Welding and Exposures to Manganese Assessment of Neurological Adverse Effects

Manganese (Mn) is a required trace element for growth and reproduction with Mn deficient diets resulting in growth retardation, skeletal and joint cartilage abnormalities, fetotoxicity, testicular degeneration, diabetes and vestibular dysfunction. Mn requirements range from 30-100 ppm in the diet for various species. Man ingests 2.3-7 mg Mn/day. Calculated absorbed doses of Mn for man range from 80-450 μg/day from dietary sources. Excessive body burden of Mn can occur from excessive exposures from intravenous supplementation or from occupational exposures or from decreased Mn excretion seen with chronic liver disease. Exposures during welding have ranged from 0.024-0.22 mg/m in the last decade with higher exposures when there was GMAW welding, no ventilation or welding on manganese alloy steel. Exposures associated with manganism have been above 3 mg Mn/m and have occurred primarily in the mining and manganese alloy fabrication industries. Manganism is an illness that includes extrapyramidal symptoms and findings (parkinsonism), psychiatric symptoms, intentional tremor, dystonia, and incoordination. In non-human primates, no clinical or pathological changes occur when exposures are limited to 3 mg Mn/m or less. Clinical findings of manganism respond readily to treatment with chelating agents (EDTA or PAS), even when treatment is first started years after exposure ceases. Manganism differs both clinically and pathophysiologically from Parkinson’s Disease (PD). The latter involves damage to dopaminergic neurons in the substantia nigra while with manganism there is damage downstream from this dopaminergic pathway. In PD, where there are decreased levels of dopamine, treatment with L-dopa can result in clinical improvement while in manganism, where dopamine levels are generally normal, L-dopa is usually ineffective. Studies by Racette and his colleagues have been used to hypothesize that welders may develop classical PD at an earlier age than the general population. The age distribution in these populations was, however, similar to other PD populations seen in physician offices. Further, studies of PD patients have not identified an increased prevalence of either welders or exposure to Mn in these populations. Exposures to Mn at levels below those associated with a risk of manganism can result in changes found on sensitive neurophysiological studies including tests of eye-hand coordination, hand steadiness (tremor), and simple reaction time. When exposure to Mn is discontinued, neurophysiological abnormalities improve or disappear. These neurophysiological studies are used for worker monitoring as a supplement to industrial hygiene monitoring studies. Further these studies have been used to define acceptable exposure levels to Mn. Various agencies have set acceptable continuous (environmental) exposures to Mn at levels ranging from 0.05-31.5 μg/m. In adults, such exposures, assuming 100% absorption of inhaled Mn, would result in daily absorbed doses of Mn ranging from 0.05-470 μg/d, similar (at least at the upper range) to those seen from dietary sources. The current workplace threshold limit value for Mn is 200 μg/m. Welding and Exposures to Manganese. Assessment of Neurological Adverse Effects Table of