What a biological exposure index is not: lessons from benzene for South Korea's special health examination.

Biological exposure indices (BEIs) shape South Korea's special health examination system, yet the epistemological assumptions underpinning their regulatory use have received little scrutiny. In 2023, the American Conference of Governmental Industrial Hygienists (ACGIH) reduced the benzene threshold limit value-time-weighted average (TLV-TWA) from 0.5 to 0.02 ppm, a 25-fold decrease that exposed serious practical limitations in biomarker frameworks for low-level carcinogen monitoring. This narrative review analyzes the ACGIH BEI and DFG (Deutsche Forschungsgemeinschaft) systems using primary sources, with benzene as the paradigm case for structural and operational differences between frameworks. ACGIH and DFG operationalize biological monitoring differently: ACGIH assigns benzene BEI values, whereas DFG declines to set a health-based tolerance value and instead provides exposure-response correlations (EKA) and nonsmoker population reference values (BAR). South Korea's system has structural defects, including the listing of S-phenylmercapturic acid (S-PMA) only as a recommended indicator, while the second-tier options retain confounding-prone biomarkers such as trans,trans-muconic acid (t,t-MA). No currently validated benzene biomarker can reliably monitor compliance at 0.02 ppm. BEIs are exposure assessment tools, not diagnostic thresholds or legal standards. Restoring this distinction requires interpretation within integrated surveillance, evidence-based biomarkers such as S-PMA, and analytical capacity for the 0.02 ppm era.