NCI Press Office
A comprehensive study by researchers from the National Cancer Institute (NCI) and the Children's Cancer Group (CCG) found no evidence that magnetic fields (EMFs)* in the home increase the risk for the most common form of childhood cancer.
In this case-control study, the researchers found that, in general, children who lived in homes with high measured magnetic fields were not significantly more likely to be diagnosed with acute lymphoblastic leukemia (ALL) than children living in homes with lower magnetic field levels. Nor was ALL found to be more likely among those whose homes were classified in high categories of "wire-code," a surrogate measure of magnetic fields that is based on the thickness, configuration, and distance from the home of nearby power lines.
"The results of our study differ from three earlier U.S. studies in that we found no evidence of a significantly increased risk of ALL among children whose main residence or residence during pregnancy was classified in the highest wire code category," said lead investigator Martha S. Linet, M.D., of NCI's Radiation Epidemiology Branch. The results are published in the July 3 issue of the New England Journal of Medicine.**
Whether power frequency magnetic field exposures (EMFs) may increase cancer risk has been a controversial question, and nearly two decades of research has produced conflicting results. EMFs exist naturally inside the human body and in the surrounding environment. But stronger fields are produced by power lines and electric appliances, which have been the focus of most research. Recent research has focused on magnetic fields, specifically the 60 cycle-per-second (60 hertz) fields produced by alternating current (AC) in household electrical power.
The first study to suggest a risk from magnetic fields was published in 1979, when researchers reported that children who had died from leukemia or other cancers were about two to three times more likely than other children to have lived within 40 meters of a high-current power line. Several other groups of investigators later described similar findings based on proximity to power lines. When researchers have actually measured magnetic fields in children's homes, however, they have not found significantly increased risks of leukemia or other cancers.
Previous studies on magnetic fields and childhood cancer have had one or more shortcomings that make interpretation of their results difficult. These include small numbers of leukemia cases, measurements limited to a single residence, long intervals between leukemia diagnosis and magnetic field measurement, and data collectors aware of which children had leukemia (cases) and which did not (controls).
The NCI/CCG researchers designed their study to overcome these limitations as much as possible. A large study population covering nine states was chosen, and measurement technicians were unaware of case or control status. For most subjects, measurements were made within two years after diagnosis, and were obtained in both current and former residences. The measurements covered homes in which the child had lived for at least 70 percent of his or her life, or 70 percent of the five years immediately before diagnosis for children age 5 and older.
The researchers compared magnetic field exposures of 638 children with leukemia and 620 children without leukemia who were similar in age and race. About 58 percent of the children were under age 5, the age group in which ALL is most common. The participants lived in Illinois, Indiana, Iowa, Michigan, Minnesota, New Jersey, Ohio, Pennsylvania, and Wisconsin.
The researchers estimated magnetic field exposures in two different ways: by measuring fields in current and former homes of the children (including homes their mothers lived in during the pregnancy) and by assigning wire codes to the homes. Slightly less than half of all subjects had summary residential magnetic levels less than 0.065 microtesla (µT)***, close to 20 percent had levels ranging from 0.065 to 0.099 µT, 23 percent had levels ranging from 0.100 to 0.199 µT, and the remaining 12 percent had levels of 0.200 µT or higher.
If magnetic fields increased risk for ALL, the researchers would expect that the higher the measured level of magnetic fields in homes, and the higher the wire code category, the more ALL cases they would find. But in general, they did not see either of these patterns. For children living in homes with magnetic fields measured at 0.2 µT or above, the researchers calculated a slightly elevated, but not statistically significant risk for ALL compared with risk for children living in homes with magnetic fields below 0.065 µT. While risk of ALL appeared to be slightly higher among children residing in homes with high levels, the absence of a consistent pattern of increasing risk with increasing exposure level suggests that the slight increase seen could be due to chance.
In addition, the researchers found no relationship between wire code classification and risk for ALL. Children with ALL were no more likely than controls to live in homes with high
The researchers also interviewed mothers of case and control children about the children's electrical appliance use and the mothers' appliance use during pregnancy. Results from this part of the study are still being analyzed and are expected to be published separately in 1998.
"This important study would not have been possible without the close collaboration and commitment of the physicians, nurses, and researchers of the Children's Cancer Group, and the cooperation of the families who participated," said Leslie L. Robison, Ph.D., a co-investigator in the study and professor of pediatrics at the University of Minnesota, Minneapolis. The Children's Cancer Group is a multicenter network of pediatric oncologists and other researchers from 38 institutions and affiliated hospitals who diagnose and treat approximately 50 percent of children with cancer in the United States.
The NCI/CCG study is part of a larger CCG investigation of ALL comprising more than 1,900 ALL cases and 1,900 controls. The larger study, overseen by Dr. Robison, is designed to evaluate the risk of ALL associated with a wide range of factors, including maternal diseases and medication use during pregnancy, childhood diseases, and other exposures such as parental occupation. Results are expected within the next two years.
Questions & Answers about the National Cancer Institute/Children's Cancer Group Study of Magnetic Fields and Childhood Leukemia
*Both electric and magnetic fields are present around power lines and electrical appliances. Recent interest and most research studies have focused on potential health effects of magnetic fields. The reason is that several prior epidemiologic studies have found associations between surrogate measures, particularly wire codes (based on power line thickness, configuration, and distance between power lines and homes), which are more closely related to magnetic than electric field levels.
**The study is titled "Residential Magnetic Field Exposures and Childhood Acute Lymphoblastic Leukemia." The authors are Martha S. Linet, Elizabeth E. Hatch, Ruth A. Kleinerman, Leslie L. Robison, William T. Kaune, Dana R. Friedman, Richard K. Severson, Carol M. Haines, Charleen T. Hartsock, Shelly Niwa, Sholom Wacholder, and Robert E. Tarone. NEJM, July 3, 1997.
***1 µT = 10 milligauss (mG). Some studies have reported measurements in mG.
For more detailed information on EMF and research on possible health effects, call the National Institute of Environmental Health Sciences' (NIEHS) Environmental Health Clearinghouse at
1-800-NIEHS-94 (1-800-643-4794) to get a copy of the booklet Questions and Answers About EMF: Electric and Magnetic Fields Associated with the Use of Electric Power. This publication, produced by NIEHS and the U.S. Department of Energy, is also available on the World Wide Web at http://www.niehs.nih.gov/oc/factsheets/emf/emf.htm.
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