How is roentgen (R) exposure primarily measured?

The roentgen (R) exposure is primarily measured in air because it is defined based on the amount of ionization produced in air by X-rays or gamma rays. Specifically, one roentgen is the amount of radiation that produces one electrostatic unit of charge in one cubic centimeter of dry air at standard temperature and pressure. This definition emphasizes that air serves as the fundamental medium for measuring exposure, as it reflects the radiation's ability to ionize particles, which is critical for understanding radiation dose and its potential biological effects.

While other materials such as soil, water, and solid objects can interact with radiation and have associated measurement metrics, they are not the basis for the roentgen unit. Measurements in these materials require different approaches and calculations, making air the standard reference point for roentgen exposure quantification.