Ok, so this fragment of a fic just popped into my head this morning, but I have no clue as to the author, title or location of the fic.

All I can remember is that Sam was driving and Dean was smoking a cigarette. Dean starts to cough and Sam tells him he should quit smoking. Dean rolls down the window to try to breathe better but it doesn't help and he tells Sam he can't breathe. Sam momentarily thinks he's exaggerating, but then panics and pulls over, dragging Dean out of the car and laying him on the ground. There's some kind of light traveling along Dean's skin indicative of the curse/spell/whatever. Sam realizes that they've waited to late to run from the curse and that he could have killed Dean if they'd passed the invisible boundary.

So yeah, that's all I have. Anyone know what this story is or where to find it?