The community life of fungi and algae in need of moisture, protects itself against evaporation and radiation of sunlight by special provisions of the supporting tissue. By aggressive air constituents such as nitrogen oxides and heavy metals, this protective features of lichens are destroyed. Photosynthesis in the symbiotic algae is significantly impeded. The result is a slow dehydration and destruction of the lichen.
Although many species of lichens can live under extreme conditions (well below -20 °C and at temperatures above +70 °C), some species react particularly sensitively to pollutants in the air. Therefore they are considered as so-called bio-indicators of contaminated air. As long as these kinds of lichens occur frequently on trees or woodland, this indicates no or little contaminated air. Some lichens would be able to reach an age of more than 100 years. Lichens are characterized by a very wide variety of shapes and colors.