Toluene, a homolog of benzene that contains a single methyl group, is a clear, colorless liquid at room temperature. The molecular formula of toluene is C7H8 and the molecular weight is 92.13. Other physical properties of toluene include a melting point of -95xC, a boiling point of 110.6xC, a flash point of 4.44xC, a vapor pressure of 28.7 torr at 25xC, and a density of 0.8669 g/ml at 20xC. Toluene is a slightly soluble in both fresh and salt water (535 mg/l and 379 mg/l, respectively) at a temperature of 25xC. The physical properties of toluene indicate that toluene in the environment is likely to be present in the air, and that toluene originally present in water may be transferred to the atmosphere. Toluene can undergo photochemical reactions, particularly under atmospheric smog conditions. In aqueous media under the conditions of water chlorination, toluene may be chlorinated followed by subsequent hydrolysis to benzaldehyde. This reaction may account for the benzaldehyde detected in some finished drinking waters. Effects on mammals: Available evidence associated with effects upon humans and experimental animals indicates that the health effect of primary concern is dysfunction of the central nervous system (CNS). However, observed effects are associated with exposure levels greatly in excess of those levels in the environment. Dysfunction of the CNS may occur during short- term ( and lt;8 hours) exposure to 100-300 ppm. Toluene has not demonstrated any overt signs of kidney or liver damage upon animal experimentation. It was non-carcinogenic in rats exposed to 300 ppm for 24 months. However, the full extent of toluene's carcinogenic potential is currently being evaluated, at higher exposure levels, in a lifetime bioassay of rodents in the National Toxicology Program. Toluene is classified as provisionally non-mutagenic, and its teratogenic potential has not been fully explored. The results of the available evidence indicate that exposure to environmental levels of toluene is unlikely to constitute a significant hazard to the general population. Effects on ecosystems: The ecological effects on toluene have been investigated using aquatic and terrestrial microorganisms, aquatic invertebrates, fish, and higher plants. Toluene can both stimulate and inhibit growth of bacteria and algae, depending on the species and the concentration of toluene. The growth of most species of bacteria, algae, and other microorganisms is not inhibited until the toluene concentration exceeds 10 to 100 mg/l. Toluene is acutely toxic to aquatic invertebrates and fish at concentrations ranging between 3 and 1180 mg/l. The 48-hr LC50 value for Daphnia magna was 60 mg/l. The lowest concentration shown to cause sublethal effects in aquatic animals was 2.5 mg/l. Chronic toxicity data are available for two species of fish; marine sheepshead minnows were affected at 6 mg/l but not at 4 mg/l. Chronic effects occurred at concentrations that were about 2 to 18 times lower than the acute LC50 for these species. Toluene concentrations between 0.1 and 1.0 ppm have been reported occasionally in surface water (0.1 to 1.0 mg/l) and sediments (0.1 to 1.0 mg/kg). These concentrations are sufficiently close to the toxic concentrations for sensitive species to indicate that acute or chronic toxic effects may occur in some polluted habitats, especially after accidental spills of toluene. Toluene has only a low bioconcentration potential and is metabolized and rapidly depurated from fish, which indicates that toluene is unlikely to biomagnify through aquatic food webs (calculated log Pow = 2.69). Toluene, however, has been shown to impart an unpleasant taste to fish that inhabit contaminated water. The impact of toluene spills or chronic low-level pollution on ecosystems in unknown. Adverse effects may occur but probably are limited by rapid rates of loss of toluene through evaporation and biodegradation.