Occupational Electric Shocks, Electromagnetic Fields and Amyotrophic Lateral Sclerosis
This dissertation examines the association of neurodegenerative diseases and electric occupations and evaluates electric shocks and magnetic fields exposures. First, we conducted a meta-analysis of occupational electromagnetic fields (MF) and neurodegenerative diseases (NDD) to systematically explore methodological differences between studies. Second, we developed an electric shocks job exposure matrix (JEM) to characterize occupations exposed to electric shocks. Finally, we examined the association between occupational electric shocks, MF and amyotrophic lateral sclerosis (ALS) mortality.
We conducted a meta-analysis of epidemiologic studies on occupational MF exposure and NDD. We found weak associations of occupational MF exposures with both AD and MND, but not with Parkinson's disease, dementia, and multiple sclerosis. Risk of developing MND was associated with electric occupations, while AD risk was associated with estimated MF levels. Nonetheless, there is extensive result variation related to aspects of study design, with dissimilarity in this variation across diseases. Our results do not support MF as the explanation for the observed electric occupation and MND association. Misclassification of disease, particularly for AD, and imprecise exposure assessment affected most studies.
To evaluate a consistent association between jobs in "electric" occupations and ALS, we developed a comprehensive JEM that includes electric shocks and MF. Electric shocks were based on two data sources along with expert judgment. Main occupational groups experiencing the electric shocks were precision production, craft and repair occupations. Specific jobs with high electric shock exposure were electrical apprentices, mechanic and repairer helpers, hoist and winch operators and electrical power installers. Examples of job titles with low electric-shock exposures were administrative support occupations, data-key entry operators, and waiters and waitresses.
The relationship between occupational electric shocks, MF and ALS was investigated using cases identified in 1991-1999 U.S. mortality data. For each ALS death, 10 sex-, age-, year- and region-matched controls were selected. We linked the usual occupation reported on the death certificate to a JEM with electric shocks and MF. Increased odds ratios were observed for ALS among those in electric occupations (OR=1.23, 95% confidence interval (CI): 1.04, 1.47). For electric shocks, ALS mortality odds ratios were 0.73 (95% CI: 0.67, 0.79) for high exposure and 0.90 (95% CI: 0.84, 0.97) for medium exposure compared to low exposure. For MF, ALS mortality odds ratios were 1.09 (95% CI: 1.00, 1.19) for high exposure and 1.09 (95% CI: 0.96, 1.23) for medium exposure as compared to low. However, ALS mortality increased only in electric occupations with medium and low electric shocks exposures. Current results support an association between electric occupations and ALS, but provide no evidence that this association is explained by occupational exposure to electric shocks or MF.