noun
stuffy heat from a crowd; body heat in close quarters
Describes the warm, stuffy air that builds up when many people are packed together, such as in a crowded train or room.
満員電車の人いきれで気分が悪くなった。
I felt sick from the stuffy heat of the crowded train.
会場は人いきれでむんむんしていた。
The venue was stuffy with the heat of the crowd.
蒸し暑い refers to general hot and humid weather, while 人いきれ specifically describes the heat and stuffiness caused by many people in an enclosed space.
Compound of 人 (ひと, person) and いきれ (stuffy heat, from 息 (いき, breath) + 熱れ (いきれ, heat)). The exact derivation of いきれ is uncertain, but it likely relates to the warmth and moisture of breath accumulating in a crowd.