The air sensitivity of n-doped layers is crucial for the long-term stability of organic electronic devices. Although several air-stable and highly efficient n-dopants have been developed, the reason for the varying air sensitivity between different n-doped layers, in which the n-dopant molecules are dispersed, is not fully understood. In contrast to previous studies that compared the air stability of doped films with the energy levels of neat host or dopant layers, we trace back the varying degree of air sensitivity to the energy levels of integer charge transfer states (ICTCs) formed by host anions and dopant cations. Our data indicate a universal limit for the ionization energy of ICTCs above which the n-doped semiconductors are air-stable.