When making grab measurements, for how long is it important to conform to closed-building conditions before the measurement?

Conforming to closed-building conditions for a period of 12 hours before making grab measurements is critical because this time frame allows for the indoor radon levels to stabilize after any potential disturbances, such as ventilation changes or opening windows. This stabilization is essential for obtaining an accurate measurement that reflects the true concentration of radon in the indoor environment. By keeping the building closed without any significant air exchange, the radon emanating from the soil and building materials has an opportunity to collect adequately, thus leading to a reliable assessment.

Longer periods, such as two days or 24 hours, may be unnecessary for standard grab measurements and could complicate the measurement process without significantly improving accuracy. Conversely, a shorter period of one hour may not be sufficient for the levels to stabilize, as radon can fluctuate substantially within a very short time due to activities like heating, cooling, or people entering and leaving the building. Therefore, the 12-hour window is the most effective balance to ensure accurate grab measurements in accordance with established protocols in radon measurement practices.