Lichens and many mosses lack protective surfaces that can selectively block out elements including pollutants that are dissolved in rainwater. Lichens act like sponges , taking in everything that is dissolved in the rainwater, and retaining it. Since there is no means of purging the SO2, the sulfur content accumulates within the lichen and reaches a level where it breaks down the chlorophyll molecules which are responsible for photosynthesis in the algae. Photosynthesis is the process green plants use to convert sunlight energy to chemical energy which in turn is used by the plant. In the case of lichens, when the photosynthetic process stops in the algae, the algae die and this leads to the death of the fungus.
Many mosses assimilate much of their nutrients and water through their tiny leaves.
This allows mosses to take in water quickly through their leaves during rains or high humidity, and dry out fast when the air dries. This rapid and direct absorption is detrimental when the moisture is laden with pollutants. This is very useful because modern air quality instruments cannot measure the effects air pollution has on living cells and they are limited to measuring present conditions. One can readily tell what the air quality is in their own backyard, park or woodlot just by looking at what is growing on the bark of older trees. Take some time and look at the bark of some older trees.