Miniature ecosystems like Lichens need clean air...

...are very sensitive to pollution.

Lichens are ecosystems of algae fungi microbes insects, live long for years; can survive inhospitable conditions like periods of drought, but not pollution in the air. 

Bark of Azardicta Indica (Neem) with lichens in a homestead in Thiruvananthapuram

Cat napping with Lichens on a tiled roof of an old house in Thiruvananthapuram

Some of the chemicals in air particulates which destroy lichens are Nitrogen di oxide, Sulphur di oxide, Hydrogen Fluoride, Peroxyacetyl nitrate (PANs), metals like nickel cadmium iron lead emitted by smelters and mines, radioactive elements…

Parmelia perlata,the stone flower lichen known as Kalpoovu in Malayalam, Shilapushp in Sanskrit is used as a spice in cooking and in many kashayams (concoctions of various ingredients) in many homes - some where traditional knowledge was passed from grandmothers, mothers to daughters.

Parmelia perlata was a common sight growing on rocks, stones… under the shade of the tall Artocarpus hirsutus trees in the sacred Hindu groves of many homesteads in Kerala.

Parmelia Perlata is now found mostly in the forests.