Concerning the assessment of the toluene concentration in the urine in the hippuric acid form, some have argued that the result might vary due to metabolism. It has also been reported that the toluene concentration in urine was a more sensitive biomarker than hippuric acid; however, many studies have found an association between metabolites in the urine and the concentration of toluene. Therefore, it was widely used as a toluene-exposure biomarker.