A young laughing boy looking through the bough of a tree covered in lichens. Lichens serve as indicators of air pollution. In places of heavy pollution, few to no lichens are found. Lichen populations increase gradually with distance from these polluted places and are used in ecological studies as a measure of pollution intensity. Lichens are therefore excellent bioindicators and biomonitors.

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Royalty Free Stock Photography: Smile, the air is fresh & pure

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