Jonathan Blow

What is the contradiction? I didn't notice it!

Look up the composition of Earth's atmosphere.

And then ask, how does this thing contaminate the ship if they had to work hard to even get it to survive in 3.5% nitrogen?

I mean, as the author you would kind of want to mention that pertinent fact though? (Maybe I missed it, but if so it was a minor aside, which is not good either). I guess it did come up in the "bead exchange" thing early on, I don't remember what that part said.

I guess I vaguely remember it being 1 oxygen and 26 ammonias, as opposed to multiple elements on each side, so, maybe that was the deal. Still seems like way too big a thing to elide like that.

The bead exchange is pretty good evidence! The atmosphere was at least majority-oxygen then. The reduced-pressure paragraph had some more detail, it's possible someone with sufficient knowledge of chemistry would come away from it knowing the atmosphere was pure oxygen.