For proper ventilation in a building, what must occur?

For proper ventilation in a building, it's essential that air flows and exchanges properly throughout the building space. Effective ventilation is crucial for maintaining indoor air quality, preventing the buildup of moisture and pollutants, and ensuring a comfortable environment for occupants. Proper air flow helps to remove stale air while bringing in fresh air, which is fundamental to the health and comfort of those inside.

The other options provide elements that contribute to a comfortable environment but do not directly relate to the core requirement of ventilation. While keeping windows closed may be practical in certain scenarios, it does not inherently ensure proper air exchange. Continuous operation of cooling systems does not guarantee effective ventilation either; the systems may simply recirculate the same air. Finally, maintaining humidity levels is important for comfort and health, but it is a secondary aspect of ventilation. Hence, the primary focus for proper ventilation should be centered on the movement and exchange of air within the building space.