How polluted is the habitat if trees have Pleurococcus and crusty yellow lichens on them?

If trees have Pleurococcus and crusty yellow lichens on them, the habitat is considered to be moderately polluted.

Pleurococcus is a green alga that is commonly found in urban areas and is tolerant of air pollution. Crusty yellow lichens are also tolerant of air pollution and are often found in areas with moderate to high levels of air pollution. The presence of both Pleurococcus and crusty yellow lichens on trees indicates that the habitat is likely to be polluted, but not severely polluted.