Lichens have been recognized as being very sensitive to air pollution for many years (Hawksworth 1971; Nimis et al. 2002). In the 1800s independent observations in England, Munich, and Paris documented that lichens were already disappearing from urban areas. By the early 1900s this “city” effect was a widely recognized phenomenon in Europe and was first attributed to coal dust, which was emitted by most homes as well as many industries. Only later did the colorless gas, sulfur dioxide, become recognized as a principal phytotoxic agent. Today the list of air pollutants is much longer and includes oxidants, hydrogen fluoride, some metals (Section 12.7), acid rain, and organics. Certainly the list of potentially toxic substances is not yet fully circumscribed.

The high sensitivity of lichens is related to their biology. Most species live for decades or hundreds of years and a few longer; thus, as perennials, they are subject to the cumulative effect of pollutants. Lichens have no vascular system for conducting water or nutrients; as a consequence, they have developed efficient mechanisms for taking up water and nutrients from atmospheric sources. Fog and dew, major water sources for lichens, often have much higher pollutant concentrations than precipitation, and the lichens' nutrient concentration mechanisms also will concentrate pollutants. Unlike many vascular plants, lichens have no deciduous parts, and hence cannot avoid pollutant exposure by shedding such parts.