Lichens are related to fungi and algae and appear as crusty patches or bushy growths on trees, rocks and the ground.Lichens are very sensitive to sulphur dioxide pollution in the air. Since London have been industrialised, many lichen species have become extinct in Britain. This is because sulphur dioxide pollution. Lichens are sensitive to sulphur dioxide because they have a very efficient absorbing system. Lichens also absorb sulphur dioxide dissolved in water. They then produce food by photosynthesis. This is how they can be environmental indicators.