Mercury-induced motor and sensory neurotoxicity: systematic review of workers currently exposed to mercury vapor

The neurotoxicity of elemental mercury (Hg⁰) is well-recognized, but it is uncertain whether and for how long neurotoxicity persists; among studies that evaluated previously exposed workers, only one examined workers during and also years after exposure ceased. The aim of this review is to document the type, frequency, and dose-relatedness of objective neurological effects in currently exposed mercury workers and thereby provide first approximations of the effects one would have expected in previously exposed workers evaluated during exposure. We systematically reviewed studies of neurotoxicity in currently exposed mercury workers identified by searching MEDLINE (1950–2015), government reports, textbook chapters, and references cited therein; dental cohorts were not included. Outcomes on physical examination (PE), neurobehavioral (NB) tests, and electrophysiological studies were extracted and evaluated for consistency and dose-relatedness. Forty-five eligible studies were identified, comprising over 3000 workers chronically exposed to a range of Hg⁰ concentrations (0.002–1.7 mg/m³). Effects that demonstrated consistency across studies and increased frequency across urine mercury levels (<50; 50–99; 100–199; ≥200 μg/L) included tremor, impaired coordination, and abnormal reflexes on PE, and reduced performance on NB tests of tremor, manual dexterity and motor speed. The data suggest response thresholds of U_Hg ≈275 μg/L for PE findings and ≈20 μg/L for NB outcomes. These results indicate that PE is of particular value for assessing workers with U_Hg >200 μg/L, while NB testing is more appropriate for those with lower U_Hg levels. They also provide benchmarks to which findings in workers with historical exposure can be compared.