Unbreathing
Unbreathing creatures do not breathe. These include undead and nonliving monsters, as well as plants. Player characters can achieve this by playing a Ghoul, Vampire (regardless of satiation, they do not have to breathe), Mummy, Gargoyle, grey Draconian, or by being in lichform via Necromutation.
Unbreathing creatures are not affected by mephitic or poisonous clouds. They are also immune to asphyxiation damage (such as through curare needles or water elementals), and take reduced damage from Primal Wave.
Note, however, that unbreathing creatures can still drown in deep water (with the exception of grey Draconians, who specifically have the ability to move through deep water, albeit slowly and awkwardly). Also, the unbreathing attribute will not protect you from miasma.