General properties: Beta-hexachlorocyclohexane (beta-HCH) is a by-product 7-10%) in the manufacture of lindane ( and gt; 99% gamma-HCH). Its solubility in water is low, but it is very soluble in organic solvents such as acetone, cyclohexane, and xylene. is a solid with a low vapour pressure. The n-octanol/water partition coefficient (log Pow) is 3.80. It is an environmental pollutant. Beta-HCH can be determined separately from the other isomers by gas chromatography with electron capture detection and other methods after extraction by liquid/liquid partition and purification by column chromatography. Environmental transport, distribution, and transformation: Biodegradation and abiotic degradation (dechlorination) by ultraviolet irradiation occur in the environment and produce pentachlorocyclohexane, but at a much lower rate than in the case of lindane (gamma-HCH). Beta-HCH is the most persistent HCH isomer. Its persistence in soil is determined by environmental factors such as the action of microorganisms, content of organic matter and water, and co-distillation and evaporation from 1. Owing to the persistence of beta-HCH, rapid bioconcentration takes place in invertebrates (the bioconcentration tor is approximately 125 within 3 days), fish (250-1500 a dry weight basis or approximately 500 000 times on a id basis within 3-10 days), birds and man (approximately 525). The bioconcentration is higher and the elimination is slower for beta-HCH than for the other HCH isomers. Effects on organisms in the environment: Beta-HCH generally has moderate toxicity for algae, invertebrates, and fish. The acute LD50 values for these organisms are of the order of 1 mg/litre, but the EC50 values are lower (0.05-0.5 mg/litre). The no-observed-effect level for Oryzia latipes and Poecilia reticulata, two freshwater fish exposed for 1 or 3 months, was 0.03 mg/litre (Daphnia, NOEL for reproduction = 0.32 mg/l). No data are available on effects on populations and ecosystems. Effects on experimental animals and in vitro test systems: The acute oral LD50 values for mice and rats were reported in 1968 to lie between 1500 and 2000 mg/kg body weight. However, more recent studies yielded values of 16 g/kg body weight for mice and 8 g/kg body weight for rats. Signs of intoxication were mainly of neurological origin. Two short-term mouse studies, with dose levels of up to 600 mg/kg diet for 26-32 weeks, showed increased liver weight and nodular hyperplasia and atypical proliferations in the liver. In a third study, dose levels of up to 500 mg/kg diet for 24 weeks did not result in liver tumours or nodular hyperplasia. A 90-day study with rats fed 50 or 250 mg/kg diet revealed liver changes, i.e. hypertrophy and proliferation of smooth endoplasmic reticulum and increased activity of microsomal enzymes. Changes in the gonads occurred at the highest dose levels but these were associated with severe effects on body weight. Hormonal changes associated with the gonadal atrophy showed no consistent endocrine effect. There were no adverse effects at a dose level of 2 mg/kg diet (equivalent to 0.1 mg/kg body weight). In a long-term rat study (reported in 1950), doses of 10 mg/kg diet (equivalent to 0.5 mg/kg body weight) or more led to liver enlargement and histological changes. In a two-generation reproduction study on rats, the same effects were found as in the 90-day study. There were no effects at 2 mg/kg diet (equivalent to 0.1 mg/kg body weight), but a dose level of 10 mg/kg diet resulted in increased mortality and infertility. No compound-related teratogenic effects were found in an extension to this study. A weak "estrogenic" effect has been described. The uterus was the target organ for this effect; there were no clear effects on endocrine control systems. The mechanism and significance of this effect are uncertain. The mutagenicity studies reported did not show any increase in mutation frequency in Salmonella typhimurium strains. An in vivo bone marrow metaphase analysis in rats yielded positive results. Two studies have been carried out on mice to determine carcinogenic potential. In one study, 200 mg/kg diet was given for 110 weeks, and liver enlargement, hyperplastic changes, and an increase in benign and malignant tumours were reported. In the other study, where 500 mg/kg diet was administered for 24 weeks, no tumours were observed. Studies in which rats were fed combinations of beta-HCH with polychlorinated biphenyls suggested a promoting effect of beta-HCH. At 300 mg/kg diet, beta-HCH caused significant changes in several immune functions in mice within one month. Effects on humans: When workers at a lindane-producing factory, with a geometric mean exposure of 7.2 years (1-30), were investigated, it was concluded that occupational HCH exposure did not induce signs of neurological impairment or perturbation of "neuromuscular function".