Why are lichens called ecological indicators?

Lichens can live in rather unfavorable conditions, but they cannot put up with air pollution. Lichens are also called bioindicators, because they grow only under good environmental conditions. Lichen is a living organism, and the state of the environment can be determined by its habitat. Previously, the presence of lichen, for example on a tree, was equated with a parasitic organism, but in fact, this indicates the health of the tree and clean air.