I recently purchased a tritium keychain, composed of a small glass vial of tritium gas partially enclosed in a stainless steel fob. Here are the Amazon links so you can see a specific example:

The glass vial is 12mm long and 2mm diameter. Here's a picture of what they look like for future-proofing against link rot:

Tritium vial keychains

The seller claims (in slightly broken English) that this product is perfectly safe:

Because the beta decay of tritium will only emit electronic high-speed mobile, do not penetrate the human body, there is no harm to human body. The half-life is 12.3 years, electrons produce beta decay of tritium is very weak, a piece of paper can be blocked, so the tritium gas in the lamp is closed extremely safe, even if the glass tube rupture, the release of tritium gas, and the use of people to complete inhalation, but also far less than people in the normal life of the day is the amount of radiation. Tritium gas emission technology has been applied in many civil fields.

I'm aware from my own research that inhaling or ingesting the contents of the vial won't kill me, but won't be a good thing. However, my question is about the radiation emitted from the vial inside the fob, assuming it does not break.

The seller claims that the beta particles cannot penetrate the human body. However, doing a little basic reading into beta particles led me to this:

Beta particles are able to penetrate living matter to a certain extent and can change the molecular structure of molecules exposed to this type of radiation. In many cases, such changes can be considered to be damaging with results possibly as severe as cancer or death. If the struck molecule is DNA, it can cause spontaneous mutation.

Additionally, I found some forum posts online saying that the vial itself is safe, but when encased in stainless steel it emits Bremsstrahlung radiation in the form of X-Rays. That seems to be supported by this Physics SE answer about detecting X-Rays from similar tritium keychains.

All of this leads to the question: how do the beta radiation and Bremsstrahlung radiation emitted by the tritium gas compare to other common sources of background radiation I receive? How do those levels compare to the standard safety guidelines for radiation doses?

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    $\begingroup$ The betas from tritium decay are fairly low in energy. They will not penetrate the glass vial, and would not penetrate the dead layers of skin on your outside. Breathing or drinking tritium would not be a good thing. $\endgroup$
    – Jon Custer
    Mar 2, 2018 at 1:25
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    $\begingroup$ While it doesn't answer your question, you may be interested in XKCD's radiation chart I find it does a good job of helping calibrate and intuitive understanding of radiation dosages. $\endgroup$
    – Cort Ammon
    Mar 2, 2018 at 2:19
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    $\begingroup$ The hazard (or lack thereof) depends on the energy, type, and intensity of the radiation. You might want to see if they state how intense the source is. To get a feel for the units, I believe the US government is pretty permissive about sources below about 10 microcuries. They can be sent in the mail and discarded in the trash. $\endgroup$
    – user4552
    Mar 2, 2018 at 2:39
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    $\begingroup$ I also bought one of these. The betas (max energy 18.6 keV) are all absorbed by the glass. There is some x-ray (and I measured the spectrum), but that is less than what an old CRT tv would produce. $\endgroup$
    – user137289
    Mar 2, 2018 at 7:27
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    $\begingroup$ @DavidZ The estimation of the radiation emitted by this source and the corresponding dose deposited on tissue are squarely within physics, and the guidelines for safe dosages are standard bits of biophysics. The rationale for those dosages does stray away from physics, but that's not the question here. $\endgroup$ Mar 2, 2018 at 10:07

2 Answers 2


The beta electrons have a maximum energy of 18.6 keV and all of them are absorbed by the glass or plastic. But there is some Bremsstrahlung x-ray intensity. I measured the spectrum in 2015 with an Amptek silicon energy-dispersive detector, see below. The intensity was low - it took several days to collect these data. The maximum of the continuum is consistent with the maximum beta energy. There are also characteristic peaks of zinc $K_\alpha$. I attribute those to x-ray fluorescence from a zinc-oxide phosphor.

The spectrum is similar in energy to what one would receive from old-fashioned cathode-ray tv-screens with a similar energy of the electron beam. But the visible light from these key chains is many orders of magnitude weaker than that of a CRT screen. The "current" of beta electrons is only $3.7\cdot10^5 \times 1.6 \cdot 10^{-19} = 0.06$ picoampere, which is much less than the typical electron beam current of a CRT display, about a milliampere. (But CRT screens often have heavy (lead etc) glass on the front.)

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    $\begingroup$ On the other hand, few people carried CRT TVs in their pockets, turned on, all day every day. If your measurements can be expanded to an estimate of the dose to nearby biological tissue then the answer would be much stronger. $\endgroup$ Mar 2, 2018 at 10:00
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    $\begingroup$ @EmilioPisanty I added some quantitative considerations about the dose. But this was a nice suggestion - I will give this as a lab experiment for students a few weeks from now, to measure x-ray spectra of CRT displays and oscilloscopes. $\endgroup$
    – user137289
    Mar 2, 2018 at 12:17

The low-energy beta radiation of H-3 is effectively shielded by any kind of material, including the outer layers of the skin. Therefore, external exposure to H-3 is generally not taken into account in radiation protection. Typical tabulated dose coefficients for external exposure to H-3 are all zero – except for submersion in air containing gaseous H-3 if the small contribution of radiation from H-3 present in the air volume of the lungs is taken into account ($h_\text{lungs}=2.75\times10^{-18}\ \mathrm{Sv\ s^{-1}\ Bq^{-1}\ m^3}$, which corresponds to an effective dose coefficient of only $e=3.31\times10^{-19}\ \mathrm{Sv\ s^{-1}\ Bq^{-1}\ m^3}$).

The limiting exposure pathways for H-3 are due to internal exposure, usually after inhalation or ingestion. In case of some consumer products like the one in the question, also penetration of H-3 through the skin may be relevant. If the H-3 is contained in a sealed glass vial, it is safe as long as the vial isn’t damaged. For luminous dial wrist watches with plastic cases, however, the intake of H-3 can be shown by measuring the H-3 activity in urine.

Nevertheless, the dose coefficients for internal exposure to H-3 are rather low compared to many other typical radionuclides. The assumed values depend on the considered chemical form of H-3. According to the old but still widely used publications ICRP 68 and 72 (which were based on the 1990 Recommendations of ICRP 60), the effective dose coefficient for H-3 as tritiated water (HTO) is $1.8\times10^{-11}\ \mathrm{Sv\ \ Bq^{-1}}$ for inhalation as well as for ingestion for adult members of the public as well as workers. The corresponding effective dose coefficient for H-3 as organically bound tritium (OBT) are $4.2\times10^{-11}\ \mathrm{Sv\ \ Bq^{-1}}$ for ingestion and $4.1\times10^{-11}\ \mathrm{Sv\ \ Bq^{-1}}$ for inhalation.

New values based on the 2007 recommendations of ICRP 103 can be found in ICRP 134. For H-3 as tritiated water (HTO) or soluble organic vapour (other than biogenic tritiated organic compounds), the effective dose coefficient is $2.0\times10^{-11}\ \mathrm{Sv\ \ Bq^{-1}}$ for inhalation and $1.9\times10^{-11}\ \mathrm{Sv\ \ Bq^{-1}}$ for ingestion. A similar value may be assumed for H-3 that penetrates the skin.


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