Indoor climate is often only experienced unconsciously. You may not notice it until suddenly the air seems bad, the room dusty, the noise levels high, etc.
Is there actually a ventilation system here?
This could be a good question for the person in charge of the building, if you notice a poor indoor climate - in the home, at school or at work. Maybe no ventilation is installed, or maybe the existing ventilation system is inadequate or poorly maintained.
The home
Homes need ventilation to remove humidity from showers and cooking etc.
At school or in the workplace
At the workplace, or at school and other institutions where may people gather, ventilation is needed both to keep temperatures down and to secure good air quality with low CO2 content.
If the air quality is poor, this may cause fatigue and headache. Studies have shown that both learning and working efficiency are reduced when the CO2 level is too high, so there is every reason to keep an eye on it.