The exposure dose of X and gamma rays is very nearly equivalent to the kinetic energy of the ionizing rays they release per unit mass (KERM) in air. The KERM at a point is related to the absorbed dose at nearby points by an influence function. The influence function is used to find the difference between the KERM and the absorbed dose due to lack of electronic equilibrium resulting from exponential absorption and inverse square. When the difference is small, it can be calculated accurately. The free air chamber is analyzed to show its relation to a cavity extrapolation chamber. The KERM concept can be applied to all radiations emitted in matter. It is a useful concept when the mean free path of the nonionizing rays emitted is very large compared to the range of the ionizing rays.