From publisher blurb:
Toxic gas is one of the most common hazards faced by explorers of sewers, dungeons, and other subterranean spaces. Some of these foul vapors are naturally occurring, while others are the byproducts of magical catastrophes, alchemical experiments gone awry, or interdimensional incursions into the mortal plane. Assassins and tyrants alike often weaponize such gases to subjugate or terrorize those who oppose them.
20 Vile Vapors presents twenty rare and unusual vapors to terrorize your players, including ghoul fog, lavender reek, and stygian dioxide. These gasses that dissolve memories, transform flesh to copper, or grant mind-bending visions.
Plus it includes a new spell Niznoc's exhalation, which lets players harness the vapors from this book.