Exposure is a measure of the ionization of air due to ionizing radiation from photons, that is, gamma rays and X-rays. It is defined to be the electric charge freed by the radiation divided by the  mass of the air. As a measure of radiation damage it is less useful than the analogous concept of absorbed dose, nevertheless since exposure is convenient to measure directly in gaseous ionization detectors and since it is easily converted to dose, it is commonly used in the nuclear industry.