Reframing welders' parkinsonism: from the "idiopathic" label to biological causation and legal recognition.

In Korea, welders diagnosed with "idiopathic Parkinson's disease (iPD)" often face denial of workers' compensation, as "idiopathic" is interpreted as excluding occupational contribution. This narrative review examines whether a categorical distinction between iPD and "toxic parkinsonism" is defensible on diagnostic, pathological, neuroimaging, epidemiological, causal-inference, and legal grounds. The 2024 NSD-ISS (Neuronal α-synuclein disease Integrated Staging System) and SynNeurGe (Synuclein-Neurodegeneration-Gene) frameworks challenge categorical use of the "idiopathic" label by defining Parkinson's disease (PD)-related disease biologically, irrespective of etiology. Nonhuman primate studies show that chronic low-dose manganese exposure can produce nigrostriatal deficits and α-synuclein aggregation overlapping with PD biology. Dichotomous use of FP-CIT dopamine transporter imaging or levodopa responsiveness to separate manganese poisoning from iPD is therefore scientifically unsound, as chronic welding exposure can induce presynaptic dysfunction and treatment responses indistinguishable from PD. Epidemiological findings diverge methodologically: direct clinical examination has reported elevated parkinsonism prevalence among welders, whereas administrative cohorts have generally found no increased PD risk. This divergence is plausibly explained by differences in case ascertainment, exposure assessment, healthy-worker-survivor selection, and disease latency. Bradford Hill and Rothman frameworks support a causal interpretation, although validated biomarkers and dose-response thresholds remain undefined. Under Korea's Industrial Accident Compensation Insurance Act, causation is evaluated as a "proximate causal relationship" rather than strict scientific proof. A multi-hit model integrating genetic predisposition, aging, and cumulative environmental exposure supports recognition under this standard, and compensation eligibility should not depend on whether the diagnosis label is 'PD' or 'parkinsonism.' When parkinsonian symptoms occur in welders with at least approximately 20,000 exposure-weighted welding hours, or with substantial high-intensity exposure such as confined-space welding, inadequate ventilation, or flux-cored arc welding, occupational contribution should be considered under a weight-of-evidence approach. This criterion is sufficient for recognition but is not a necessary condition.