Many lichens show varying degrees of sensitivity to man-made pollutants. Lichen are very sensitive to pollution such as the presence of sulfur dioxide in the atmosphere.
Scientists have learned to use this variability as an indicator of the air quality of any given site.
Also, because the presence or absence of certain lichen species is easily recorded, and has in some cases been recorded at different times in the past, lichens can be a record through time of changing patterns of air quality.
Many lichens show a remarkable sensitivity to air quality. Some forms growing almost anywhere, others living in only the purist of atmospheres. When lichens disappear, they give early warning of harmful conditions.
Studies carried out in the 1970s in Europe were able to create a graded series of 10 zones with indicator species for each zone, varying from highly polluted to basically unpolluted or pristine.
The first category was no lichens, the second contained algae only, the third zone has Hypogymnia physodes and Parmelia caperata. The last zone was characterised by rare species such as Usnea articulata and Teleoschistes flavicans.
Lichens can also indicate past pollution by faded or abnormal colouring and patchiness in the centre of the thallus.