Figure 2
 Levels of phenol (a) and hydroquinone (b) in the urine of various groups of individuals. Phenol and hydroquinone levels in the blood and urine are commonly analyzed by gas chromatography in combination with various methods of initial purification and hydrolysis of the specimen. In the earlier studies colorimetric methods of Gibbs or Theis-Benedict were used to quantitate phenol (reviewed in Ref. 48), which were less specific and sensitive compared to gas chromatography. The other important factors affecting reported phenol values are variation in sampling and adjustment for gravity and creatinine. Phenol levels are typically reported as a concentration, mass per mass creatinine, or total mass excreted in 24 h. Data from Refs 47505154575875107108
