Epiphytic mosses and lichens are very sensitive to air pollution. Their gradual disappearance from large cities and from the vicinity of industrialized complexes is due, mostly, to phytotoxicants. A simple method to map the long-range effect of air pollution on corticolous epiphytes is described. An I.A.P. or "index of atmospheric pollution" based on the number of species present, their coverage and frequency, and their specific tolerance to pollutants can be expressed quantitatively.