What must be present in a measurement device for it to be considered exposed?

For a measurement device to be considered exposed, it must have contact with radon. This is essential because the fundamental purpose of a radon measurement device is to detect and quantify the presence of radon gas in an environment. The device needs to be in an area where radon can be present in order to provide an accurate measurement. Without this contact, the device would not be able to fulfill its role as a radon measurement tool.

The emphasis on contact with radon highlights the nature of radon as a gas that is typically found in various concentrations in different locations. Therefore, ensuring that the measurement device is placed in a radon-conducive environment is crucial for effective monitoring and assessment of radon levels, which is a significant part of ensuring safety and making informed decisions in radon management.