Do Extremely Low-Frequency Magnetic Fields From Power Lines Increase Cancer Risk?
What the science says: Almost 30 years ago, a study published in the American Journal of Epidemiology reported a higher risk for leukemia in children who lived near power lines. The authors suggested a possible explanation for this could be magnetic fields, sparking concern that emissions from power lines could cause cancer.
Power lines expose people to extremely low-frequency magnetic fields (ELF MFs) generally within the 50- to 60-Hz range. These fields are even less energetic than the nonionizing radiation produced by cell phones. Given the low levels of radiation emitted, ELF MFs have been generally perceived to be harmless.
But after decades of investigation, epidemiologic studies have consistently found associations between ELF MFs at relatively high levels and childhood leukemia. A meta-analysis published in 2000 examined the results of nine studies to determine whether an association between magnetic fields and childhood leukemia did indeed exist. The researchers found that 99.2% of children living with exposure levels below 0.4 microtesla (μT) showed no increased risk, but the 0.8% of children with exposures of 0.4 μT or more had an elevated risk. A second meta-analysis, which pooled data from 15 studies, found that exposure to ELF MFs of at least 0.3 μT was associated with an almost twofold increased risk for childhood leukemia. To put this risk into context, about 1 in 20,000 US children per year is at risk for leukemia, and proximity to a power line may double those odds to 2 in 20,000 per year, according to statistics from the Centers for Disease Control and Prevention.
In 2002, the IARC classified ELF MFs as possibly carcinogenic to humans on the basis of studies of childhood leukemia (group 2B), because the large body of epidemiologic evidence consistently shows an association between ELF MFs and childhood leukemia; however, the mechanism for how ELF MFs could cause childhood leukemia is still not understood. In addition, no animal model for childhood leukemia exists, making the association difficult to study in animals. Such models have been developed only recently, but they have not yet been tested.
After 2002, subsequent epidemiologic analyses have continued to find an association between ELF MFs and childhood leukemia. In a 2010 analysis, Dr Kheifets and colleagues pooled seven studies performed after 2000, which included 10,865 cases and 12,853 controls. In most individual studies and in the combined results, the risk for childhood leukemia increased with greater exposure, but the association was weaker in the most recently conducted studies. Dr Kheifets and her colleagues concluded, "Our results are in line with previous pooled analyses showing an association between magnetic fields and childhood leukemia."
Still, causality remains uncertain. A 2013 literature review aimed to estimate the cases of childhood leukemia that might be attributable to ELF MF exposure in the European Union, if the association were indeed causal. The researchers mapped out several exposure/response scenarios and estimated that about 50-60 cases of childhood leukemia could be attributed to ELF MF exposure, depending on the model used. This estimate corresponded to about 1.5%-2.0% of all cases of childhood leukemia. The authors concluded that "according to the current state of evidence, residential exposure to ELF MF may contribute to cases of leukemia in children, but this contribution is relatively small and is characterized by considerable uncertainty."
According to the National Cancer Institute, "Overall, these [epidemiologic] analyses suggest that if there is any increase in leukemia risk from magnetic fields, it is restricted to children with the very highest exposure levels."
Regarding a potential mechanism, a 2011 study found that exposure to 50-Hz magnetic fields at 100 µT increased genotoxicity in a human tumor cell line, but also found the rate of DNA repair increased in exposed cells. A subsequent 2014 study from the same group identified cellular changes induced in a human tumor cell line exposed to a 50-Hz magnetic field for 24 hours. Overall, the exposure disturbed the balance in the cell and increased production of chemically reactive molecules. Although the exposure levels in these studies are orders of magnitude above those used in epidemiologic studies, they do start to hint at how magnetic fields may alter cellular activity and potentially induce genomic instability.
What the experts say: "There is consistent epidemiologic evidence for such an association, but support for causality is missing from experimental research," Martin Röösli, PhD, head of the Environmental Exposures and Health Unit, Swiss Tropical and Public Health Institute, Associated Institute of the University of Basel, Switzerland, told Medscape in an email.
According to Dr Kheifets, "Research in this area indicates that there is no increased risk for major cancers, such as breast cancer, from ELF MFs." But, she said, the evidence that remains unresolved is the link to childhood leukemia. "The epidemiologic studies have consistently found an association between childhood leukemia and magnetic fields, but a plausible explanation for this association is lacking—which means it could be due to chance, bias, or other (yet to be identified) factors related to both magnetic fields and childhood leukemia, or it could be a true association. Even if we were to confirm a relationship between ELF MFs and childhood leukemia, the risk appears to be very low."
Dr Henshaw, however, believes that the link to childhood leukemia is a robust one. "When we come to cellular studies, there is extensive evidence of DNA damage in cells exposed to magnetic fields. For instance, recent work shows that low-level MFs can increase the rate at which free radicals are produced in the cell."
Dr Kheifets noted that it's important to resolve this issue, because any factor that may raise the risk for childhood leukemia is important to understand. "What we need now are some novel approaches to really understand the association."
Verdict: Uncertain, according to Dr Kheifets. Very likely, according to Dr Henshaw. The evidence is highly consistent for an increased risk for childhood leukemia at higher-intensity ELF MFs, but a precise mechanism and animal models for leukemia are still needed.
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