What form of pollution can lichens be a good indicator of?

Lichens can be used as air pollution indicators, especially of the concentration of sulphur dioxide in the atmosphere. Lichens are organisms that grow in exposed places such as rocks or tree bark. They need to be very efficient at absorbing water and nutrients to grow there.

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).