours to accumulate the 1 mSv allowed to members of the public from nuclear power plants, and 200 hours to equal background radiation. And this is not a whole body dose, which those figures are for. However, when working on a watch one is very close to it, and the eyes and fingers would be most exposed. Wearing an eyeglass or similar would protect the eyes from alpha and beta radiation, but would not stop the gamma.
The figure of 10 µSv/hr was rather plucked from the air because my radiation detector had gone off-scale at 9.99 µSv/hr. The issue of "Radiation Protection News" that I linked to above also has a section about vintage aircraft which notes that many aircraft built before circa 1950 have instrument dials luminised with radium paint. Dose rates on contact with these instruments can sometimes exceed 200 µSv/hr and dose rates in the pilot's position may be as high as 15 µSv/hr. The note goes on to give guidance that where dose rates in excess of 2.5 µSv/hr exist, steps should be taken to reduce radiation levels and/or prohibit access by members of the public. On the face of it, the radiation received from a watch would be broadly similar in extent to the radiation received from an aircraft instrument while sitting in the pilot's seat, and remember that 2.5 µSv/hr is less than I measured off the second watch while it was in its zip-lock bag.
The NRPB man was also concerned about removing the paint from the dial and hands, as this could be seen as creating "radioactive waste". The view seemed to be that as far as the authorities are concerned the paint is OK so long as it is on the dial inside the watch case, but that you shouldn't open the timepiece or remove the paint from the dial. The official view seems to be, it's OK to have one of these watches, but don't get too close to it, don't wear it, don't work on it, and don't remove the radioactive paint, but this doesn't apply if you are a private individual with a related horological hobby/collection.
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What are the Dangers?
There are three sources of potential danger from radioluminescent paint on a watch dial: gamma radiation, alpha radiation and radon gas.
Gamma radiation will easily pass through the watch case or crystal, but since it also passes through most materials without much interaction it is less damaging than alpha radiation. A watch with radioluminescent paint is a "point source" of gamma radiation, which you can reduce your exposure to by simply moving away from it. The published medical effects are usually based on "whole body" irradiation, the sort of thing that you would get from entering an area contaminated with radioactive material, like the site around Chernobyl. Simply being in a room along with a watch that is emitting a certain level of radiation is not the same as being in an area where every surface is covered with radioactive material.
Alpha radiation is stopped by the case or crystal, but if the watch case is opened, flakes of paint or paint dust can escape, and these are alpha emitters. Little flakes of radium paint obviously aren't going to give you a radiation dose over your whole body, but a flake of alpha emitting radium paint breathed in could potentially cause a tumour in your lungs; radium paint swallowed and digested is processed by the body similarly to calcium, and has been termed a "bone seeker" because it is selectively accumulated and retained in bones, which may cause long term effects. I don't know of any study into the actual health effects on watchmakers who have been working for years with radium paint; it may be not a lot, because some old watchmakers say that they have restored or replaced the lume on old watches and are not dead yet, but this is not a good reason to ignore the dangers.
When radium decays, it emits an alpha particle and turns into the element radon, which is a gas at room temperature. Radon gas is an alpha emitter and, as a gas, will escape from all but the best sealed, fully waterproof, watch cases.
Until recently the radon given off by radioluminescent paint on watch dials has not been recognised as a significant health hazard. However, a study by Dr Crockett and Professor Gillmore is the first to quantify the danger posed by radon gas given off by timepieces and found that it could be significant. The study found that the levels of radon were high enough to be dangerous even in large spaces, such as whole houses.
Radon gas is given off by many natural materials such as granite rocks, and it is now known that breathing radon in homes from these natural sources is the second most frequent cause of lung cancer after smoking, and the leading cause among non-smokers. This was recognised some years ago, and concerns that houses in granite rock regions could have dangerously high levels of radon resulted led to people being urged to test their houses for radon. New build houses in regions where this is a concern have radon gas barriers built into their foundations.
Release of radon from a watch with radioluminescent paint depends not only on the luminous paint, but also on the type of case. A ‘jointed case’ with a hinged back will allow all the radon produced to escape, whereas a well-sealed watch will contain the radon until the case is opened. However, seals deteriorate over time, and watches that were once waterproof may not still be well-sealed. Radon has a half life of about four days, so when a new source is introduced the radon level will build up over a period of days to a maximum when the rate of decay of the gas in the room is equivalent to the rate of its production and release from the source. Effective ventilation can reduce the level of radon gas in a space, but it needs to be continuous to be effective. However, if the room is not well ventilated, radon gas given off by the decay of radium in the paint could accumulate above the recommended level.
When radon gas decays, daughter particles of solid materials including polonium, isotopes of lead and bismuth are formed. These are also radioactive and themselves decay, emitting alpha and/or beta particles as they do. Radon daughters are solids and will stick to surfaces, including airborne dust particles. This means that any enclosed space where there has been radium or radon will most likely have some of these radioactive elements around in the form of very fine particles or dust.
Alpha emitters such as radium and radon are particularly dangerous if inhaled because alpha radiation can damage the delicate tissue of the lungs and cause lung cancer. Radon gas is given off by many natural materials such as granite rocks, and it is now know that breathing in radon in homes from these natural sources is a leading cause of lung cancer deaths. Some years ago it was realised that houses in regions of granite rock could have dangerously high levels of radon and people were urged to test for radon. New build houses in regions where this is a concern have radon gas barriers built into their foundations.
At the request of a correspondent I tested a watch for radon gas emissions. The watch is visible in the picture. It is an anonymous ‘non-brand’ with a screw back that I would have expected to be reasonably well sealed. I would guess it dates to the 1950s. The hands have been relumed with modern light-charged paint, the only sources of radiation are the small hour marker dots and the amount of radiation from these measured by a Geiger counter was relatively low compared to some watches I have seen.
The watch was put into a small airtight box with the radon detector for five days. As you can see from the image, the radon in the box took the 7 day reading off scale on the detector, which answered the basic question; it is giving off radon gas. It is difficult to quantify this because the radon detector takes at least 24 hours to settle down, and even opening a door affects the amount of gas in a room which is why I did the test in a sealed box. The box is only a tiny fraction of the air volume of a whole room, so the steady reading after four days is likely to be lower than recorded in the box, but how much lower I don't know. If the watch were mine I would store it in an air tight box and not open the box inside the house.
Any items that are likely to give off radon gas should be kept in sealed containers and only taken out in places where there is plenty of ventilation. If you are a serious collector it is also worth considering getting a radon detector to monitor the radon levels in your working and living spaces to make sure they are below the advised thresholds.
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Is there a possibility that the radiation from radium luminous paint will induce radioactivity into the other parts of the watch, or something nearby? As far as I am aware, only neutron radiation is able to induce radioactivity in something else, and radium does not emit neutrons, so the short answer is no.
However, aluminium is a weak converter of alpha radiation to neutron radiation. Watches contain some aluminium oxide in the form of jewel bearings and sapphire glass. For early watches with high levels of radiation from radium paint, sapphire glass can be discounted because it wasn't used until the 1930s. The jewel bearings in jewelled watches will generate a small amount of neutron radiation which, over the years, possibly might induce radioactivity in nearby metal. My feeling is that this effect will be small and not something to worry about.
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What's the Risk?
So the question is, should you avoid old watches with radium paint on them, because of the risk to your health from the radiation? This is something only you can decide, and depends on your attitude to risk.
Although I have worked in the nuclear industry I don't have a qualification on the health effects of radiation, so these are just my thoughts based on my experience and on what I have picked up over the years. All nuclear radiation carries some risk to human health, and as radium gives off nuclear radiation, radium paint must be a health risk. However, the question here is: what are the risks from the small amount of radiation emitted by radium paint on a watch dial?
Radiation is scary, conjuring up images of atomic bombs, mutation and cancer, but many people are either not aware, or forget, downplay or overlook, that we are all continuously exposed to natural radioactivity from soil and rocks, and cosmic radiation coming from space.
We are all exposed to radiation all the time, natural radiation from cosmic rays, from radon gas escaping from the ground, etc.; and also to man made radiation from dental xrays, etc. etc. (there is no difference between natural and man made radiation, it's just that the source is different). Every animal on the planet including the human body has evolved repair mechanisms for coping with the damage caused by natural levels radiation, or else they and we wouldn't be here. These mechanisms can cope with levels of man made radiation that are similar in intensity to the natural radiation, but obviously if the intensity of the radiation is higher, then more damage can be caused than the repair mechanisms have evolved to cope with, and that is when health effects occur.
The Chernobyl accident that occurred in 1986 was one of the worst nuclear accidents that have occurred. But people who visit the Chernobyl Exclusion Zone, from where the inhabitants were evacuated, generally receive more radiation from cosmic radiation on their flight to Ukraine, and their inhalation dose from breathing the air is less than they would get in places with granite rocks like Cornwall.
When I worked in the nuclear nuclear power industry the safety culture was that radiation doses to the general public should not only be low, i.e. well below the level at which health effects were known to occur, they should be be ALARP - As Low As Reasonably Practical. The reasoning behind this is that exposure to nuclear radiation can, but doesn't necessarily, cause cancer. The higher the dose of radiation, the greater is the risk. Small doses of radiation carry only a small risk, but governments who have the health of millions of people to worry about get concerned about small risks because a small risk times millions of people will result in some positive outcomes, that is why members of the public are not allowed into the cockpits of old aeroplanes if the instruments have radium paint on them, because it is a small but easily avoidable risk.
Just wearing a watch with radium paint means that you get a dose of gamma radiation, most intensely to your wrist, but also all over your body. The dose is not high enough that you will see any immediate health effects such as reddening of the skin, but it adds to the dose that you get from cosmic radiation, natural background radiation, medical x-rays, etc. The dose will be small, especially if you don't wear the watch very often, and is probably small enough that the risk will be very low, smaller than the risk from other sources, and probably very much smaller than the risk you get from driving to work each day. So I personally don't worry too much about it, but I am always aware that it is there.
Working on watches with radium paint presents an entirely different level of risk to simply owning one. Opening the watch case obviously opens the possibility of doses of alpha and beta radiation simply because the paint is no longer behind the crystal. There is also the danger from loose flakes of the old paint that can fall off at any time. If you do not take precautions, such as wearing a mask, you might breath in or swallow flakes of radium paint, and then you would have radioactive, alpha emitting, material inside your body, which is definitely something to be avoided.
So my bottom line is that wearing an old watch with radium paint may involve a small health risk, but that risk is probably very low and smaller than other risks that I am exposed to in everyday life such as driving a car or regularly eating red meat. I am comfortable with that level of risk and continue with my life and my hobby, but I am always aware that an old watch may well have radium paint, and I am aware of the dangers. I am cautious about radon and do not keep watches with radioluminescent paint in my living areas, or in the open in working areas. If I was to work on an old watch, I would be aware that the dangers are greater and I would take the precautions that I outline elsewhere on this page.
If you are not comfortable with any level of risk, or want to do everything you possibly can to minimise yours, then you might want to avoid old watches - and at the same time you should also think about not driving to work, eating red meat, flying, etc. etc., which almost certainly have higher levels of risk.
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Naturally-occurring background radiation is the main source of exposure for most people. Levels typically range from about 1.5 to 3.5 millisievert per year (mSv/yr). The average background radiation dose in the UK is 2.7 mSv/yr per year, of which about half comes from radioactive radon gas leaking out of the ground.
Cornwall is a radon hotspot where the average background radiation dose is 7.8 mSv due to the presence of igneous granite which naturally contains more uranium than the average rocks or clays of the UK. Flying exposes you to more cosmic radiation because the cosmic rays are less absorbed by the atmosphere at high altitude than they are at ground level. An airline crew flying the New York to Tokyo polar route are estimated to receive an annual exposure of 9 mSv per year because of this.
A steady dose rate that gave an exposure of 1 mSv/yr would be about 0.1 micro sievert per hour (µSv/hr), 10 mSv/yr would be the result of a steady dose rate of about 1 µSv/hr. There are 8,760 hours in a year, which can be rounded to 10,000 for quick estimates. The table below uses 8,760.
|Event||Annual Dose||Dose Rate|
|UK average background radiation||2.7 mSv||0.31 µSv/hr|
|Cornwall background radiation||7.8 mSv||0.89 µSv/hr|
|Flying New York to Tokyo||9 mSv||1 µSv/hr|
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I am often contacted by people who have come into contact with, or near to, a watch, compass, or other item, that they later realise might have radium paint. Although obviously I can't offer specific advice, here are a few general thoughts.
So long as the case and glass are intact, most of the radiation from the paint, if it is in fact radioactive, will not escape. Gamma rays will pass easily through the glass, or through the case, and through you, mostly harmlessly. We are all exposed to gamma rays all the time from the environment, food, X-rays and, most importantly, cosmic rays. Our cells have evolved repair mechanisms to cope with these. It is only when a large amount of radiation causes simultaneous multiple damages to cells and to their repair mechanisms that problems occur. The small additional exposure received by being in contact with or near to a watch with radium is unlikely to do any harm. However, unnecessary exposure to radiation should be avoided and watches and instruments with radium paint should be treated with due caution.
Radium is chemically similar to calcium and active bone cells will take up the radium like calcium. If you swallow radium paint, some radium will end up in your bones; this is what happened to the famous radium girls. But they were not just tipping their paintbrushes with their lips as is often reported, they were deliberately painting their teeth and lips with radium paint. I think you would need to swallow quite a lot of paint to cause a major problem, but of course swallowing any radioactive material should be avoided if at all possible.
The greater danger is that a dust sized particle of paint gets taken into a lung when you breathe in. The alpha radiation hitting the delicate lung tissue can cause a tumour to start. In this case a very small amount can be dangerous and particular precautions should be taken to not breathe in particles of paint or dust. The lungs have a good dust removal function in the motile cilia which keep the airways clear of mucus, pollen and dust. This is impaired in smokers and gives rise to smoker's cough. Generally these mechanisms would move out any inhaled particles of paint in the mucus, which then gets swallowed and passed out. There are watch repairers who in the 1940s and 1950s worked on radioactive watches without any precautions and they weren't and aren't dropping like flies. However, radiation is dangerous and it is wise to be cautious.
Radon gas is emitted as radium decays and is itself radioactive. Radon gas is in the environment as a result of natural radioactive decay, mainly radium in granite rocks, and people in houses in granite areas can get a significant exposure from regularly breathing it in. Because of the emission of radon gas, watches containing radioluminescent paint should not be stored in the open in living or working areas. Storage of items in a sealed container would contain the radon gas, but the container would need to be opened outdoors, or in a room with continuous natural or forced ventilation; it must be noted that radon is a heavy gas, heavier than air, which does not disperse easily. A collector should give serious consideration to the purchase of a radon detector.
Whether a watch or other item actually contains any radioactive material can only be confirmed by a radiation detector. The level of radon in a room can only be established by a radon detector.
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Removing Radium Paint
If the radioluminescent paint is removed and the watch cleaned, the radioactivity of the watch will be essentially nil, lower than the level of the background radiation which is all around us all the time.
Watches do not become radioactive as a result of having radioluminescent paint. That is a phenomenon that affects nuclear reactors which emit massive amounts of neutrons and is called induced radiation.
It is illegal in the UK to work with radioactive substances or to accumulate radioactive waste outside of a licensed nuclear facility, so you will so you will not find anyone advertising commercially to remove radium paint from watch dials.
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Refinishing Luminous Dials
Over time I have changed my views about preserving or removing radium paint. I now think that unless there is some overriding reason, radium paint should whenever possible be removed and replaced with a safe modern alternative. Old radium paint is fragile due to radiation damage over the years and it is prone to flaking and disintegrating into small particles and virtually invisible dust, all of which are radioactive. Trench watches have enamel dials; the radium paint is not an integral part of the dial and so it can be stripped off and replaced without affecting the original dial or the hands.
One of the first Borgel watches I collected is shown here with its original radioluminescent paint. This was highly radioactive and I decided to get it replaced. The watch was restored by a company who replaced the old radioluminescent paint with modern luminous Super Luminova light charged paint. I don't have a problem in principle with the use of modern luminous paint, but they haven't reproduced the colour and appearance of the old paint and this has rather spoiled the vintage appearance of the watch. This has caused me to be more cautious in having dials restored.
Radioluminescent paint is made from a tiny amount of radioactive substance, radium or similar, mixed with a fluorescent material, usually zinc sulphide, in a clear, colourless, transparent binder like varnish. When it was new, all that was visible was the zinc sulphide which is a white or pale yellow in colour. However, old radioluminescent paint has a dark brown or olive green colour. This is because the radiation from the radioactive material has damaged the clear binder of the years, like an accelerated ageing. Even if the zinc sulphide wasn't burnt out from the radiation, the dark colour of the binder means that it probably would let out any luminescent light today.
The fundamental problem with modern luminous paint is that it doesn't look like old radium paint, but if it is tinted to look like the old paint, then it doesn't glow.
There are tinted paints available, or colour can be added, but when enough coloured pigment is added to make the paint look like old radium paint, it pretty well eliminates any glow from the luminous compound. Even luminous paint that is described as “vintage tint” or similar does not produce much glow in the dark and is, in my opinion, a waste of time. If you find something that looks vintage but glows well in the dark, please let me know.
The best option to retain an original appearance is to use an acrylic paint which accurately matches the colour of old radium paint and forget about making it luminous.
If you want to get a dial painter to replace and match old radium paint, I recommend that you take some close up pictures of the dial and hands and send them to several companies. Give a detailed description of what you want doing and ask them if they can do it in the way that you wish. If you want the dial and hands to look exactly like they do now, paint colour and finish included, make sure you say so. Be as detailed as you can in describing what you want, don't imagine that they will know. Take up references and do your due diligence before committing your dial to them.
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Reactivating Radium Paint
I sometimes get asked about the possibility of painting over old radium with zinc sulphide to get a glow from still active the radium, and on the face of it this is an interesting idea. Radium will be still virtually as radioactive as it was when it was first applied (although if the effect was boosted at the time by mesothorium that will have decayed long ago). And by all accounts, modern luminous paint has nothing like the same glow as radium paint. A friend of mine who is in his mid seventies remembers from the 1950s one of his brothers having a belt with the buckle carrying radium luminous paint, and he says "you could see it glowing right across the [farm] yard, it was really bright".
One problem is that zinc sulphide needs to be "doped" with something, e.g. copper, silver, manganese, to get the best phosphorescent effect and I am not sure how this could be done. Another problem is that the radium and the old zinc sulphide were bound intimately together in varnish and I think that applying the phosphorescent paint on top of the old paint will not achieve as good a glow. There is also the problem of painting onto skeleton hands. These are painted from the back and it would not look very good to paint new material onto the front, it would be impossible to get a neat result. But these are just speculative ideas and not something that should be attempted.
In reality, messing about with old radium paint in this way is asking for trouble and not something that I would do, and I would strongly recommend that you don't do it either. Old radium paint is fragile due to radiation damage over the years and it is prone to flaking and disintegrating into small particles and virtually invisible dust, all of which are radioactive. Old radium paint needs to be approached and handled with great care. Any attempt to remove old radium paint and reprocess it in some way is also illegal in the UK under health and safety laws. Although often the object of jokes, health and safety laws are intended to keep you and everyone around you safe and, in the case of radioactive substances, should be taken seriously.
Bottom line is: don't do it.
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So what's my final position on all of this, I hear you asking. Well, as a private individual with a horological hobby and collection, the rules laid down by the Environment Agency don't apply to me. However, I don't want to take any unnecessary risks with my own health so I am going to be careful when handling watches luminised with radioactive paint and take sensible precautions.
If you insist on removing radium paint yourself, then I suggest you be very careful. I can't give you any "guidelines" for obvious reasons, but here is what I would do, just for your information. These are not recommendations, just notes of what might do in a hypothetical situation - I can't and don't take any responsibility for what you do.
Before working on a watch that looks like it has radioactive luminous paint, sometimes even before opening the packaging that it was delivered in, I test it for radiation. When handling an old watch with radioactive luminous paint I am always aware that there may be loose flakes of paint under the crystal on the dial, and radioactive dust from radon decay throughout the movement. If I was going to work on it I would wear a disposable dust mask and disposable latex gloves and work on disposable paper towels, and work quickly to minimise my exposure.
If I was going to strip the paint from the dial and hands I would use a liquid paint stripper and make sure that the dial and hands remain submerged or covered by the liquid while flaking stubborn bits of paint off - the idea being to avoid any dust or tiny flakes of paint becoming airborne. (Be aware that although this is fine for an enamel dial, which will not be affected by the paint stripper, a printed dial would be ruined buy submerging it in paint stripper.) I would then wash the movement and case using an ultrasonic cleaner to remove any dust or paint flakes before dismantling the movement. I would dispose of the mask, gloves and the paper towels, folding them up carefully so that everything was contained. I would then give the work surface where I was working a good wipe down with a damp cleaning cloth and dispose of that too. And then I would check everything with a radiation detector.
Based on the HSE document discussed above I believe that the quantity of radioactive material from single watch is acceptable to put in the bin for landfill disposal - after all, this is only the same quantity of radioactive material as an unknowing member of the public putting a complete watch in the bin, which must happen all the time. I would rather the radioactive material was disposed of properly - or even reused, I am sure that radium must still have its uses - but it appears that there is no choice other than to put it in the bin.
Research performed by Nottingham University in 2018 suggests that radon gas emitted when radium decays is also a potential concern. I have bought a radon detector to monitor the levels of activity in the air to make sure that they are below the recommended levels. I discovered that the levels of radon in some areas were higher than I was comfortable with, so now I am more cautious about radon and do not keep watches with radioluminescent paint in the open in my living or working areas.
Let me know if you have any better ideas - legal ones, that is! And remember, these are not recommendations, they are just thoughts to help you decide what to do - anything you actually do is your own responsibility.
If you have any comments or questions, please don't hesitate to get in touch via my Contact Me page.
Copyright © David Boettcher 2005 - 2023 all rights reserved. This page updated July 2023. W3CMVS. Back to the top of the page.