Lichens are bioindicators of air pollution because:
They are sensitive to the levels of contaminants like sulphur dioxide
Lichens are mutualistic associations of a fungus and algae or cyanobacterium and occur as crusty patches or bushy growths on trees, rocks and bare ground. The lichens are useful bioindicators for air pollution, especially sulphur dioxide pollution since they derive their water and essential nutrients mainly from the atmosphere rather than from the soil. They react to air pollutants all year round. Lichens do not grow in a polluted area. They grow in pollution free area.