Ventilation is significantly more important than it was a decade ago. The insulation was usually nonexistent so fresh air could easily get through gaps or walls in the building. The building materials were mostly made of natural products as today much more chemicals are being used. Most common in American homes do not include ventilation. This leaves minimal opportunity to remove stale air or moisture to bring in fresh outdoor air. Natural ventilation is uncommon. Exhaust only mechanical ventilation is common strategy in which small exhaust in places such as bathrooms operate to exhaust stale air and moisture this stagey is a great choice and comes at a small cost.