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

Abstract

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.

Keywords:

• Elemental mercury
• occupational disease
• neurotoxicity
• tremor
• physical examination
• neurobehavioral testing
• motor function
• electrophysiological studies

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Erratum

Acknowledgments

We wish to thank Ms. J. Rivera for her bibliographic excellence, and Dr. M. Russi, Mr. G. Munemitsu, and Mr. E. Fiel for help and advice regarding translations. We are very grateful to the five anonymous reviewers selected by the Editor for the value of their comments in improving the manuscript.

Declaration of interest

Jonathan Borak and Cheryl Fields are employees of Jonathan Borak & Company, which provides consulting services on environmental and occupational health issues (including concerns related to mercury exposure) to government and private clients. Both Ms. Fields and Dr. Borak are part time employees of Yale University. Elan Louis is a full-time employee of Yale University. The authors have not appeared in the last five years in any legal or regulatory proceedings related to the contents of this article and have no plans to participate in such proceedings in the future. Dr. Borak and Ms. Fields contributed to the research reported in this article during their normal course of employment without supplementary funding or external support.

Dr. Louis was supported by grant R01 NS094607 from NINDS.

Supplemental material

Supplemental material for this article is available online here.