There are a a variety of warning signs you should look into house ventilation, starting with:
1. Lasting odors
If you have issues getting strong aromas out of your home, you may have a ventilation issue.
2. Musty air
The air in your house should be fresh, but stuffy sectors mean you likely have insufficient ventilation.
3. Lots of dust
Dust can get stuck more quickly in rooms with poor air movement.