Welders exposed to fumes containing low levels of the metal manganese had a high rate of Parkinson-like symptoms, which progressed with increasing cumulative exposure to the metal, according to research published in the December 28 online edition of Neurology. The results make a case for better worker protection. They also help scientists understand the underlying mechanisms by which exposure to substances in the environment may contribute to Parkinson’s disease (PD).
The law limits occupational exposure to high levels of manganese because it is known to lead to a severe movement disorder with Parkinson-like symptoms. Yet, in an earlier study, researchers led by Brad A. Racette, M.D., at the Washington University School of Medicine in St. Louis, MO, found that many welders with chronic exposure to low manganese levels had stiffness, slowness and other symptoms similar to those of PD. This is called manganism. While manganism is similar to PD in that it produces PD-like symptoms, such as slowness of movement and is characterized by a disruption of normal dopamine signaling, it is a different disease. In fact, it affects a different part of the brain than PD and not the substantia nigra.
For the new research, the scientists recruited 886 study participants who worked as welders at two Midwestern shipyards and a heavy machinery fabrication shop. Movement disorders specialists examined all participants at the start of the study and followed 398 of them for up to 10 years, using standard tests to assess Parkinson’s-like symptoms. The workers filled out questionnaires about their job types and length of employment as a basis for estimating their manganese exposure.
- The average exposure to manganese was estimated to be 0.14 milligrams per cubic meter, below the legal limit of 0.2 milligrams per cubic meter.
- Of the total number of study participants, 135 (15 percent) had Parkinsonism, meaning Parkinson-like movement problems.
- Parkinson’s symptoms were progressively more severe among participants with higher cumulative manganese exposure.
- The symptoms that progressed were slowness of movement in the arms and hands, and stiffness in arms and legs, as well as speech problems and less facial expression. Symptoms tended to appear on both sides of the body (as opposed to PD which is often asymmetric).
- Welders who did flux core arc welding in a confined space, and those who were examined within the first five years of their welding experience, had the fastest progression of PD-like symptoms.
What Does It Mean?
This study is the first to report that long-term, near-threshold exposure to welding fumes containing low manganese levels can lead to progressive parkinsonism, a disease that mirrors symptoms of Parkinson’s but is a separate disease.
A strength of the study is that it followed a large number of participants over several years. Combined with research in which study participants also had brain scans, these results shed light on the mechanisms underlying manganese exposure. It remains unknown if manganese contributes to the development of PD, which is also associated with Lewy body pathology in the brain.
The study authors also advocate that steps should be taken to broadly monitor worker exposure to manganese and to reduce workplace exposure levels to below the current limits.