Nitrogen oxide
From Memory Alpha, the free Star Trek reference.
Nitrogen oxide (frequently abbreviated NOx) refers to one or more gaseous nitrogen-oxygen compounds that are toxic to many species, including Humans, when concentrated in sufficient quantities.
In 2366, nanites that had infected the computer core of the USS Enterprise-D released nitrogen oxide onto the bridge of that starship in retaliation for Paul Stubbs' use of gamma radiation to destroy several of the nanites. (TNG: "Evolution")
