Wiktionary
unbreathably
adv. In such a way that breathe is difficult or impossible.
Usage examples of "unbreathably".
The shrill whine of the turbojet engines, the rush of unbreathably thin air just inches away on the other side of the transparent canopy, were nothing more than background music, muted, unimportant.
The bloody deck was neither new nor interesting, nor were the pillaged cabins and quarters below, but Jack led him down and down into the hold, dim in spite of the open hatches, and extraordinarily aromatic, almost unbreathably scented.