Lichens are plants composed of fungi growing in a symbiotic association with algae. About 17,000 species have been identified, and journals concerned with lichenology are published in several countries. Although lichens have thus far attracted little attention from medical mycologists, communications in the August Archives of Dermatology1 and the November Archives of Environmental Health2 indicate that usnic acid, a chemical substance found in many lichens, may cause allergic contact dermatitis in the human.
Although dermatitis from exposure to lichens is most likely to occur in forest workers, urban dwellers may be exposed by carrying fire wood, from wood-ash, and from lichens used in decorating model railroad layouts, architects' models, floral table decorations, and funeral wreaths. Campers may be exposed to rock and tree lichens both on the tree and on the ground. In the spring lichens are carried by melting snow from their original sites to the ground