After visiting Loch Lomond last week I brought this oak twig home to identify the lichens which were growing profusely in the area.
Good indicators of pollution are lichens and the tangled hair-like lichen which is a type of Usnea, often called Old Man’s Beard is a good sign of clear air and is generally found in the wetter north and west.
The other is the Oak Moss or Evernia prunastria which is a commonly found on oak trees but also on deciduous trees and it too shows there is little atmospheric pollution.
Country folk used many types of lichens as dying agents, or even to help in illnesses.  The oak moss was used also for fixing perfume and ground up to make a hair powder and as wadding in shotguns. Now some lichens are used in the production of antibiotics because Usnea contains usnic acid used in herbal formulas for sore throats and skin infections.