Most likely the actual radiation is caught and transformed to the light seen in something akin to the phosfor layer in a flourescent light. Anyway it's a fairly weak beta emitter, and it doesn't travel far in air, and is shielded by most anything. some interesting reading, pdf.It's long been used for watch/clock handles and weapon sights The people painting the watch handles, licking the brush tip to sharpen it, didn't do to well, but otherwise tritium is fairly harmless as far as radioactive things goes..

Edit: fixed the link -wfaulk