Here is just a quick summary on the epidemic that has been plaguing Korea for generations.  Ever since that wicked device, the oscillating fan, was first invented by some evil genius and pawned off on unsuspecting and fragile Koreans, thousands have died in their sleep.  No, their deaths are not due copious amounts of Soju consumed that could drown a jellyfish.  Nor are these innocent people dying from heart disease due to the fatty meat they prefer, nor the cigarettes which are puffed on without attention to the side effects.  No, these factors do not contribute to the death of many Koreans.  They are a resilient people and can overcome numerous occupations, wars and devastating typhoons.  But their reaper comes in the form of an oscillating fan.  In fact, I believe the easiest way to take over Korea, would be to put a giant dome or bubble encase the country, then merely turning on a large fan to blow a gently breeze, cooling off the populous and either causing them to die from hypothermia, lack of oxygen (because fans split oxygen molecules) or even poison.  So here is a summary of the epidemic from Wikipedia.  Please proceed with caution...

Fan death is a South Korean urban legen (also found in Japan) which states that an electric fan, if left running overnight in a closed room, can cause the death of those inside (by suffocation, poisoning, or hypothermia). Fans manufactured and sold in Korea are equipped with a timer switch that turns them off after a set number of minutes, which users are frequently urged to set when going to sleep with a fan on.  (True story.  My fan has that feature, though I never use it.  And there was an interesting pictorial warning telling me not to leave my fan on in a closed room.)

 The specifics behind belief in the myth of fan-death often offer several explanations for the precise mechanism by which the fan kills. However, as explained below, these beliefs do not stand up to logical and scientific scrutiny. Examples for possible justifications of belief in fan death are as follows:

• That an electric fan creates a vortex, which sucks the oxygen from the enclosed and sealed room and creates a partial vacuum inside.  This explanation violates the principle of conservation of matter, as indoor fans are not powerful enough to change the air pressure by any significant amount.

• That an electric fan chops up all the oxygen particles in the air leaving none to breathe. This explanation violates mass conservation and well-known properties of molecules and gases. (If a fan can chop up molecules, couldn't a really nice fan split atoms, and then be used in a weapon of mass destruction?  I'm sure the Bush White House was all over this was, as was Kim Jong Il.  Maybe North Korea doesn't even need plutonium, just a fan in an airtight box...)
  

• The fan uses up the oxygen in the room and creates fatal levels of carbon dioxide.  There is no actual conversion of oxygen to carbon dioxide happening; unlike a candle, the electric motor in a fan does not alter the chemical composition of the air (apart from creating some ozone if the motor uses brushes, and outgassing from the materials).

• That if the fan is put directly in front of the face of the sleeping person, it will suck all the air away, preventing one from breathing. This explanation ignores both the fact that a fan attracts as much air to a given spot as it is removing from it, and the fact that most people point a fan towards themselves when using one, which causes air to move past the face but does not change the amount of air present.

• That fans contribute to hypothermia, or abnormally low body temperature. As the metabolism slows down at night, one becomes more sensitive to temperature, and thus supposedly more prone to hypothermia.  (Blankets anyone?)  If the fan is left on all night in a sealed and enclosed room, believers in fan death suppose that it will lower the temperature of the room to the point that it can cause hypothermia. Empirical measurements will show, however, that the temperature in the room does not fall, at least not due to the fan; if at all, it should rise slightly because of friction and the heat output of the fan motor, but even this is generally not significant. Fans actually make one cooler by increasing the convection around a person's body so that heat flows from them to the air more easily, and by the latent heat of vaporization as perspiration evaporates from the body. However, there is no scientific study which indicates that this effect would be sufficient to cause hypothermia unless the temperature were already very low (in which case, there would be no need for a fan anyway).

Electric fans sold in Korea are equipped with a "timer knob" switch, which turns them off after a set number of minutes: perceived as a life-saving function, particularly essential for bed-time use.

• That fans contribute to hyperthermia, commonly known as heatstroke. The U.S. Environmental Protection Agency warns that during excessive heat events, people should not "use a portable electric fan in a closed room without windows or doors open to the outside." The EPA's position is based on the fact that although a fan can decrease the perceived room temperature and convection improves the ability of the human body to lose heat by sweating, a fan alone will not prevent hyperthermia brought on by hot weather, not that a fan will exacerbate hyperthermia by producing heat.

• That fans contribute to prolonged asphyxiation due to environmental oxygen displacement or carbon dioxide intoxication. In the process of human respiration, inhaled fresh air is exhaled with a lower concentration of oxygen gas (O2), and higher concentration of carbon dioxide gas (CO2), causing a gradual reduction of O2 and buildup of CO2 in a completely unventilated room.  Other indoor sources of carbon dioxide include burning fossil fuels, such as a gas-fueled water heater, and seepage through foundations in areas of high CO2 soil content.Carbon dioxide is a colorless, odorless gas, and because it weighs 1.5 times more than normal air, it tends to concentrate toward the floor, depending on temperature and air currents. In South Korea, some people sleep on traditional floor mats, called yos, while others prefer western-style beds, and floor vents may be absent when ondol radiant underfloor heating is employed. According to The Straight Dope website run by the Chicago Reader newspaper, asphyxiation is an unlikely cause of fan death because "few rooms are totally sealed, and the fan would tend to keep CO2 and other gases well mixed."

The South Korean government position: The Korea Consumer Protection Board (KCPB), a South Korean government-funded public agency, issued a consumer safety alert in 2006 warning that "asphyxiation from electric fans and air conditioners" was among South Korea's five most common seasonal summer accidents or injuries, according to data they collected.  Also included among the five hazards were air conditioner explosions and sanitation issues, including food poisoning and opportunistic pathogens harbored in air conditioners. The KCPB actually published the following:

If bodies are exposed to electric fans or air conditioners for too long, it causes [the] bodies to lose water and [causes] hypothermia. If directly in contact with [air current from] a fan, this could lead to death from [an] increase of carbon dioxide saturation concentration [sic] 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.

Fan-related hyperthermia research A 2007 meta-analysis of studies on heat wave-related deaths explains that "a fan induces air movement that increases evaporation and lowers skin temperature, but in warm environments increased wind speeds of hot air can actually raise the skin temperature and thus produce opposite results by increasing core body temperature."  The study concludes that definitive recommendations on fan use require further study.

An analysis of risk factors contributing to 700 "excess deaths" during a 1995 Chicago, Illinois, heat wave found no evidence that fans either increased or decreased mortality, but noted that "interpretation of the data on the use of fans is complicated by the need to take into account specific environmental factors (for example, whether the fan is used in a room with an open or a closed window) and the health status of individual subjects."  Fear of crime was cited as a factor in people keeping windows and doors locked shut.

Media coverage:  The phenomenon of fan death is accepted by many Korean medical professionals. In summer, mainstream Korean news sources regularly report on cases of fan death. A typical example is this excerpt from the July 28, 1997, edition of The Korea Herald, an English-language newspaper:

The heat wave which has encompassed Korea for about a week, has generated various heat-related accidents and deaths. At least 10 people died from the effects of electric fans which can remove oxygen from the air and lower body temperatures...

On Friday in eastern Seoul, a 16-year-old girl died from suffocation after she fell asleep in her room with an electric fan in motion. The death toll from fan-related incidents reached 10 during the past week. Medical experts say that this type of death occurs when one is exposed to electric fan breezes for long hours in a sealed area. "Excessive exposure to such a condition lowers one's temperature and hampers blood circulation. And it eventually leads to the paralysis of heart and lungs," says a medical expert.

"To prevent such an accident, one should keep the windows open and not expose oneself directly to fan air," he advised.

According to The Straight Dope website, when informed that the phenomenon is virtually unheard of outside of their country, "some locals claim Koreans are uniquely vulnerable due to a peculiarity either of their own physiology or of Korean fans." 
(Seriously, who would admit to being so genetically fragile?  I'd be embarassed.  Imagine if we could be killed by fog in the States due to a 'unique physiology...)

Published professional opinion:  Gord Giesbrecht, a professor of thermophysiology at the University of Manitoba in Canada is a leading expert on hypothermia:

It's hard to imagine death by fan, because to die of hypothermia, one's body temperature would have to get down to 28 [°C], drop by 10 degrees [Celsius] overnight. We've got people lying in snowbanks overnight here in Winnipeg and they survive. Maybe if someone was elderly and they were sitting there for three days in a sealed room with an electric fan turned on. Someone is not going to die from hypothermia because their body temperature drops two or three degrees overnight; it would have to drop eight to ten degrees." In addition, "the only way to verify whether someone had really died of hypothermia during the night would be to take a core body temperature the following morning. Waiting three days while the body was in the morgue wouldn't work because the corpse's temperature can drop during that time.

Dr. John Linton at Yonsei's Severance Hospital, who attended medical school at Yonsei University, is licensed to practice medicine in South Korea:
There are several things that could be causing the fan deaths, things like pulmonary embolisms, cerebrovascular accidents or arrhythmia. There is little scientific evidence to support that a fan alone can kill you if you are using it in a sealed room. Although it is a common belief among Koreans, there are other explainable reasons for why these deaths are happening.

Dr. Lee Yoon-song is a professor at Seoul National University's medical school and works with the school's Institute of Scientific Investigation. He has conducted autopsies on some of the people who have been described in Korean media as having succumbed to fan death:

When someone's body temperature drops below 35 degrees, they do start to lose judgment ability. So if someone was hiking and later found dead, that could be part of the reason. But we can't really apply this to fan accidents. I found most of the victims already had some sort of disease like heart problems or serious alcoholism. (SOJU....) So hypothermia is not the main reason for death, but it may contribute.

He blames the Korean media for the persistence of the urban legend:

Korean reporters are constantly writing inaccurate articles about death by fan, describing these deaths as being caused by the fan. That's why it seems that fan deaths only happen in Korea, when in reality these types of deaths are quite rare. They should have reported the victim's original defects such as heart or lung disease, which are the main cause of death in these cases. If a Western doctor investigated these deaths, he would say what really caused the death, and say that a fan was beside the victim.

Well, I hope you feel well informed now, and please keep in mind:  If I am ever found dead in bed, with nothing but a fan on, there is no need for a comprehensive investigation nor an autopsy. 

For great Korean T-shirts:  Check this out.