Which of the following defines closed-building conditions?

Closed-building conditions are essential for accurate radon measurement as they minimize external influences that could alter radon levels inside a building. The requirement that all windows and exterior doors must be closed helps create a stable indoor environment for the duration of the measurement period. This is crucial because open windows or doors can allow outside air, which may contain different radon concentrations, to enter the building, thereby skewing the results of the radon test.

Maintaining closed-building conditions ensures that radon levels can be measured accurately, reflecting only the radon that has accumulated within the structure over time. Other aspects of closed-building conditions, like keeping heating systems operational and ensuring the occupants are not greatly influencing the airflow indoors, are also important but secondary to the necessity of sealing off external air exchange. Therefore, the definition of closed-building conditions fundamentally rests on the requirement that all windows and exterior doors must be kept closed.