In that post I linked to an article on the dangers of yeontan in the February 1968 issue of the Peace Corps newsletter Yobosayo (pages 5 and 7), which called for two sources of ventilation. See here for more about yeontan and here and here for personal recollections of near death experiences.
Here is a July 29, 1973 Korea Times article quantifying the death toll between 1959 and 1973, stating that almost 25,000 people had died from yeontan poisoning.
Here is a July 8, 1973 Korea Times article:
This article suggests a lack of oxygen caused their deaths, which differs somewhat from an August 12, 1969 Donga Ilbo article about fan death titled "Leaving the fan running?" which noted that you can lose your life if the constant heat loss is severe enough, and that you can also have difficulty breathing:
Donga Ilbo article titled "Rainy season and heat":
We also heard a report from Kim Jin-myeon, the forcasting head of the central meteorological observatory, about how the rainy season happens every year.Handheld fans were gradually replaced by electric fans throughout the 1960s and 1970s:
When we feel hot it is more dependent on our body temperature than the air temperature at that time. It is easy to feel the difference in heat due to the wind blowing or not blowing, or using a fan or not using a fan even though the air temperature is the same. Ultimately, even if it is the same temperature, it feels more sweltering when it is very humid.
Korea Times, July 8, 1962.
Korea Times, July 19, 1970.
Here are some advertisements for electric fans from this time:
Sunday Seoul, July 11, 1971.
Sunday Seoul, June 25, 1972.
Another theory behind fan death is that the concept was encouraged by the government to urge people to turn off their fans to conserve electricity. This is plausible. While scanning the Korea Times from 1959-1976 last summer (and let me tell you how overjoyed I was to see this the other day), I was surprised by how many droughts Korea suffered. They occurred in 1962, 1965, 1967, 1968, 1970, and 1972. They not only made it impossible for Korea to grow all of its own food (necessitating use of precious foreign currency reserves to import it), they also affected Korea's power output due to its reliance in the 1960s on hydroelectric power.
Korea Times, June 26, 1962.
Korea Times, June 23, 1967.
To be sure, fans with an "automatic time controller (timer)" on them had appeared in Korea by 1967, as this article points out...
Evaluation of summer selling, blade-sprouting fans
...while this 1970 article makes clear - since a 14 inch fan with a timer cost over 16,000 won at a time when a bowl of jajangmyeon was 100 won - that they weren't cheap.
There may be something to the theory that the belief in fan death is linked to the promotion of limiting fan use to conserve electricity in the late 1960s and 1970s. It's also possible the idea caught on due to the prevalence of yeontan poisoning and the practices associated with preventing it.