You should cover your mouth when you cough.
Roy is sitting on the subway eating chips and some of the crumbs are tickling his throat.
Roy wants to let out a cough to clear his throat.
Roy leans over to cough downwards blocking it with his elbow.
Roy coughs with an open mouth and does not bother to cover it.
The passengers notice Roy covering his mouth and feel safer.
The other passengers are disgusted by Roy's lack of consideration.