Picture from the instruction manual of a fan I bought a few months ago.
It’s a widely held belief in South Korea that sleeping with a fan on could kill you, but this has generally been debunked: http://www.snopes.com/medical/freakish/fandeath.asp
It’s a belief also held in Japan and I have young friends in Asia who believe in it. The long and short of it is, in this Junior Scientist’s opinion from reading about fan death and sleeping with her fan on many a night, it’s not true. Unless! You have a plain ol’ fan in a room that doesn’t have any windows open and it is excruciatingly hot that night. Because there’s no breeze, you can cook yourself. Maybe. Possibly. If you have the worst luck in the universe.
More and more, younger Koreans don’t really believe in fan death, but I do, on occasion, meet someone who will defend the theory. Whether or not they really believe it or they’re just defending a cultural aspect is another thing. Korea has a lot of national pride that can extend into many, many other aspects of life (sports, art, testing, etc) including urban legends!
Typically if someone believes in fan death I just kind of shrug and change the subject. It’s so often an ingrained belief from childhood that they’re not going to change their mind any time soon. And, frankly, you can come off as quite the douche who is trying to ‘educate’ the poor, misguided Korean, adding further insult.
Besides, every culture has these little quirks. As far as I know, I never went cross-eyed from sitting too close to the TV or caught a cold from going out in the winter with damp hair..!