Bio-indicator are living organisms that respond in an especially clear way to change in the environment. The hardly lichens are useful bio-indicators for air pollution, especially sulphur dioxide pollution, since they derive their water and essentials nutrients mainly from the soil. They are able to react to air pollutants all year round. Compared with most physical / chemical monitors, they are inexpensive to use in evaluating air pollution.
Lichens can also be used to measure toxic elemental pollutants and radioactive metals because they bind these substances in their fungal threads where they concentrate them over time. Environmental scientists can then evaluate the accumulation to determine the history of the local air.
