Which unit is used to express radiation intensity in the air according to the British system?

The unit used to express radiation intensity in the air according to the British system is the Roentgen. This unit specifically measures the amount of ionization produced in air by X-ray or gamma radiation. In practical terms, it quantifies the exposure of air to radiation and is particularly relevant in radiological contexts.

The Roentgen is defined based on the ionization of 1 cubic centimeter of dry air at standard temperature and pressure, where the radiation has sufficient energy to produce one electrostatic unit of charge. This makes it a direct measure of radiation exposure in air, distinguishing it from other units that may apply to different contexts or materials.

While Gray, Rad, and Rem are all related to radiation measurement, they refer to absorbed dose and biological effects or dose equivalence, and thus do not specifically address radiation intensity in air as the Roentgen does. This specificity is what makes the Roentgen the correct choice for this question.