Tritium decays by beta-decay (an electron). The electron can not travel very far in air (1/4 inch), and is stopped by even the thinnest piece of metal. It's even stopped by the dead outer layer of your skin.

i.e. it's completely harmless unless you eat it.

Not quite: beta decay will penetrate the skin enough to damage living tissue - beta burns are what caused the fatalities of the Chernobyl first responder fire fighters.

They spent a few hours covered in dust on their coats, and did a bunch of subsurface skin damage which manifested as third degree burns. Sepsis, not radiation poisoning, generally killed them.

Well, it lasts for several years, but considerably less than even a human lifetime: Tritium's half-life is only about 11 years, so gun sights, dark-proof glow-in-the-dark signage (usually reserved for critical industrial plants, ships and offshore platforms due to expense), etc, will become seriously degraded in just a few years. (Since the glow is directly proportional to the remaining low-level beta radioactivity, which can barely penetrate the glass envelope in the first place - you'd get more radiation (from radium) living in a brick house than carrying 24-7.)

FWIW, tritium and a phosphor granule encapsulated in glass microspheres have been developed for self-illuminating runway paint, but again, no one really uses it because tritium is stupid expensive, and again, it' loses half its brightness in only a decade.

On the other hand, I've been told that Trijicon will replace their tritium gun sights for the lifetime of the original owner. I plan to live long enough to cost them money...

> Tritium's half-life is only about 11 years, so gun sights ... will become seriously degraded in just a few years.

We'll all be using RDSs by then ;) ?

> ...I've been told that Trijicon will replace their tritium gun sights for the lifetime of the original owner.

I didn't know this, thanks.