UPSC NDA II 2014 General Ability Paper

Lichens can be used as environmental indicators as they are very sensitive to sulphur dioxide pollution. Sulphur dioxide (SO2) is a gas which is produced when a material containing sulphur is burned. If the air is badly polluted with sulphur dioxide, there may be no lichens present. The most tolerant lichens are those that are crusty in appearance whereas the leafy lichens are not very pollutant tolerant.