In December 1984, the world's worst chemical accident occurred at Union Carbide Corporation's chemical plant at Bhopal in the state of Madhya Pradesh, India. More than 401 of methyl isocyanate (MIC) gas leaked from the tank of the plant and was released into the atmosphere. The immediate cause was a buildup of pressure in the tank due to an exothermic reaction caused by water in the tank. This pressure caused the safety valve to rupture and the gas to escape. This accident killed at least 3000 men, women, and children after breathing the lethal gases and caused significant morbidity and premature death for many thousands more. Extensive literature reports are available relating to the Bhopal accident, but only environmental and ecological issues are considered here for the purpose of this article.
Unlike TCDD in Seveso, MIC is highly reactive and rapidly hydrolyzed, so the initial effects of MIC were much more devastating than those of TCDD. The rapid degradation of MIC, unlike that of TCDD, however, meant that the long-term environmental impact of the accident was less serious than that at Seveso. The biodegradation of MIC and its anticholinesterase activities in different tissues of various species of fish have been studied and the concentrations of the degradation product of MIC, methylamine, in the water of the lakes from Bhopal over a period of 6 months after 3 weeks of MIC leakage into the environment have been estimated. There are two lakes in Bhopal. One ofthem, the Upper Lake, is the water source for the city and is far away from the accident site, whereas the Lower Lake is very close to the accident site and is completely eutrophic. The concentrations of methylamine in the Lower Lake decreased from the first sampling (23-30 December 1984) to the second sampling (25-26 February 1985), and an increase in concentrations with depth was observed. Methylamine was not detected during the third sampling (1-2 May 1985). Methylamine was detected in the fish samples of both lakes, and the concentrations in Puntius ticto from the Lower Lake were fairly high (9.2 mg kg~ wet weight) at the first sampling, and methylamine was not detected during the third sampling. No methylamine was detected in other fish species from the Lower Lake. In the first sampling, the acetyl-cholinesterase activities of different tissues of fishes from the Lower Lake were significantly lower than those from the Upper Lake, implying the effects of MIC on their environment, but no clear trend in inhibition was determined.
Various aspects of the Bhopal accident, including its effects on animals and plants, have been reported. The animal death toll caused by MIC in this accident was massive, numbering over 1000 domestic animals, including 240 cows, 280 buffaloes, 18 bullocks, 84 calves, 288 goats, 60 pigs, 12 horses, 99 dogs, 2 cats, and 3 chicken. More than 7000 animals were reported to have some symptoms of varying degrees. Autopsies of the animals showed swollen livers and lymph glands, bloated digestive tracts, enlarged blood vessels or edema, and congestion of the heart and kidney. As for the damage to plants, broadleaf trees such as Azadirachta indica and Ficus religiosa showed total defoliation within 1 km of the accident site. Plants with thicker leaves and shrubs were less affected. Reaction with the leaf surface was the main cause of damage to the trees, whereas stems or other hardy parts of the tree were not affected.
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