These are the chemical changes involved. The room heats up perceptibly. Humidity increases because of the sweat and the exhaled air. Sweat also produces odour. The flow of air is impeded and this makes the room stuffy.
People also release bacteria into the air, when they cough or sneeze.
All these cause physical changes in the air. Apart from this, very specific pollutants are found inside rural and urban buildings. Many of us feel uncomfortable in crowded rooms without really being able to say what it is that annoys us. The above factors, apart from psychological, are responsible for this annoyance factor. In poorly ventilated rooms people feel unable to concentrate, complain of drowsiness and headaches.