Radioactive substances in watches

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 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 StrlSchV. In Germany, this requires a licence according to § 106 StrlSchV. According to § 108 StrlSchV 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 § 107 StrlSchV 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.

Furthermore, according to § 4 StrlSchV, 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).