Origins of belief
The genesis of this misconception is unclear, but fears about electric fans date almost to their introduction in Korea, with stories dating to the 1920s and 1930s warning of the risks of nausea, asphyxiation, and facial paralysis from this "new technology".
Some have speculated that the South Korean government created or perpetuated the myth as propaganda in order to curb the energy consumption of Korean households during the 1970s energy crisis. This period was marked by short supply and high prices of oil, and coincided with the rule of President Park Chung-hee, who listed a self-reliant economy and modernization as his top goals, as announced in his Five Year Economic Development Plan. This theory is based on the fact that reports of fan death first appeared in the 1970s.
There are several purported explanations for the precise mechanism which might result in death.
"Fans cause hypothermia"
Hypothermia is abnormally low body temperature caused by inadequate thermoregulation in humans. As the metabolism slows down at night, one becomes more sensitive to temperature, and thus supposedly more prone to hypothermia. People who believe in this theory think that a fan operating in a closed room all night will lower the temperature of the room to the point of causing hypothermia.
Empirical measurements show, however, that the fan does not cause the room temperature to drop; if anything, it should rise slightly because of friction and the heat output of the fan motor, but even this is negligible. Fans actually lower body temperature by increasing the convection around a person's body so that heat flows into 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. The perception of warmth and cold is physiologically different from actual empirical measurement of heat, as is shown by natural phenomena like the wind-chill factor. The movement of the air from fans has similar effect to that of the wind blowing on the skin, in fact that is the primary functionality of cooling fans.
"Fans cause asphyxiation"
It is alleged that fans may cause asphyxiation in humans due to oxygen displacement and 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 in rooms equipped with radiant underfloor heating, called ondol.
According to The Straight Dope website run by the Chicago Reader, 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 US EPA does not warn of fan death, but it discourages people from using fans in closed rooms without ventilation during excessive heat, specifically when the heat index is above 99 °F (37 °C). Although the increased air movement will increase evaporation of sweat, the fact that the blown air is warmer than body temperature increases the heat stress placed on the body and can speed the onset of heat exhaustion and other detrimental conditions. The EPA does, however, approve of using a fan if a window is open and it's cooler outside, such as during overnight conditions, or if the indoor heat index in a closed room is below 90 °F (32 °C).
Fans are necessary in zero G environments where the exhaled gasses of different densities do not rise or fall away from the person breathing, but instead mix more slowly via diffusion. Sleeping on the International Space Station without a fan or ducted airflow will cause asphyxiation after an extended period of time.
In summer, mainstream Korean news sources regularly report on alleged cases of fan death. A typical example is this excerpt from the July 4, 2011, edition of The Korea Herald, an English-language newspaper:
A man reportedly died on Monday morning after sleeping with an electric fan running. The 59 years-old victim, only known by his surname Min, was found dead with the fan fixed directly at him.
However, this article at least nominally repudiates the fan death myth, saying that there is "no evidence" for it.
Ken Jennings, writing for Slate, says the belief is fading among younger Koreans: "A decade of Internet skepticism seems to have accomplished what the preceding 75 years could not: convinced a nation that Korean fan death is probably hot air." 
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. The KCPB 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.
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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 [Celcius] 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.
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. So hypothermia is not the main reason for death, but it may contribute.
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.
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- "Strange Harm From Electric Fans", Jungoe Ilbo (Domestic and International Daily), July 31, 1927, "The rotating fan blades create a vacuum directly in front, and the intensity of the resulting air flow always results in an insufficient supply of oxygen to the lungs." (Korean)
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