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

<p>The neurotoxicity of elemental mercury (Hg<sup>0</sup>) is well-recognized, but it is uncertain whether and for how long neurotoxicity persists; among studies that evaluated <i>previously exposed</i> 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 <i>currently exposed</i> mercury workers and thereby provide first approximations of the effects one would have expected in <i>previously exposed</i> workers evaluated during exposure. We systematically reviewed studies of neurotoxicity in <i>currently exposed</i> 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 Hg<sup>0</sup> concentrations (0.002–1.7 mg/m<sup>3</sup>). 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<sub>Hg</sub> ≈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<sub>Hg</sub> >200 μg/L, while NB testing is more appropriate for those with lower U<sub>Hg</sub> levels. They also provide benchmarks to which findings in workers with historical exposure can be compared.</p>