Air pollution has a direct influence on stone decay but also and indirect one by favouring lichen development. This is specially perceptible for nitrophilous lichens which proliferate on rainexposed walls. Thus an historical monument, Notre-Dame de l'Epine's Basilica, was colonized by nitrophilous lichenic population between 1975 and 1980. These nitrophilous species with orange and yellow thallus are responsible for the disfigurement of the Basilica. The spread of nitrophilous lichenic populations is related to a recent increase in the use of fertilizers and, above ali, to the recent methods of spraying fertilizers.
