Responses
I really do not remember which chapter - it's one of their first kisses. Yeon was holding in the smoke and then kissed Jun and Jun has then completely inhaled the smoke of the cigarette. After that he bought the pack and tried to smoke himself and became weirdly obsessed with the fact that Yeon smokes and that specific brand of cigarettes. You can also look into Jun's expressions, it becomes kinda odd when they have sex, somebody said like he's on drugs on like he's controlled. He has similar expression after inhaling the smoke.
I guess the title of his popular book is partly a clue - Evaporation - everything the hands kill (or "eat") disappears.
It might be partly influenced by Stephen King's book The Shinning, like the house has something that controls people (gives them power) etc, like the hotel did to Johnny.
I just hope they do not die both at the end or Jun goes mad and Yeon controls him fully or something along those lines.