Xylene is an aromatic hydrocarbon which exists in three isomeric forms: ortho, meta and para. Technical grade xylene contains a mixture of the three isomers and also some ethylbenzene. The estimated world production in 1984 was 15.4 million tonnes. Xylene is a colourless liquid at room temperature with an aromatic odour. The vapour pressure lies between 0.66 and 0.86 kPa for the three isomers. Approximately 92% of mixed xylenes is blended into petrol. It is also used in a variety of solvent applications, particularly in the paint and printing ink industries. The majority of xylene released into the environment enters the atmosphere directly. In the atmosphere the xylene isomers are readily degraded, primarily by photooxidation. Volatilization to the atmosphere from water is rapid for all three isomers. In soil and water, the meta and para isomers are readily biodegraded under a wide range of aerobic and anaerobic conditions, but the ortho isomer is more persistent. The limited evidence available suggests that bioaccumulation of the xylene isomers by fish and invertebrates is low. Elimination of xylene from aquatic organisms is fairly rapid once exposure has ceased. Typically, mean background levels of all three xylene isomers in ambient air are around 1 ug/m3, but in suburban areas they are around 3 ug/m3. Higher levels have been measured in urban and industrialized areas, mean concentrations ranging up to 500 ug/m3. However, concentrations are generally below 100 ug/m3. Estimated daily exposure of the general population through inhalation is 70 ug in rural areas and less than 2000 ug in urban areas. The concentration in drinking-water ranges from not detectable to 12 ug/litre. The data on the level in food are too limited to estimate daily oral exposure. Mean background concentrations of xylenes in surface water are generally below 0.1 ug/litre. However, much higher values have been measured in industrial areas and areas associated with the oil industry (up to 30 ug/litre in polluted waters and up to 2000 ug/litre near to discharge pipes). Similar background levels have been reported for groundwater although high levels have been reported due to localized pollution from underground storage tanks and pipes. After inhalation exposure the retention in the lungs is about 60% of the inhaled dose. Xylene is efficiently metabolized. More than 90% is biotransformed to methylhippuric acid, which is excreted in urine. Xylene does not accumulate significantly in the human body. Acute exposure to high concentrations of xylene can result in CNS effects and irritation in humans. However, there have been no long-term controlled human studies or epidemiological studies. The chronic toxicity appears to be relatively low in laboratory animals. There is suggestive evidence, however, that chronic CNS effects may occur in animals at moderate concentrations of xylene. Xylene appears not to be a mutagen or a carcinogen. The critical end-point is developmental toxicity, which has been demonstrated at an exposure level of 870 mg/m3 (200 ppm) in rats. Based on this end-point, the recommended guidance value for xylene in air is 0.87 mg/m3 (0.2 ppm). The xylene isomers are of moderate to low toxicity for aquatic organisms. For invertebrates the lowest LC50 value, based on measured concentrations, is for o-xylene at 1 mg/litre (Daphnia magna). The lowest LC50 values recorded for fish are 7.6 mg/litre for o-xylene (rainbow trout; based on measured concentrations), and 7.9 and 1.7 mg/litre for m- and p-xylenes respectively (both for striped bass; based on nominal concentrations). Limited information is available regarding chronic exposure of aquatic organisms to xylenes; however, rapid volatilization makes chronic exposure in water unlikely. The acute toxicity of xylene to birds is low.