The paper by Racette et al. [1] shows the effect of manganese, as measured by UPDRS score, in a population of welders. The interesting point is that the workers were not (as yet) diagnosed with parkinsonism. Multiple follow-ups show a slow progression over many years but not, for most at least, being severe enough to get medical attention.

Reference:

[1] "Dose-dependent progression of parkinsonism in manganese-exposed welders"
Brad A. Racette, MD, Susan Searles Nielsen, PhD, Susan R. Criswell, MD, MSCI, Lianne Sheppard, PhD, Noah Seixas, PhD, Mark N. Warden, MS and Harvey Checkoway, PhD
Neurology, Dec 2016.
Dose-dependent progression of parkinsonism in manganese-exposed welders

John