Used as a biomarker for exposure to phthalates, particularly diethyl phthalate (DEP), which is commonly found in personal care products like perfumes, lotions, and cosmetics. Monitoring monoethyl phthalate levels in urine helps assess human exposure to phthalates and evaluate potential health risks associated with endocrine disruption. It is also used in environmental health studies to track the prevalence and sources of phthalate contamination in populations.