Biomarkers of Heavy Metal Exposure and Hearing Loss: A Narrative Review

A BSTRACT Background: Environmental heavy metals are increasingly recognized as potential contributors to hearing loss beyond age and noise exposure. Objective: To synthesize human epidemiologic evidence (2016–2026) on associations between biomarkers of heavy metal exposure and hearing outcomes. Methodology: A structured narrative review guided by the Scale for the Assessment of Narrative Review Articles was conducted. PubMed and Web of Science were searched through January 2026. Eligible studies included original human research reporting quantitative metal biomarkers in blood, plasma/serum, urine, hair, or toenail and audiometric or physiologic outcomes (pure-tone audiometry, otoacoustic emissions, auditory brainstem response). Findings were synthesized descriptively. Results: Twenty-seven studies were included. Lead (Pb) and cadmium (Cd) showed the most consistent associations with hearing impairment, particularly at high frequencies (≥3 kHz), often demonstrating dose–response patterns after adjustment for noise. High-frequency pure-tone averages appeared more sensitive than speech- or low-frequency metrics. Mercury (Hg) findings were largely null or inconsistent, while evidence for arsenic (As), manganese (Mn), and hexavalent chromium was limited or heterogeneous. Few studies formally evaluated metal–noise interaction. Conclusions: Contemporary evidence most strongly supports associations of Pb and Cd with high-frequency hearing loss. Standardized frequency-specific outcomes, improved cumulative exposure assessment, and prospective designs are needed to clarify causality and inform environmental and occupational hearing conservation strategies.