"Fan death" is such a widely believed superstition in Korea that almost all fans come with a timer that switches them off after a given time. The belief is as follows:
If you leave your fan on (without a timer) in a room with no open windows, you could be asphyxiated and die. Weird or what?

Fans with Timers
This belief is so commonplace that the South Korean government recently released a statement about fan death:
"If bodies are exposed to electric fans or air conditioners for too long, it causes bodies to lose water and [causes] hypothermia. If directly in contact with [air current from] a fan, this could lead to death from [the] increase of carbon dioxide saturation concentration and decrease of oxygen concentration. The risks are higher for the elderly and patients with respiratory problems. From 2003 [to] 2005, a total of 20 cases were reported through the CISS involving asphyxiations caused by leaving electric fans and air conditioners on while sleeping. To prevent asphyxiation, timers should be set, wind direction should be rotated and doors should be left open"
Many South Korean doctors believe in Fan Death, despite the strong evidence against such phenomenon. Dr. John Linton of Severance Hospital said that "There is little scientific evidence to support that a fan alone can kill you if you are using it in a sealed room". An expert on hypothermia Gord Giesbrecht can't see how fan death occurs: "It's hard to imagine, because to die of hypothermia, [one's body temperature] would have to get down to 28, drop by 10 degrees overnight," he said. "We've got people lying in snowbanks overnight here in Winnipeg and they survive."

Death By Fan
Despite this evidence, fan death is deeply entrenched in Korean culture. It was one of the top five summer dangers according to the korean government! It doesn't seem that the "phenomenon" of fan death will be going any time soon.