But the findings are limited by variable evidence and conflicting results, according to the report by Ximena Vergara, PhD, of the Electric Power Research Institute, Palo Alto, Calif., and colleagues.

The researchers analyzed past studies on the association between MF exposure and the risk of neurodegenerative diseases such as Alzheimer's and motor neuron disease (MND), including amyotrophic lateral sclerosis. Magnetic field exposure is common among workers in electrical occupations.

The results suggested significant but weak associations between measures of MF exposure and the risk of Alzheimer's disease and MND. Other neurodegenerative diseases -- including Parkinson's disease and multiple sclerosis -- were unrelated to MF exposure.

There were some notable inconsistencies regarding the link between MF exposure and neurodegenerative diseases. Associations for MND were stronger in studies based solely on job titles, and for Alzheimer's disease in studies using estimated MF exposure.

Few studies included direct measurement of MF exposure or data on other potentially relevant job exposures, such as electrical shocks. There was also evidence of possible "publication bias," with studies reporting finding no link between MF and Alzheimer's disease being less likely to be published.

Because of the weaknesses in the evidence, no "reliable inferences" can be drawn about the effects of occupational MF exposure, according to Dr Vergara and coauthors. They write, "In light of these problems we believe that conclusions about the relations of occupational MF exposure to neurologic disease will require improvement in exposure assessment, disease classification, [and] more complete reporting of results."