"Exhale" as a Noun

Silly question but when did "exhale" become a noun? I've been seeing it everywhere in fan fic lately, everywhere an "exhale," not one fic with an "exhalation"--or a "he exhaled." I figured it was a fan fic thing.

Then I saw an "exhale" in the poem "Forgotten Portraits," on my son's AP test study list.

The dictionaries are pretty much still telling me "exhale" is a verb.

This is, of course, all my language snobbery and utterly irrelevant, but when did this happen? What memo did I miss?