Crustose Lichen on hickory bark

Look closely at the trees around you. Do you see lichen? If not it may be because you are breathing polluted air. Irwin Brodo writes in Lichens of North America: “Undoubtedly, the most important modern use of lichens is for monitoring air quality. For over 140 years, lichens have been known to be extremely sensitive to air pollution. … Close to a pollution source lichens are completely absent, creating a “lichen desert.”