Is a radium dial watch dangerous to wear or keep?

A radium dial watch is not dangerous to wear in the way the headlines suggest, but it is not harmless to keep. Two answers circulate: an intact radium watch is perfectly safe, or wartime watches are a serious cancer risk. Each drops half the picture. "Intact is safe" is about the dose from wearing one, and misses that a closed case does not hold radon in. The "serious cancer risk" headline came from a 2018 university press release about radon from a collection of 30 watches in a small, poorly ventilated room, not from the dose of wearing one.

What US law allows in a watch

The Nuclear Regulatory Commission's exemption for timepieces, 10 CFR 30.15(a)(1), sets how much of each radioactive material a watch may hold before a license is needed. Radium came under it after the Energy Policy Act of 2005 put discrete radium-226 sources under NRC authority, at the 1 microcurie that model state standards already used, according to a 2008 Oak Ridge report for the NRC.

The 25,000-fold gap is about the radiation, not the quantity. Tritium's beta particle, the Health Physics Society's fact sheet says, travels about 6 mm in air and "cannot penetrate the typical thickness of the dead layer of skin". Radium's exposure comes mainly from gamma rays emitted by its decay products, lead-214 and bismuth-214, which pass through a case. The tritium figure also explains a dial marking: "SWISS T<25" declares less than 25 mCi of tritium, the same 25 millicuries the US rule allows per timepiece.

Two words in the radium line carry the weight. A watch that is no longer "intact", with a loose dial or a broken crystal, falls outside the exemption. And 1 µCi is a cap, not a typical value.

How much radium is actually in one

A 2012 study in Environment International by Gavin Gillmore, Robin Crockett and colleagues at Kingston and Northampton universities measured 15 pocket watches and 18 wristwatches, most bought on eBay, and estimated each one's radium from its dose rate:

Military issue watches "produced ambient equivalent dose rates at least twice that of non-military watches", and those are the pieces collectors most want. Readings through the front ran about 70 percent above the back, and taking the glass off one Waltham pocket watch raised its reading from 13 to 18 µSv/h. The authors call these readings "indicative", and conclude that radium dial watches "individually are a modest health risk to wearers".

The dose from wearing one

The 2012 paper reports the Oak Ridge (ORISE) dose scenarios prepared for the NRC beside its own estimates:

• Wristwatch at the 1 µCi limit: 0.61 mSv a year (ORISE). A separate ORISE scenario puts the skin under a wristwatch worn 16 hours a day at 16 mSv a year.
• Pocket watch at 1 µCi: 1.1 mSv a year in a trouser pocket, 4.8 mSv in a vest pocket (ORISE).
• The most active pocket watch in the study: 5.1 mSv a year in a chest pocket, 1.169 mSv in a trouser pocket (Gillmore et al.).
• Typical watches: "a few tens of microsieverts per year", in the answer of the health physicist Ray Johnson for the Health Physics Society.

For scale, the EPA puts the average American's yearly dose at 6.2 mSv, and the public dose limit the paper cites from UK regulations is 1 mSv a year. In the ORISE model a wristwatch sits below that limit even at the legal maximum. The paper's warning is for regularly carrying the most active pocket watches in a chest pocket, which can exceed it. It adds that sleeping in a radium watch is "highly inadvisable", because the wrist can rest by the nose and mouth all night, where the radon leaving the watch is most concentrated.

Radon: why storage is the bigger hazard

Radium-226 decays to radon-222, a gas, and the case does not keep it in. The 2012 team put the least radioactive complete wristwatch in their sample, a Newmark, into an alpha spectrometer chamber and detected radon escaping from it. Every watch is a small radon source, opened or not. Here are the published figures beside the levels at which the US and UK tell householders to act:

The 2018 headline figure needs reading carefully. The University of Northampton's release says the collection reached "134 times greater than the UK's recommended 'safe' level", but in the same release that is "more than 130 times greater than the Domestic Target Level" of 100 Bq/m³, and "approximately 67 times the Domestic Action Level of 200". Both describe about 13,400 Bq/m³. The 2018 paper's abstract says the collection was "capable of giving rise to radon concentrations two orders of magnitude greater" than the Action Level in "unventilated or poorly ventilated rooms".

Single watches count too. In the same poorly ventilated room the Swiss pocket watch alone passed 1,200 Bq/m³, and two wristwatches each passed 200. The 2012 authors point out that their 67 m³ test room is "likely to be considerably larger than" a typical collector's storage space, and that collectors who keep watches in small closed rooms or sealed cabinets expose themselves to "very high concentrations of radon" when they are with them.

How to store a radium watch

Volume and air change keep concentrations down: the 2012 room stayed in the low hundreds only while the ventilation ran at full rate.

• Not in a closed drawer, safe, watch box or cabinet in a room you use. Our guide to watch storage covers ordinary pieces; radium pieces belong apart, somewhere ventilated and little used.
• Air the space before you spend time with a collection. The 2012 authors add that collectors have "a duty of care" to visitors.
• Test the room. A household radon test gives a number to hold against the 150 and 200 Bq/m³ levels above.
• Do not sleep wearing one.

Why you should not open the case

"The main concern with radium has to do with ingestion", Ray Johnson writes for the Health Physics Society, because radium in bone "will basically stay there forever". The EPA's advice is direct: "Do not attempt to take apart radium watches or instrument dials", and radioactive antiques "are usually not a health risk as long as they are intact and in good condition".

Opening the case frees loose paint, which the Northampton release warns "will contain radium particles which could be ingested", and removing the glass raises the external dose, as the Waltham reading showed. Hand a radium watch to a watchmaker for a service only after telling them what it is, so they can decide whether and how to do the work. Never relume, scrape or clean a radium dial at home.

If a radium watch is broken, with the glass gone or paint loose inside, the EPA says to dispose of it and to ask your state or local radiation control program how.

How to tell radium from tritium

Four checks, the last the only conclusive one:

1. Age. Tritium and promethium-147 replaced radium from the 1950s and 1960s, says the Oak Ridge report; the 2012 paper puts the change in the late 1960s, after a 1967 IAEA recommendation; and the EPA says that by the 1970s radium was no longer used on dials. A self-luminous dial from before about 1970 is a candidate. A refinished dial may not match its case, as our page on spotting a redialed watch explains, so the case's age alone does not settle it.
2. Dial markings. A "T" beside "SWISS" or "SWISS MADE" at six o'clock declares tritium: "T SWISS MADE T" for under 7.5 mCi and "SWISS T<25" for under 25 mCi. An "L" on both sides means non-radioactive photoluminescent lume. Our guide to Swiss origin marks shows where to look. No marking proves nothing.
3. Glow. Not a test. With a 1,600 year half-life, a 1940 dial still holds about 96 percent of its radium, so a dark dial is not a safe one; with tritium's 12.3 years, a 1975 dial keeps about 6 percent of its tritium. Our page on why lume fades covers modern pigments.
4. A radiation detector. Geiger counters are "usually not capable of detecting tritium", the Health Physics Society's fact sheet says, while the 2012 team read radium wristwatches on a handheld meter at an average of 3.0 µSv/h at 2 cm. A clear reading well above background on an old dial means radium or another emitter, not tritium; naming the isotope took the 2012 team laboratory spectrometry. Without a meter, the Health Physics Society suggests a health physicist, your state radiation control agency, a hospital nuclear medicine department or a radiation safety officer.

Buying and selling a radium watch

The US exemption covers anyone who "receives, possesses, uses, transfers, owns, or acquires" an exempt timepiece, so buying, owning and reselling an intact pre-2007 radium watch under 1 µCi needs no license; the exclusions are for those who apply radioactive material or first distribute such products. The 2012 team found radium watches "relatively common on sites such as eBay", and our pages on buying on eBay, buying pre-owned and the vintage buying guide cover the other checks. When you sell one, or ask what it is worth, say what the dial is, so the next owner can store it properly. Each wartime piece added to a collection adds to the radon in the room that holds it.

When this answer stops applying

• The watch is not intact. With a loose dial, missing glass or paint loose inside, ingestion becomes a real risk and the US exemption no longer applies.
• The item is a compass or instrument gauge. Gauges "typically have more radioactivity than timepieces", says the Oak Ridge report, and the 2012 team read a First World War pocket compass at 20 µSv/h.
• The paint is mesothorium. Cheaper 1930s products using it now have "less than one thousandth of their original activity", the 2012 paper says.
• You live outside the US. The 1 µCi figure is a US federal rule. In the UK, for example, the Health and Safety Executive noted in 2002 that traders were no longer free to dispose of damaged luminised watches with general refuse. Check your own regulator.

For the chemistry of radium paint and what replaced it, see our guide to watch lume and the comparison of Super-LumiNova, tritium and LumiBrite; the history of the wristwatch covers the trench watches with radium numerals.