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Radioactive substances in watches

  • Luminous paints are used in watch dials to ensure that they can also be recognised in the dark. The luminous paints are stimulated to glow by a radioactive substance.
  • Until well in the 60ties, luminous paints containing Radium (Ra-226) or Promethium (Pm-147), until the middle of the 90ties, paints that were enriched with Tritium were used for this purpose.
  • In watches manufactured today, tiny, narrow glass tubes filled with tritium gas (GTLS = gaseous tritium light sources) are used. For normal use of such a watch the annual effective dose is far below 0.1 µSv.

Among others, luminous paints are used in watch dials to ensure that they can also be recognised in the dark. The luminous paints are stimulated to glow by a radioactive substance.

Until well in the 60ties, the luminous dials of wrist watches and alarm clocks were marked with luminous paints containing Radium (Ra-226) or Promethium (Pm-147). These kind of watches are no longer manufactured today. This is not due to the radiation exposure to the persons wearing the watches but because of the radiological risk for the persons manufacturing them.

Until the middle of the 90ties, Tritium was used instead of Radium

Until the middle of the 90ties, paints that were enriched with Tritium (H-3), a radioactive isotope of Hydrogen, were used for marking luminous dials. Tritium is a beta-emitter with low energy of up to 19 keV and a half-life of 12.3 years. The paint was stimulated to glow by the Tritium beta radiation. The radiation is nearly entirely absorbed in the luminous paint itself and in the watchcase or the watch glass. However, Tritium as a volatile substance could diffuse through the underside of the watch casing, which was made from plastic in these watches, and could be incorporated through the skin by the person wearing the watch. Watches containing Tritium-luminous paints show a Tritium activity of about 0.2-0.3 GBq on average. The dose caused is mostly below 20 µSv per year. This corresponds to about 1/100 of the annual natural radiation exposure which is 2 mSv on average in Germany.

Today narrow glass tubes filled with tritium gas (GTLS = gaseous tritium light sources) are used

In watches manufactured today, tiny, narrow glass tubes filled with tritium gas (GTLS = gaseous tritium light sources) are used whose inner surface is coated with a special phosphorescent colour. Watches containing radioactive material are usually labelled as H3, T or T25 on the dial. The tritium beta-radiation stimulates this paint to generate continuous glowing. The glass tubes themselves, the metallic watch case and the watch glass serve as radiation shielding. The wall material of the GTLS-glass tubes is much less permeable to tritium than e. g. a plastic case. In case of a complete destruction of the watch, containing 1 GBq tritium (all tritium sources in the watch do break), only a dose of 20 µSv would be incorporated through inhalation [3]. In general, during normal use such a damage is unlikely, so the annual effective dose by everyday wearing of this type of wrist watch would be far below 0.1 µSv.

Some of the commercially available watches contain up to 15 GTLS and can have a Tritium activity up to 1.9 GBq, exceeding the exemption limits of 1 GBq of the German Radiation Protection Ordinance (StrlSchV). These watches are not allowed to be sold in Germany.

Legal prerequisites for manufacturing consumer goods in Germany according to the German Radiation Protection Act (StrlSchG) and Radiation Protection Ordinance (StrlSchV)

Wrist watches are considered "consumer goods". Consumer goods containing radioactive substances can basically be manufactured only, if the radioactive substances used do not exceed the exemption limit according to the German Radiation Protection Ordinance (StrlSchV). In Germany, this requires a licence according to § 40 Radiation Protection Act (StrlSchG). According to § 42 StrlSchG the transboundary trade of such consumer goods requires licensing too. Do not deposit a "radioactive" watch into the garbage bin. All watches suspected to have radioactive paint or gaseous Tritium must be returned to the manufacturer or to the responsible distributor (vendor) free of charge. According to § 41 StrlSchG these consumer goods must have a product instruction sheet to provide information about the radioactive content and the way it has to be disposed off. If the watch can no longer be taken back by the manufacturer free of charge according to § 44 StrlSchG, e.g. because the company no longer exists, the consumer goods can also be handed over to a state collection point for radioactive substances.

Furthermore, according to § 38 StrlSchG, the principle of "justification" applies to the use of radioactive substances. This principle obliges to compare the benefit of using radioactive substances with the possible health impairments. This consideration especially must take into account, if there are technical options available for the same purpose, that do not need radioactive substances.

Nowadays, there are very good luminous, non-radioactive paints available, which are manufactured on the basis of e.g. Strontium Aluminate (SrAl2O4).

Dealing with vintage watches (potential radium watches)

Radium was only used in old clocks and watches before around 1970 (see above) and decays in equilibrium with its shorter-lived daughter nuclides with a half-life of 1,600 years. As a result of the decay of radium and its daughter nuclides, alpha, beta and gamma radiation are emitted, with the alpha radiation being completely shielded by the watch case. One of radium's daughter nuclides is the gaseous gas Radon-222 (Rn-222), which couldan escape from the watch and be inhaled. The radioactive gas Rn-222 is only produced in watches containing radium; other nuclides used in watches cannot lead to radon contamination.

As long as there is no damage, the radiological risk posed by these consumer goods is relatively low. Since the dial and hands are enclosed in the housing, there is no risk of contamination for people or objects in the area. Radioactive substances in watches cannot activate other substances (i.e. make them radioactive).

However, due to the gamma radiation emitted, there is radiation exposure when wearing a radium watch, which depends very much on the activity (amount of radium on the hands and numbers) and the length of time it is worn. Calculations by the U.S. Nuclear Regulatory Commission (NRC) show that for example, a wristwatch with an activity of 37,000 Bq of radium, which is worn 16 hours a day, 365 days a year, receivescauses an additional local skin dose of 16 mSv. Although this is below the limit for the local skin dose according to the Radiation Protection Act (StrlSchG) Section 80 ParagraphSubsection 2 No. 2 (50 mSv per calendar year), from a radiation protection perspective it is an unnecessary additional local dose that can be avoided.

Even if no immediate health risk is expected from occasionally wearing and/or storing intact radium watches in the form of a collection, e.g. in a display case, it is advisable to remove items containing radium from your own household. In principle, these items should not be disposed of with household waste, but should be taken to a state collection point. The responsible state authority must be involved for this.

Is my watch radioactive?

In order to quickly and easily rule out the use of radioactive substances in your watches, they can be sealed light-tight for a certain period of time (2-3 days) (e.g. wrapped in aluminum foil inside a box or a drawer) and then unpacked and observed in absolute darkness whether these continue to light up. Since no excitation by UV light was or is now possible, the (after)glow should also have disappeared.

If the test confirms a suspicion that radioactive substances are being used in the watches and there is a desire to have this measured, the responsible state authority must be contacted for a professional examination or measurement of the watches.

State of 2024.01.22

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