High performance liquid chromatography (HPLC) and spectrophotometry (Star Dust MC 15) were used to analyze the metabolite concentration of xylene and toluene, namely, methyl hippuric acid (mg/gram creatinine) and hippuric acid
(mg/gram creatinine) in the urine of the workers exposed to the solvents, at the Occupational and Environmental
Department of the Ministry of Public Health in Bangkok. The calibration and quality control was performed as per the NIOSH method 8301, fourth edition. The standard biological exposure indices (BEI) of methyl hippuric acid in the urine are 1.5 g/g creatinine, and BEI of hippuric acid in the urine are 1.6 g/g creatinine.