Exposure is a radiation measurement used to quantify which of the following?

• A. Radiation energy deposited in tissue
• B. Absolute number of ionization events in air
• C. Speed of radiation in air
• D. Absolute number of ionization events in air

Answer: (B)

Exposure measures the ionization produced in air by ionizing radiation. It is defined as the electric charge liberated per unit mass of air by X- or gamma rays, with units of coulombs per kilogram. Since each ionization event creates charge, the total exposure is directly tied to how many ion pairs are formed in that mass of air. In other words, exposure reflects the amount of ionization happening in air, not how much energy is deposited in tissue (that would be absorbed dose) and not the speed of the radiation. So the best match is that exposure quantifies the absolute number of ionization events in air, recognizing the relationship through charge per mass.