Well, they were not about to drink heavy water, they were just breathing air with some percentage of heavy hydrogen. How much D2 can you breathe in before you

Well, they were not about to drink heavy water, they were just breathing air with some percentage of heavy hydrogen. How much D2 can you breathe in before you suffocate anyway? Of whatever amount you do breathe, how little is absorbed? Of the tiny amount that is in fact absorbed, how much ends up exchanging D atoms with your body? I don't know the answers to these questions, but my bet is the airborne process is orders of magnitude less efficient than drinking D2O. Their main concerns should be: 1. Suffocation from lack of O2 2. Risk of explosion if the D2 percentage is in the danger zone 3. If that think was leaking out of a reactor, maybe there were some byproducts leaking out with it? Like, I dunno, tritium? That would be really, really nasty to breathe in.