Occupational Exposure to Electric Shocks and Magnetic Fields and Amyotrophic Lateral Sclerosis in Sweden
Fischer H, Kheifets L, Huss A, Peters TL, Vermeulen R, Ye W, Fang F, Wiebert P, Vergara XP, Feychting M. Occupational Exposure to Electric Shocks and Magnetic Fields and Amyotrophic Lateral Sclerosis in Sweden. Epidemiology. 2015 Nov;26(6):824-830.
Abstract
BACKGROUND: Amyotrophic lateral sclerosis (ALS) has been consistently related to "electric occupations," but associations with magnetic field levels were generally weaker than those with electrical occupations. Exposure to electric shock has been suggested as a possible explanation. Furthermore, studies were generally based on mortality or prevalence of ALS, and studies often had limited statistical power.
METHODS: Using two electric shock and three magnetic field job-exposure matrices, we evaluated the relationship of occupational magnetic fields, electric shocks, electric occupations, and incident ALS in a large population-based nested case-control study in Sweden. Subanalyses, specified a priori, were performed for subjects by gender and by age (less than and more than 65 years).
RESULTS: Overall, we did not observe any associations between occupational magnetic field or electric shock exposure and ALS. For individuals less than 65 years old, high electric shock exposure was associated with an odds ratio (OR) of 1.22 (95% confidence interval [CI] = 1.03, 1.43). The corresponding result for the age group 65 years or older was OR = 0.92 (95% CI = 0.81, 1.05). Results were similar regardless which job exposure matrices, exposure definitions, or cutpoints were used. For electric occupations, ORs were close to unity, regardless of age. For welders, no association was observed overall, although for welders <65 1.52="" 2.21="" b="" ci="1.05," or="" the="" was="" years="">
CONCLUSIONS: In this very large population-based study based on incident ALS case subjects, we did not confirm previous observations of higher risk of ALS in electrical occupations, and provided only weak support for associations between electric shocks and ALS.
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Excerpts
Within the study base, we conducted a nested case–control study. We defined ALS cases as those with a discharge record of either an inpatient or outpatient clinic visit in the Patient Register with ALS as the underlying or a secondary diagnosis (ICD-9: 335C and ICD-10: G12.2). Date of first ALS diagnosis was defined as the case index date. Using incidence density sampling, we randomly selected five control subjects per ALS case subject from the study base, individually matched by birth year and sex. Control subjects had to be alive, living in Sweden, and have no diagnosis of ALS as of the case index date.
We identified a total of 5,020 ALS cases and initially selected 25,100 controls for the analysis. Only case and control subjects employed during at least one census were included in the analysis, leaving 4,709 case subjects and 23,335 control subjects.
Our results indicate that the working population less than 65 years old, with high or medium occupational exposure to electric shocks, may have a slightly higher risk of ALS. Our results were similar regardless which job-exposure matrices, exposure definitions, or cutpoints were used.
While we did not see any relationship between ALS and electric occupations, we found an increased risk of ALS among welders, but only restricted to those less than 65 years. However, we still observed a raised OR associated with electric shock exposure in this age group after excluding welders; thus, they do not explain the observed relationship between ALS and electric shock exposure.
There are several possible explanations for a higher risk in the younger age group. Exposure misclassification might be reduced when restricting to age groups that have not yet retired. Older persons (65 years or more) may be considered retired in the most recent censuses, and may be classified as unexposed even if they had been exposed before the first available census. In addition, their cumulative exposure will be lower for the same reason. There is also a possibility that timing of exposure is critical, if electric shocks are of importance mainly for early onset ALS. We found some evidence that exposure in the 1980s conferred higher risk, although the OR was still slightly raised for those exposed in 1990 but not 1980. An alternative explanation is random variation, especially considering that some of the ORs in the older age group were slightly reduced.
Earlier studies, which mainly focused on electromagnetic fields, have consistently found an increased risk of ALS associated with “electric occupations,” but not with magnetic fields.7 Many of these studies were, however, limited by the use of occupational and outcome information from death certificates and limited control of confounding, or were based on small numbers of case subjects occurring among workers in the utility industry. Two clinically-based studies included prevalent ALS case subjects, and control subjects were not population based,12,27 while the only previous population-based study with incident case subjects was very small (four cases in electricity work).28 The latter study found no association with electric shock assessed through questionnaires.
Within the study base, we conducted a nested case–control study. We defined ALS cases as those with a discharge record of either an inpatient or outpatient clinic visit in the Patient Register with ALS as the underlying or a secondary diagnosis (ICD-9: 335C and ICD-10: G12.2). Date of first ALS diagnosis was defined as the case index date. Using incidence density sampling, we randomly selected five control subjects per ALS case subject from the study base, individually matched by birth year and sex. Control subjects had to be alive, living in Sweden, and have no diagnosis of ALS as of the case index date.
We identified a total of 5,020 ALS cases and initially selected 25,100 controls for the analysis. Only case and control subjects employed during at least one census were included in the analysis, leaving 4,709 case subjects and 23,335 control subjects.
Our results indicate that the working population less than 65 years old, with high or medium occupational exposure to electric shocks, may have a slightly higher risk of ALS. Our results were similar regardless which job-exposure matrices, exposure definitions, or cutpoints were used.
While we did not see any relationship between ALS and electric occupations, we found an increased risk of ALS among welders, but only restricted to those less than 65 years. However, we still observed a raised OR associated with electric shock exposure in this age group after excluding welders; thus, they do not explain the observed relationship between ALS and electric shock exposure.
There are several possible explanations for a higher risk in the younger age group. Exposure misclassification might be reduced when restricting to age groups that have not yet retired. Older persons (65 years or more) may be considered retired in the most recent censuses, and may be classified as unexposed even if they had been exposed before the first available census. In addition, their cumulative exposure will be lower for the same reason. There is also a possibility that timing of exposure is critical, if electric shocks are of importance mainly for early onset ALS. We found some evidence that exposure in the 1980s conferred higher risk, although the OR was still slightly raised for those exposed in 1990 but not 1980. An alternative explanation is random variation, especially considering that some of the ORs in the older age group were slightly reduced.
Earlier studies, which mainly focused on electromagnetic fields, have consistently found an increased risk of ALS associated with “electric occupations,” but not with magnetic fields.7 Many of these studies were, however, limited by the use of occupational and outcome information from death certificates and limited control of confounding, or were based on small numbers of case subjects occurring among workers in the utility industry. Two clinically-based studies included prevalent ALS case subjects, and control subjects were not population based,12,27 while the only previous population-based study with incident case subjects was very small (four cases in electricity work).28 The latter study found no association with electric shock assessed through questionnaires.
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Joel M. Moskowitz, Ph.D., Director
Center for Family and Community Health
School of Public Health
University of California, Berkeley
Electromagnetic Radiation Safety
Website: http://www.saferemr.com
Facebook: http://www.facebook.com/SaferE
News Release: http://pressroom.prlog.org/
Twitter: @berkeleyprc
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