Abstract: The chemical lifetime of the mesospheric ferrum and sodium layers is calculated using the EigenAnalysis （EA） method. For comparison, their meteoric smoke adsorption lifetime and vertical transportation lifetime are also computed. The results show that （1） at the altitudes around the peak number density of Fe layer, the EA chemical lifetime for Fe atom is several days, much longer than the traditionally defined chemical lifetime for Fe of several minutes, （2） below 84km chemical process prevails whereas above that height, vertical process dominates, and （3） among the four of meteoric injection process, chemical process, vertical transportation process, and meteoric smoke adsorption process, the most possible one that can make the cutoff heights of Fe and Na layers to be identical is meteoric smoke adsorption.