Bull. Environ. Contam. Toxicol. (1990) 45:280-287 9 1990 Springer-Verlag New York Inc. Environmental Contamination ~and Toxicology Comparative Toxicity of an Effluent from a Chlor-Alkali Industry and HgCI~ B. P. Shaw, A. Sahu, and A. K. Panigrahi Laboratory of Environmental Toxicology, Department of Botany, Berhampur University, Berhampur-760007, Orissa, India Release of waste waters from industries severely affect aquatic flora and fauna. Effluents from industries involved in the production of pesticides, or from industries utilizing toxic substances in their production processes are of particular concern. One such industry, the chlor-alkali industry, utilizes mercury as a cathode in an electrolytic process. Though no mercury is being utilized in the production process, losses are inevitable during washings (Skei 1978). Release of mercury into the aquatic environment has received much international attention following the tragedies in Japan in the 1950s and the ornithological disaster in Sweden in the 1960s. Since then, attempts have been made to evaluate the toxicity of effluents in a number of ways. One of the most important ways is to determine the minimum concentration of industrial effluents that kill fish in a particular period. Only with the help of such bioassay experiments can the toxicity of any pollutant in an effluent be established (Sprague 1973), though its toxic effect on aquatic systems also depends on the latter's quantity, volume and flow of the water, as well as the nature of the bottom sediment. Still, more valuable is the evaluation of the LC ^, i.e. the concentration of the substance which kills 50% 5o~f the organisms. Long-term exposure experiments of Mount & Stephan (1967), Herbert et al. (1965), Edwards & Brown (1967) and Warren (1971) support the view of the development of applica- tion factors which are no doubt important, but the LC.~ values are also quick and useful for asserting harmful effe~c~s of a pollutant. The present study reports the toxicity assessment of the effluent from a chlor-alkali factory, Jayshree Chemicals Ltd., Ganjam, Orissa (30 kms away from the town of Berhampur) The reported production capacity of the plant is 50 tons of NaOH per day (Satpathy 1984). The effluent discharged, 50,000 1 per day, contains mercury ranging up to 1.5 mg/l (Shaw et al. 1988), much more than the permissible limit of 0.01 mg/l set by the Central Board for the Prevention and Control of Water Send reprint request to Dr. B.P.Shaw at the above address 280