Wiktionary
unbreathable
a. 1 Not suitable for breathing. 2 Not letting air seep through.
Usage examples of "unbreathable".
Think of the way plantlike bacteria released oxygen into an otherwise unbreathable atmosphere.
The Unbreathable had ways and knowledge, I warned in my most formidable Pisces speech.
Dense with neutrons, the waste would merge with the unbreathable underMantle and, perhaps, sink at last into the Quantum Sea.
Enclosing the whole of this space station was a massive space shield dome, which enabled them to live in an otherwise unbreathable atmosphere.