Adjective
Not suitable for breathing.
The atmosphere on Venus is an unbreathable mixture of carbon dioxide and sulphuric acid, amongst other things.
Not letting air seep through.
an unbreathable fabric
If America was a person, - and it sat down, - Lancaster town would be plunged into a Darkness unbreathable. Thomas Pynchon
Irons and the unbreathable air of this world strip us of everything, except the freedom to kill ourselves; and this freedom grants us a strength and pride to triumph over the loads which overwhelm us. Emil Cioran
Let us speak plainly: everything which keeps us from self-dissolution, every lie which protects us against our unbreathable certitudes is religious. Emil Cioran