A method for rapidly determining t,t-muconic acid (MA) by high performance liquid chromatography was developed and successfully applied to urine samples from 152 workers exposed to benzene (64 men, 88 women) and 213 non-exposed controls (113 men, 100 women). The MA concentrations in urine correlated linearly with time weighted average benzene concentrations in the breath zone air of workers. A cross sectional balance study showed that about 2% of benzene inhaled is excreted into the urine as MA. The MA concentrations in the urine of the non-exposed was below the detection limit ( < 0 1 mg/l) in most cases, and the 95% lower confidence limit of MA for those exposed to benzene at 5 ppm (5-0 mg/l as a non-corrected value) was higher than the 97'5%-tile values for the non-exposed (1-4 mg/l). In practice, it was possible to separate those exposed to 6-7 ppm benzene from the non-exposed by means of urine analysis for MA. The urinary MA concentration was suppressed by coexposure to toluene.Benzene, an established human leukaemogen,' is an important material in the chemical industry, and it is also known to be present in petrol2 and in petroleum distillate solvents of low boiling point.3 Accordingly, efforts have been made to identify exposure markers, preferably in urine, which are sensitive enough to detect benzene exposure at or below 10 ppm. 45 The present report is an extension of three previous studies on the biological monitoring of exposure to benzene by means of urine analysis for metabolites such as phenol,5 catechol, and quinol,6 and on the effects of combined exposure to toluene.' A time and work saving method of urine analysis for t,t-muconic acid (MA) has been developed and successfully used for the biological monitoring of workers exposed to 6-7 ppm benzene. Materials and methods WORKERS STUDIED AND URINE COLLECTIONThose who participated in this study were 152 workers exposed to benzene (64 men exposed up to 92 ppm, 88 women up to 210 ppm), 55 male workers exposed to a mixture of benzene (up to 1 6 ppm) and toluene (up to 114 ppm), and 213 non-exposed workers (113 men, 100 women) in factories in China. The jobs of the Accepted 2 February 1988 exposed workers have been described previously8; the benzene workers were mostly shoemakers whereas the workers with mixed exposure were predominantly painters. The urine samples were collected at 1500 in the second half of a working week, when the concentrations of benzene metabolites in urine are expected to reach a maximum.9 URINE ANALYSIS FOR t,t-MA The high performance liquid chromatographic (HPLC) method of Karim et all' for t,t-MA was simplified for urine analysis as follows: an aliquot of urine sample, kept frozen and thawed immediately before analysis, was well mixed with an equal volume of methanol. The mixture was spun at 3000 rpm for 10 minutes to remove precipitates as in urinary hippuric acid determination." The supernatant layer (5 to 10 pl per injection) was introduced into a HPLC (Hitachi Model 635) equipped with a Spherisorb ODS 5 im colu...

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The exposure intensity during a shift and the metabolite levels in the shift-end urine were examined in male workers exposed to either benzene (65 subjects; the benzene group), toluene (35 subjects; the toluene group), or a mixture of both (55 subjects; the mixture group). In addition, 35 non-exposed male workers (the control group) were similarly examined for urinary metabolites to define background levels. A linear relationship was established between the intensity of solvent exposure and the corresponding urinary metabolite levels (i.e. phenol, catechol and quinol from benzene, and hippuric acid and o-cresol from toluene) in each case when one of the three exposed groups was combined with the control group for calculation. Comparison of regression lines in combination with regression analysis disclosed that urinary levels of phenol and quinol (but not catechol) were lower in the mixture group than in the benzene group when the intensities of exposure to benzene were comparable, indicating that the biotransformation of benzene to phenolic compounds (excluding catechol) in man is suppressed by co-exposure to toluene. Conversely, metabolism of toluene to hippuric acid was suppressed by benzene co-exposure. Conversion of toluene to o-cresol was also reduced by benzene, but to a lesser extent. The significance of the present findings on the mutual suppression of metabolism between benzene and toluene is discussed in relation to solvent toxicology and biological monitoring of exposure to the solvents.