noun
heat; hot air
Literal physical heat, often the hot air or oppressive warmth in a room or space.
部屋の中は熱気でむんむんしていた。
The room was stuffy with heat.
noun
Figurative heat: a charged, passionate atmosphere created by a crowd, event, or intense activity.
会場は熱気に包まれていた。
The venue was filled with excitement.
noun
Rare, somewhat literary use for bodily fever; modern Japanese normally uses 熱 (ねつ) or 発熱.
熱気にうなされる、という古風な表現がある。
There is an old-fashioned expression, 熱気にうなされる, meaning to be delirious with fever.