Toxic Miasma var addthis_config = {"data_track_addressbar":true};

Toxic Miasma is a Tyranid biomorph consisting of vents sprouting from the backs of some Tyranid creatures to release toxic gases of microscopic algal formations ¹ into a prey world's atmosphere, most notably the Venomthrope species. Even brief exposure to the miasma gases can cause a victim to choke to death on its own blood.²

Sources

• ¹:Codex: Tyranids (4th Edition), p.33
• ²: Codex: Tyranids (5th Edition), p.84