Academic journal article Scandinavian Journal of Work, Environment & Health

Neurological Outcomes Associated with Low-Level Manganese Exposure in an Inception Cohort of Asymptomatic Welding Trainees

Academic journal article Scandinavian Journal of Work, Environment & Health

Neurological Outcomes Associated with Low-Level Manganese Exposure in an Inception Cohort of Asymptomatic Welding Trainees

Article excerpt

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Elevated exposure to manganese (Mn) has long been an occupational health concern; excess Mn in the body crosses the blood-brain barrier, accumulates in the brain, and can cause adverse neurological health effects (1,2). Manganism is a syndrome characterized by parkinsonism features (bradykinesia, rigidity, postural instability, tremor), dystonia, and cognitive dysfunction, clinical features also common to idiopathic Parkinson's disease (3).

Welders typically experience 8-hour mean personal breathing zone Mn concentrations ranging from <0.02->1.0 mg/m3, depending on welding process and control methods (4-7). The American Conference of Governmental Industrial Hygienists' (ACGIH) 8-hour time-weighted average threshold limit value (TWATLV) for Mn in total inhalable dust was recently reduced to 0.1 mg/m3 and the respirable TLV was reduced to 0.02 mg/m3, based on neurological outcomes seen in workers exposed to low levels of Mn (8). Although welding fume is almost entirely respirable, other metalworking tasks commonly done by welders generate larger, inhalable particles. However, the Occupational Safety and Health Administration's Permissible Exposure Limit (OSHA PEL) for Mn in total inhalable dust of 5.0 mg/m3 (ceiling) is the only enforceable Mn standard in the United States. In Europe, the European Commission Scientific Committee on Occupational Exposure Limits (SCOEL) recommends an 8-hour TWA of 0.2 mg/m3 for Mn in the inhalable fraction and 0.05 mg/m3 for Mn in the respirable fraction, though enforceable standards would vary between European countries (9).

Manganism has historically been associated with long-term, high levels of occupational exposure to Mn, yet neurological effects have been observed at Mn levels below the OSHA PEL and closer to the ACGIH TLV or SCOEL recommended 8-hour TWA (10-12). Recently, Laohaudomchok et al (13) reported parkinsonian effects among welders who experience low Mn exposures (median: 0.013 mg/m3). Ellingsen et al (14) found welders with a mean Mn exposure of 0.21 mg/m3 to have poorer performance than unexposed referents on Grooved Pegboard, Finger Tapping, and Simple Reaction Time tests. In contrast, among both mineworkers and smelter workers exposed to Mn, Myers et al (15,16) found no relationship between Mn exposure and nervous system effects, even at levels above the ACGIH TLV.

While neurological function tests, such as UPDRS3 and Grooved Pegboard, are classified as biomarkers of Mn effect, T1 -weighted magnetic resonance images (MRI) have been utilized as a biomarker of Mn in-vivo brain exposure. Mn is a paramagnetic element that strongly influences the magnetic resonance properties of surrounding tissues. Mn shortens tissue T1 relaxation times resulting in increased signal intensity detectible on T1 -weighted imaging. Human studies have confirmed the intensity of the T1 pallidal signal correlates with blood Mn levels and cumulative exposure to Mn, but it may not be as accurate at low levels of chronic exposure (17-19). Arecent paper by Criswell et al (20) found intensity changes in the caudate and putamen to be better markers of cumulative exposure. In this study we attempt to evaluate caudate, putamen, and pallidal T1 signal as markers of low-level Mn exposure in a longitudinal study of welder trainees.

Many tests exist to assess neurotoxicity associated with Mn exposure, both specific (ie, functional imaging techniques, neuropsychiatrie testing for domains of memory) and general (ie, some tests of mood, dexterity), that have been used either singularly or as part of a battery (3, 21-23). Epidemiologic studies assessing neurological outcomes associated with Mn exposure among welders and other occupationally exposed groups are most commonly cross-sectional. Although non-exposed referent groups are frequently included, to our knowledge no prior studies have included pre-exposure baseline measurements of CNS function on a well-characterized Mn-exposed occupational cohort. …

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