闷 (mēn): stuffy; shut indoors; to smother; to cover tightly

This room hasn't had a window opened for a long time; the air is stuffy. Let's go out for a walk.

On a summer afternoon, the classroom is both stuffy and hot; the students are all looking forward to class ending soon.

There are too many people in the subway car, and the air doesn't circulate, making it feel especially stuffy.

It has rained for several days in a row; the house is damp, and the stuffiness is so bad that it's hard to breathe.