As we know that lichens are the pollution indicators,then why they do not grow in polluted areas?

Hii there,i think u have misunderstood the concept behind this.LICHENS DO NOT GROW IN POLLUTED AREAS.That is why they are known as pollution indicators.

Hello, while it is a well-established fact that lichens are good indicators of air pollution, the irony behind this premise is that their contribution to indicating air pollution happens through their absence, not presence. In other words, you can establish that the air is polluted in a particular area if the growth of lichens is not possible there. Unlike most organisms which form single stable units of life, lichens are complex life forms arising from the symbiotic relationship between separate organisms, primarily a fungus along with an alga or cyanobacteria. They mainly take root in exposed places such as walls, rocks or tree barks as this enables them to gain access to rainwater which contains the essential nutrients necessary for their survival. If pollutants present in the air, particularly sulphur dioxide, nitrogen, acids, fluoride or heavy metals dissolve with rainwater, lichens are unable to grow or survive. This trait of theirs makes it possible for them to be considered as reliable bio-indicators, and therefore if a particular area is devoid of lichens or is unable to facilitate their growth, it can be considered as polluted. To completely understand the scientific concept behind this, it is recommended that you carefully go through the Ecology and Environment section of your Class 12 NCERT Biology textbook. Hope this clears your doubt!