The half-value time of absorption from meta-stable electronic atomic states is calculated on the hypothesis that the whole decay is due to collision phenomena. Definite assumptions, which include the atomic energy level scheme, are made as to the efficiency of collisions and the interaction of two meta-stable states in these processes. The assumption adopted as the only one producing an approximation to experimental facts shows that the probability of an exchange is dependent on the absolute value of the energy difference between the states concerned. The results calculated from the final formula, for helium, neon and argon at normal and very low temperatures depend on one arbitrary constant, which is the same for all states and gases considered, and show fair agreement with any experimental values so far published. An estimate is made of the energy tolerance of a quantum transition.