Can using an air conditioner while sleeping be harmful?

Question

When I used to live in Japan, the summers were really too much for me, especially when going to bed. The strange thing is that all the (Japanese) people would strongly advise me against leaving the air conditioner running while I sleep because this is somehow very bad for people's health.

On the other hand, I also used to live in Thailand for a while, and I never heard anybody say something of that nature-- and their weather is steaming hot. Similar with other countries I've lived in. I only heard this air conditioner thing while in Japan.

So my question is: Is there really anything at all to be concerned about when running the air conditioner while sleeping? Or is this more like a Japanese urban myth?

By the way, I always used to set the air conditioner in air drier mode, rather than air cooler mode. It felt as cool, except that it was dry.

Answer 1

This is a common urban legend in South Korea as well. It's called "Fan death" though, see Wikipedia's fan death article).

Some quotes from that Wikipedia article outlines the otherwise outlandish claims that are associated with this phenomenon. Generally, many people die for various reasons, such as bad health, alcoholism, heart attack, etc. and it's easier to blame something external and unrelated. This is the main source and reason the myth propagates:

Gord Giesbrecht, a professor of thermophysiology at the University of Manitoba in Canada,¹⁴ 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.²

Dr. John Linton at Yonsei's Severance Hospital, who attended medical school at Yonsei University, is licensed to practice medicine in South Korea:²

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.

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.

Answer 2

In many places, air conditioners are not cleaned or maintained regularly. Bacteria do build up easily within aircon units and filters need to be changed occasionally. Flu viruses spread better in dry and cold air (flu season normally has such weather conditions).

Manufacturers are aware of this and do make improvements, so as long as you're using a relatively recent unit and changing the filter regularly, there should be no health hazard.

You mentioned Thailand -- I doubt most hotels there ever change the filters (even the top-notch places would probably just clean the outside). While traveling in SE Asia, I frequently experienced minor sore throat or nasal congestion that might have been due to aircon. This is anecdotal and it's difficult to determine the precise cause, but waking up slightly sick could be some indication.