The researchers found that cumulative exposure to manganese in welding fumes among shipyard and fabrication workers was linked to worsening of movement symptoms similar to those seen in Parkinson's disease.
The researchers suggest that the findings show a need for tighter control of exposure to manganese in the workplace.
The study - led by the Washington University School of Medicine in St. Louis, MO - is published in the journal Neurology.
Lead author Brad A. Racette, a neurology professor at the School, says that the welders they studied "are developing parkinsonian symptoms even though their exposure to manganese is below the current regulatory limits."
Welding is a way of joining metal components using special equipment that heats them to a high temperature until they melt and fuse. The process generates fumes containing tiny metal particles - or particulate matter - that often include a small percentage of manganese.
Manganese is an essential nutrient and a healthy person can usually excrete any excess that comes into the body through dietary sources. However, inhaled manganese is of greater concern because it bypasses our natural defense mechanisms.
Workers in a range of industries could be at risk
Welders work in a range of industries, including the construction and maintenance of ships, aircraft, oil rigs, cars, buildings, and bridges. The job requires a high degree of skill and training in the use of advanced technology.
The welding process that generates the highest level of particulate matter is flux core arc welding in a confined space.
Previous research has already linked welding fumes to parkinsonism. In 2011, for example, Prof. Racette and others reported how they found that workers exposed to welding fumes may be at risk for brain damage in an area of the brain that is also affected in Parkinson's disease.
Labor statistics show that there are more than 380,000 welders, cutters, solderers, and brazers employed in the United States.
However, this figure does not include other jobs that may also involve some welding. Prof. Racette suggests that the total number of workers in the U.S. who perform welding as part of their job is approximately 1 million.
Cumulative exposure tied to progression of symptoms
The new study concerns 886 Midwestern workers at two shipyards and a heavy machinery fabrication shop. At the start of the study, all the participants were examined by neurologists who specialize in movement disorders. After that, 398 of the participants had further follow-up examinations for up to 10 years.
The researchers assessed exposure to manganese from questionnaires filled in by the participants. These included questions about the types of job they had and how long they served in them. The researchers found that the average exposure was a manganese concentration of 0.14 milligrams of manganese per cubic meter.
The results showed that 15 percent (135 individuals) of the participants had parkinsonism, with scores of at least 15 on a scale of 0 to 108 points in a movement test known as the Unified Parkinson Disease Rating Scale motor subsection part 3.
The researchers also found that cumulative manganese exposure was linked to a yearly increase in the movement test scores. For every extra milligram of manganese per cubic meter of exposure per year, there was an additional 0.24 points on the test score.
"For example, a worker who had been a welder for 20 years before the first examination had an estimated 2.8 milligrams manganese per cubic meter years exposure and would be predicted to have nearly a seven-point increase on the movement test related to that welding fume exposure," explains Prof. Racette.
Strongest link to flux core arc welding in confined space
The researchers found little change in the results when they took into account other factors known to affect risk of developing movement disorders, such as smoking, alcohol consumption, and exposure to pesticides.
They found that the strongest link between increased parkinsonism symptoms and cumulative manganese exposure was in workers who did flux core arc welding in a confined space.
They also found that the links were strongest in workers who had their first follow-up exam within 5 years after they started welding. Prof. Racette suggests that this could be because workers with higher exposure may develop parkinsonism and then move to other jobs.
The symptoms that progressed the most as the welders' exposure to manganese accumulated over time were stiffness in the arms and legs, slowness of arms and hands, speech problems, and reduced facial expression.
"This study suggests that we need more stringent workplace monitoring of manganese exposure, greater use of protective equipment and monitoring and systematic assessment of workers to prevent this disabling disease."
Prof. Brad A. Racette
Prof. Racette points out that the study has some limitations. For instance, it did not use a direct measure of cumulative manganese exposure but derived it from information provided by the workers. Also, the researchers could not rule out the effect of other metals in the fumes, or those that may have come from other sources such as paints and degreasers.