"열기"

Translation: Heat

[CF00l]

熱氣

[avanade]

폭염 = heat wave = sweltering heat

[oee16]

Thanks DLG. You have just answered my question on #-기 때문에.

So: 열기 means "the heat" itself, while 열 on its own can be taken to mean "fever". The 기 here is unrelated to the "because of" structure...