12.14: Magnetic Fields and Cancer
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Late in the 1970s, researchers investigating a cluster of cancers in children in Colorado found an association with living near high-voltage power lines. The cancers, a form of leukemia called acute lymphoblastic leukemia (ALL), had long been associated with exposure to ionizing radiation. But power lines do not generate ionizing radiation. What they do generate is a weak magnetic field that oscillates at the frequency of the alternating current (60 hertz in the U.S.; 50 hertz in Europe). Follow-up studies elsewhere continued to find a weak association between living near power lines and the incidence of ALL in children. Note that the association was with the proximity to power lines; not to the strength of the magnetic fields. The nature of the wiring (e.g., voltage, proximity) was used as a surrogate to the actual agent under suspicion (the magnetic field).
The National Cancer Institute (NCI) Study
On July 3, 1997, The New England Journal of Medicine published the largest and best study of the question (Martha S. Linet, et al, "Residential Exposure to Magnetic Fields and Acute Lymphoblastic Leukemia in Children").
Their conclusion: "Our results provide little support for the hypothesis that living in homes with high time-weighted average magnetic fields or in homes close to electrical transmission or distribution lines is related to the risk of childhood ALL."
How the NCI study differed from earlier studies
The NCI study differed from the earlier studies in 4 important ways:
- It involved a much larger sample size (624 children with ALL and 615 children chosen at random to compare their homes with those of the patients.
- The strength of the magnetic fields in the homes were actually measured (including continuous measurement for 24 hours under the child's bed). They also evaluated the nearby power lines as the earlier studies had done.
- The collection of data was "blinded"; that is, the people doing the measurements did not know whether they were in the house of an ALL patient or in the house of a control.
- The investigators had no axe to grind. None had any connection to the power industry or to grieving parents seeking to find an explanation for the tragedy that had struck their family.
The Magnetic Field Results
Patients and controls were grouped in 7 classes ranging from a magnetic field of less than 0.065 microteslas (µT) to greater than 0.5 µT. The tesla is a unit of magnetic field strength; the earth's magnetic field, which makes a compass needle turn, is about 50 microteslas (but does not fluctuate at 60 hertz as the much smaller fields near alternating current lines do). The Odds Ratio is a calculation of how likely it is that the results for the patient group differ from that of the control group. The total number of patients and controls in each class is shown within each bar. The blue lines show the 95% confidence limits; that is, that there is a 95% probability that the "true" mean (the height of the bar) is somewhere within the range shown in blue.
Interpreting the results
Only one class of exposure (0.400 - 0.499 µT) showed a statistically significant difference between patients and controls. Is it truly significant? Perhaps. But note that only 19 children of the 1,239 enrolled in the study lived in homes with this level of magnetic field.
How do this and the earlier studies meet the 5 standards of epidemiology?
- High Relative Risk
Not met. Every group but one had a relative risk whose 95% confidence limit included 1.00; that is, no relative risk at all.
- The earlier studies did not measure magnetic fields.
- Most of the earlier studies, which simply evaluated the nature of the nearby power lines, showed a 2–3 fold increase in relative risk whereas this study showed no increase.
- A graded response to a graded dose
Not met. There is no steady increase in ALL with increasing exposure to magnetic fields. The possible increased risk of ALL with exposure to 0.400 - 0.499 µT is followed by no increase at exposures above 0.5 µT.
- Temporal relationship
Met. In fact, built into the design of the study. All the patients were selected after they had developed ALL.
- A plausible mechanism Not met. In vitro studies have failed to reveal any mechanism to explain how such weak magnetic fields could produce oncogenic changes in cells. (Note that the Y axis of this graph of representative magnetic fields is logarithmic: the magnetic field directly under a high-voltage power line is only 1/10 that of the earth's own magnetic field.)
Two papers published in 1992 claimed that weak magnetic fields increase the flow of calcium ions into lymphocytes. Such a response may trigger mitosis and thus provide a plausible mechanism for a tumor-promoting effect. However, in June 1999 the author was censured by the Office of Research Integrity for falsifying his data, and the author retracted the papers.
The Bottom Line
In the words of Edward W. Campion, M.D. (New England Journal of Medicine, 337:44, July 3, 1997):
"there is no convincing evidence that high-voltage power lines are a health hazard or a cause of cancer...18 years of research have produced considerable paranoia, but little insight and no prevention. It is time to stop wasting our research resources. We should redirect them to research that will be able to discover the true biologic causes of the leukemic clones that threaten the lives of children."