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The paper discusses the application of lichen diversity in the form of Index of Atmospheric Purity (IAP) for environmental stress study (air pollution) around a paper mill in Assam, India. Seventeen sites were selected within an area of approximately 1800 km(2) around the mill to prepare a stress zone map using IAP; the values ranged between 17 and 113, delineating the area into five zones. The zones are categorised as zone II (IAP: 1 - 24), zone III (IAP: 25 - 49), zone W (IAP: 50 - 74), zone V.(IAP: 75 - 99), and zone VI (IAP > 99). Zone I, which represents IAP = 0 (or lichen desert) is not found in the study. The resultant map from interpolation of IAP exhibits resilience by lichens to the multiple stress sources like stone crushers, cement industry and urban sprawls. IAP, derived from number of species, frequency of occurrence, coverage, and sensitivity is found to be an efficient and cost effective method depicting long term stress effects on lichens and is one of the most suitable biomonitoring tools. for a developing country like India.