Here's a December 7, 1975 Stars and Stripes article about the dangers of carbon monoxide titled "Gas Takes 10 More Victims:"
SEOUL — Ten persons in Seoul and Uijongbu were killed by carbon monoxide gas Wednesday and Thursday and 14 others were hospitalized in serious condition, police in the capital said.A more detailed contemporary look at the dangers of yontan poisoning can be found in the February 1968 issue of the Peace Corps newsletter Yobosayo (page 5 and 7 - note also the hat tip they give to James Wade on page 5), which makes clear that at least two sources of ventilation were needed.
Some 60 persons were treated for carbon monoxide gas poisoning in hospitals in Seoul over the same period, police reported. Among the latest victims, bringing to 24 the total of persons killed by the gas from Korean hot-floor heating systems, was a family of three which died in its sleep Wednesday night in a home in western Seoul. Two of the fatalities were reported in Uijongbu, north of Seoul, police said.
Police said some of the latest deaths in Seoul have resulted from improper ventilation of rooms or faulty maintenance in structures using the hot floor heating system.
Thinking about this (after coming across several accounts of such deaths), I wondered if the need to keep windows open in the winter had an influence on the belief that ventilation was needed in the summer when running a fan all night. I really haven't a clue if there's a connection, but out of interest, I managed to find an August 12, 1969 article about fan death here, titled "Leaving the fan running?" It notes that you can lose your life if the constant heat loss is severe enough, and that you can also have difficulty breathing (and it's the source of the illustration at the top of this post). Perhaps there are earlier articles than this, but it's the earliest I've come across so far.