Stability of UV filter 2-ethylhexyl salicylate and its glucuronide in human urine - A challenge for robust human biomonitoring

Background: 2-Ethylhexyl salicylate (EHS) is used as a UV filter in personal care products, especially sunscreens. The elimination of EHS and its metabolite 2-ethyl-5-hydroxyhexyl salicylate (5OH-EHS) after dermal exposure has been previously investigated and showed that both are excreted in high amounts. However, the low stability of the reference substance for EHS highly affected the analysis of this parameter. Since EHS was assumed to be excreted primarily as a conjugate to glucuronic acid in vivo, we compared the stability of unconjugated EHS to that of glucuronidated EHS (EHS-GlcA) in urine at 8 degrees C and -20 degrees C. Results: The relative recovery of unconjugated EHS decreases significantly when stored at both 8 degrees C and -20 degrees C. Stored as EHS-GlcA, the relative recovery remains steady over the course of the experiment. In addition to the stability experiment, the fraction of unconjugated EHS in human urine samples directly after dermal exposure to EHS was assessed, and only a low effective portion of unconjugated EHS was found. The same samples were reassessed after several months of storage at -20 degrees C and showed relative recoveries within the acceptance criteria, which also endorses the low initial level of unconjugated EHS. The hydroxylated EHS metabolites, 5OHEHS, 4OH-EHS and 2OH-EHS, were also included in the experiment and showed slightly decreasing relative recoveries at 8 degrees C, but stable relative recoveries at -20 degrees C. Significance: Despite unconjugated EHS being excreted only in a low portion in vivo, its low stability in urine may affect the effectiveness of EHS biomonitoring, because only the use of EHS glucuronide guarantees a stable and reliable calibration in urine. However, the hydroxylated EHS metabolites have proven to be suitable standards for calibration as well as parameters for biomonitoring when stored at -20 degrees C.