To the Editor:
The cohort mortality study by Håkansson and colleagues1 suggests that extremely low-frequency magnetic fields are a risk factor for Alzheimer disease and amyotrophic lateral sclerosis (ALS). The cut points were arbitrarily defined as the 25th, 75th, and 90th percentile of exposure. However, even the highest exposure group to magnetic fields (>0.530 μT) can hardly be considered as “very high exposure.” Furthermore, no excess risk for Alzheimer's disease as primary cause of death was observed (relative risk = 0.94 at “very high exposure”), and no test for trend in risk is given.
The workers included in the study are engineering industry workers, mainly welders, who are exposed to metals and solvents. A risk of developing ALS has been reported for welders in numerous studies.2–6 Welding exposes workers to metal fumes containing well-established7 neurotoxicants such as lead, aluminum, and manganese. There have long been data linking neurotoxic metals to ALS and Alzheimer disease. Moreover, these observations have been recently supported in vitro8,9 (ie, metal-triggered aggregation of Cu/ZnSOD and amyloid) and in vivo10,11 (ie, aluminum chloride-induced motor neuron disease in experimental animals such as the New Zealand white rabbit). Therefore, occupational exposures should be considered with care, and exposure to established neurotoxicants (such as lead and manganese) should not be underestimated or neglected. No data about such exposures is presented in the article.
The job matrix used by the authors assesses only electromagnetic fields. In our opinion, the authors ignore the role of definite neurotoxicants in these neurodegenerative diseases and unreasonably attribute the association to electromagnetic field exposure. A recent review of electromagnetic fields and ALS12 has stated that “Most studies did not consider work-related factors potentially associated with ALS in the analysis.” This observation would seem to apply to the present study as well.
1.Håkansson N, Gustavsson P, Johansen C, Floderus B. Neurodegenerative diseases in welders and other workers exposed to high levels of magnetic fields. Epidemiology
2.Armon C, Kurland LT, Daube JR, O’Brien PC. Epidemiologic correlates of sporadic amyotrophic lateral sclerosis. Neurology
3.Gunnarsson LG, Bodin L, Söderfeldt B, Axelson O. A case-control study of motor neurone disease: its relation to heritability, and occupational exposures, particularly to solvents. J Ind Med.
4.Herrero Hernandez E, Discalzi G, Chiò A, Palmos A, Bosio D. Occupational and environmental factors in amyotrophic lateral sclerosis: preliminary data of a case-control study [Abstract]. Int J Neuropsychopharmacol
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5.Strickland D, Smith SA, Dolliff G, Goldman L, Roelofs RI. Amyotrophic lateral sclerosis and occupational history. A pilot case-control study. Arch Neurol
6.Terreni AA, Herrero Hernandez E, Discalzi G, et al. Lifelong occupational exposition and amyotrophic lateral sclerosis (ALS): results of a case-control study [Abstract]. J Neurol
. 2002;248(suppl 2):59.
7.Noonan CW, Sykes L, Hilsdon R. Motor neuron disease/amyotrophic lateral sclerosis: preliminary review of environmental risk factors and mortality in Bexar County, Texas.
March 5, 2002. Agency for Toxic Substances and Disease Registry. Available at: http://www.atsdr.cdc.gov/NEWS/alsreport.html
. Accessed 4 February 2003
8.Stathopulos PB, Rumfeldt JAO, Scholz GA, et al. Cu/Zn superoxide dismutase mutants associated with amyotrophic lateral sclerosis show enhanced formation of aggregates in vitro. Proc Natl Acad Sci USA
9.Rakhit R, Cunningham P, Furtos-Matei A, et al. Oxidation-induced misfolding and aggregation of superoxide dismutase and its implications for amyotrophic lateral sclerosis. J Biol Chem
10.Strong MJ, Gayran-Garcia S, Jakowec D. Reversibility of neurofilamentous inclusion formation following repeated sublethal intracisternal inoculums of AlCl3 in New Zealand white rabbits. Acta Neuropathol
11.Wakayama I, Nerurkar VR, Strong MJ, Garruto RM. Comparative study of chronic aluminium-induced neurofilamentous aggregates with intracytoplasmic inclusions of amyotrophic lateral sclerosis. Acta Neuropathol
12.Li CY, Sung FC. Association between occupational exposure to power frequency electromagnetic fields and amyotrophic lateral sclerosis: a review. Am J Ind Med