eh, tritium and its decay products float off into space unless incorporated into a heavier molecule (e.g. burning concentrated tritium into water). even then hy

eh, tritium and its decay products float off into space unless incorporated into a heavier molecule (e.g. burning concentrated tritium into water). even then hydrogens are usually pretty labile and don't bioaccumulate unlike e.g. calcium/strontium/phosphorous. so the risk of exposure is negligible except maybe in manufacturing, and the risk of environmental accumulation is also negligible. with tritium/phosphor, the brightness is super dim but more than sufficient in actual darkness. it's simple, stable under extreme environmental conditions, and there's nothing microscopic or macroscopic to break besides the containment vessel itself. it lasts for decades and fails by dimming gradually (unless the containment vessel is broken at least). not sure about exit sign construction but the little bitty capsule form factor is quite resilient.. by contrast LEDs and electronics in general seem to be less tolerant of challenges (i.e. side reactions in the battery over 10 years at elevated temperature, it gets zapped by an electrical arc etc). also the manufacturing is just as nasty and the stuff is less likely to just float off into space