What are three reasons lichens are useful to humans?

Lichens are useful as they act as pollutant indicators, like food and medicines.

Explanation:

In lichens, High SO2 concentrations were said to be the main reason for habitat loss, reductions in some species. The most sensitive lichens are shrubby and leafy. Lichens are very sensitive to pollution such as nitrogen because they receive water and nutrients from wet and dry atmospheric deposition. this property makes them a pollutant indicator.

Lichens provide food and also help algae to produce food. they make of their own food production. Bryoria is a genus of lichen used by native Americans.

Lichens in traditional medicine are generally used for treating skin disorders, wound, and digestive issues.

i won't lie i don't know so go online and ask google thats what i do