State-of-the-Science Review: Does Manganese Exposure During Welding Pose a Neurological Risk?

Abstract

Recent studies report that exposure to manganese (Mn), an essential component of welding electrodes and some steels, results in neurotoxicity and/or Parkinson's disease (PD) in welders. This “state-of-the-science” review presents a critical analysis of the published studies that were conducted on a variety of Mn-exposed occupational cohorts during the last 100 yr, as well as the regulatory history of Mn and welding fumes. Welders often perform a variety of different tasks with varying degrees of duration and ventilation, and hence, to accurately assess Mn exposures that occurred in occupational settings, some specific information on the historical work patterns of welders is desirable. This review includes a discussion of the types of exposures that occur during the welding process—for which limited information relating airborne Mn levels with specific welding activities exists—and the human health studies evaluating neurological effects in welders and other Mn-exposed cohorts, including miners, millers, and battery workers. Findings and implications of studies specifically conducted to evaluate neurobehavioral effects and the prevalence of PD in welders are also discussed. Existing exposure data indicate that, in general, Mn exposures in welders are less than those associated with the reports of clinical neurotoxicity (e.g., “manganism”) in miners and smelter workers. It was also found that although manganism was observed in highly exposed workers, the scant exposure-response data available for welders do not support a conclusion that welding is associated with clinical neurotoxicity. The available data might support the development of reasonable “worst-case” exposure estimates for most welding activities, and suggest that exposure simulation studies would significantly refine such estimates. Our review ends with a discussion of the data gaps and areas for future research.

The authors thank Lori Stewart Wharton and Donna Westmoreland for their help with the manuscript, Rick Nelson for editing it, and Dr. Diane Miller and Dr. Mark Roberts for providing thorough reviews. This work was not funded by any source or group.

Dr. Finley has testified as an expert witness, in cases involving alleged asbestos exposures during welding rod use.

The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Notes

¹“Manganism” is a syndrome characterized by a mask-like face, fine tremor of the hands or gross rhythmical movements of the arms and legs, emotional disturbances, and a gait described as a “cock-walk” associated with exposure to manganese.

²Dystonia is a condition in which there is abnormal muscle tone, characterized by prolonged, repetitive muscle contractions that may cause twisting or jerking movements of the body or a body part.

³In this review, “subclinical” symptoms are those signs that are without clinical manifestations and may occur without progressing to the manifestation of overt clinical symptoms. Subclinical symptoms may also occur before symptoms become apparent or detectable by clinical examination or laboratory tests. “Clinical” symptoms are defined as signs or indications of disorder or disease that may be detected by physical examination or laboratory tests. They typically involve a change from normal function, sensation, or appearance in the subject.

⁴The current TLV-TWA is intended to minimize adverse effects in the central nervous, respiratory, and male reproductive systems (ACGIH, 2001). The original TLV for welding fume (not Mn-specific) was 5 mg/m³ for total particulates as a TWA and was reserved for metal arc or oxy-acetylene welding and cutting of aluminum, mild steel, and iron. ACGIH acknowledged that fumes from stainless steel and cadmium- or lead-coated steel, as well as base metals containing copper, nickel, and chromium, are appreciably more toxic, and as such, the governing criteria should be the substance-specific TLV. In 2003, ACGIH published a proposal to reduce the TLV-TWA for respirable Mn to 0.03 mg/m³ (ACGIH, 2003). In 2004, ACGIH retracted the proposed TLV of 0.03 mg/m³ for welding fume without giving an explanation for the retraction.

⁵Soldering and brazing refers to metal-joining techniques where solder (usually tin–lead or copper–zinc alloys) and nonferrous metals are used to unite metals, respectively. For purposes of this article, brazers and solderers are considered in the analysis of welders, because the American Welding Society identifies this method as a welding process. Brazing involves use of a molten filler material that is introduced between two metals to form a bond when heated above 800˚F (Beckett, 1996).

⁶Other welding techniques discussed in the literature are gas welding and plasma arc welding. As the name implies, gases are used to obtain the heat of fusion in gas welding. Specifically, combustion of oxygen and several gases (including acetylene, methylacetylene, propadiene, propane, butane, and hydrogen) creates a flame that melts the work piece. A hand-held torch is used as a flame, while a metal rod is manually fed into the joint to create the weld (Kura & Mookoni, 1998). For plasma arc welding, an ionized stream of gas (plasma) carries the arc through a torch (Kura & Mookoni, 1998).

⁷Studies attempting to determine the chemical form of manganese, as well as its location within a welding particle, are lacking. Due to its vapor pressure, manganese tends to be found within the particle core when mild steel welding alloys are used (Voitkevich 1995). When welding alloys contain fluorine (e.g., when welding with flux-cored or flux-coated alloys), manganese can also be found at the particle surface in the form of MnF₂ or MnF₃ (Voitkevich 1995). Other possible forms of manganese observed in welding fumes include MnFe₂O₄, MnF₂, MnSiO₃, KMnO₂, KMnF₃, MnO, K₂MnO₄, Mn₃O₄, γ-Mn₂O₃, MnO, and MnFe₂O₄.

⁸Patellar clonus refers to a rhythmic jerking movement of the patella produced by grasping it between the thumb and forefinger and pushing it suddenly and forcibly toward the foot. This is an abnormal reflex with alternate contraction and relaxation of the quadriceps muscle.

⁹The PHS also conducted a study in 1940 of workers in an ore-crushing mill exhibiting symptoms of extrapyramidal involvement of the CNS and pronounced changes in speech and gait (Flinn et al., 1940). A health survey of 50 workers, 11 of whom were exposed and “affected” and 23 classified as “exposed/not affected,” showed no cases of Mn poisoning in workers exposed to <30 mg/m³ of Mn; however, the authors note that the small sample size and intermittent exposure experienced by the workers suggest further research was required, recommending that quarterly medical exams be conducted, ventilation continue to be used, and workers exhibiting symptom be transferred from areas with possible Mn exposures.

¹⁰The “symptomatology” section of the medical record form states, “NERV.: Vert. ….. Parest. …. Insom. …. Irritab. ….” These are presumed to be nervous symptomology, including vertigo (vert), paresthesia (parest), insomnia (insom), and irritability (irritab).

¹¹Dry battery powder consists of 65–70% manganese as MnO₂, which serves as a depolarizing agent. Other components of dry battery powder include ammonium chloride, zinc oxide, graphite, acetylene black, ammonium hydroxide, cerium-thorium nitrate, magnesium nitrate, and mercuric chloride (Emara et al., 1971).

¹² Carlsson et al. (1957, 1958) were the first to discover that dopa, the inactive precursor to dopamine, was converted to dopamine in vivo in animal brains and could be used as a neurohumoral agent; however, it was not until 1967 that Cotzias discovered a mode of administration wherein the L-dopa was successfully incorporated into the CNS (Mena, 1979), and “marked improvement” was observed in 8 of 16 patients with PD treated with oral L-dopa (Cozias et al., 1967).

¹³“Parkinsonism” is a term that is used to describe a condition that causes any combination of the types of movement abnormalities seen in Parkinson's disease. The symptoms of parkinsonism include tremor, bradykinesia, rigidity, and postural instability or impaired postural reflexes. Idiopathic PD is the most common cause of parkinsonism and is thought to account for more than two thirds of the cases. Parkinsonism can result from many other disorders, such as hereditary diseases (e.g., Wilson's disease), multiple system atrophies (e.g., Shy–Drager syndrome), other degenerative diseases (e.g., Creutzfeldt–Jakob disease), other CNS disorders (e.g., tumor), infections (e.g., viral encephalitis), exposure to toxins (e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP], carbon monoxide), metabolic disturbances (e.g., hepatocerebral encephalopathy), and the use of medications (e.g., neuroleptics) (Stacy & Jankovic, 1992). Manganism is considered by some investigators to be a form of parkinsonism because the disorder involves abnormal motor movements.

¹⁴In addition to the generation of fumes and gases, the welding process generates variable amounts of ultraviolet, visual, and infrared radiation (Burgess, 1991; Sferlazza & Beckett, 1991; Beckett, 1996). There is also frequently spatter of hot metal and slag particles during the welding process (Palmer & Eaton, 2001). These molten, spattered particles may also contribute to a small fraction of the composition of the welding fume. Other by-products of welding include heat, electrical current, noise, and vibration.