What is air kerma?

• A. Kinetic energy absorbed by tissue per unit mass.
• B. Charge collected by an ionization chamber per exposure.
• C. Kinetic energy released in air per unit mass; used as a measure of exposure for photons.
• D. Number of photons per unit area.

Answer: (C)

Air kerma is the kinetic energy released in air per unit mass by ionizing radiation. For photon beams, this energy transfer to charged particles (mainly electrons) in air creates ionizations, so kerma in air is used as a practical measure of exposure because ionization chambers respond to that energy transfer. It differs from absorbed dose, which is the energy actually deposited in tissue per unit mass. It’s not the charge collected by an ionization chamber per exposure, nor the number of photons per unit area (fluence). The essence is energy transferred to matter (air) per mass, which serves as a way to quantify exposure for photons.