INDIAN scientists find lichens can be used to monitor urban pollution levels. Lichens are disappearing within the core zone of the highly industrialised Haldia township in West Bengal, due to increasing air pollution. (Pollution Research, Vol 11, No 1).

Sensitivity of lichens - essentially symbiotic associations of fungi and algae, often found growing on tree trunks and rocks - to air pollution, particularly gases like sulphur dioxide and particulate matter like dust, is fairly well-known.

At HaIdia, effluents from companies like the Indian Oil, Eureka Chemicals, Hindustan Fertilizer, Shaw Wallace and Hindustan Lever and smoke from marine vessels pollute the atmosphere.

S G Mina and S C Santra at the School of Environmental Sciences, University of Kalyani, and K N Roychowdhury, of the Botanical Survey of India, Howrah, discovered more lichen species are present in areas where pollution levels are low. The scientists have drawn up an index of atmospheric purity (IAP) using lichens to classify levels of pollution in various areas of Haldia.

Both the lichen constituents are affected by gaseous and particulate pollutants. Matter accumulates in the tissues bringin bout degenergation and a number of morphological changes. Mitra and Santra also discovered more pollution-tolerant species, like Bacidia convexula and Parmelia caperato, are found in areas with higher sulphur dioxide levels, while more sensitive species like TrypetheIium trot m and Gyrostomum scyphiliferum are found where pollution levels are lower.