What characteristic of the testing environment is crucial for accurate radon measurement?

Controlling for external weather conditions is vital for accurate radon measurement because fluctuations in atmospheric pressure, temperature, and humidity can influence radon levels indoors. For example, high winds or heavy rainfall can affect the pressure differential between the indoor and outdoor environments, which in turn can alter radon entry points. Ensuring a controlled testing environment minimizes these external influences, providing a more reliable assessment of radon levels in a given space.

While historical radon levels may offer insights into potential radon risk, they do not reflect the current testing conditions, making them less critical for immediate measurements. The time of year and building materials can also impact radon levels, but they do so indirectly and are not as directly controllable as weather conditions at the time of testing. By prioritizing control over external weather influences, technicians can ensure a more accurate representation of radon levels within the home or building being tested.