Edgar Allan Poe's "The Conversation of Eiros and Charmion" (flash fiction, apocalypse, free): Too much oxygen can be very harmful!
This is a frame story. Told as conversation between Eiros & Charmion, names in heaven (or hell) of men who once lived on earth, were known with different names there, & are now dead in the sense we understand. Eiros is updating Charmion of how life on earth ended.
Disaster itself is of the same kind as H G Wells' "The Star", Arthur Clarke's "The Hammer of God", & the Hollywood movie "Armageddon". Only it's not a solid asteroid about to hit earth; it's a tenuous cloud of gas in the form of a comet. Even the core of this comet is not solid.
A part of the story is devoted to earthmen's reactions of approaching object - this part is far more readable & shorter than Wells' version.
End comes as earth passes through the cometary gas. Effects on atmosphere are catastrophic - nitrogen is drained & oxygen is substantially increased. People feel more vigorous. Even plants are happier (why?).
But this atmosphere is far more combustible too. The passing of cometary nucleus triggers massive fires - the reason for apocalypse.
I'm not qualified to comment on some of the premises of the story, but it was an entertaining read.
Fact sheet.First published: Burton's Gentleman's Magazine, December 1839 (via Wikipedia).
Download full text from The Wondersmith.
Related: All stories of Edgar Allan Poe.