Exposure is a measure of the ability of a particular photon to produce ionizations in air. Exposure, symbolized as X, is equal to the total number of either negative or positive ions produced in air per unit mass of air as a result of irradiation from photons. Only the negative or positive ions produced are counted, not both. The sum of ions produced is symbolized Q, and the mass of air is symbolized m. Therefore, exposure can be written as:
Q
X = ---
m
The historical unit of exposure is the roentgen, R, of which one R is equal to 2.58 coulombs per kilogram of air. Roentgens are used to measure exposure from x-ray and gamma radiation, and apply only in air.