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1. Why was the study done?
Acute lymphoblastic leukemia (ALL) is the most common form of cancer in children, accounting for 70 to 80 percent of all childhood leukemias and one-third of all childhood cancers in the United States. Only a small proportion of cases have an identifiable cause. Beginning in 1979, a number of studies have suggested that magnetic fields (EMFs)* may increase risk for ALL, while other studies have found no evidence of risk.
2. What have been some problems in studying magnetic fields and childhood leukemia? What have earlier studies shown about the relationship of magnetic fields to childhood leukemia?
The possible relationship of magnetic fields to childhood leukemia has been difficult to study, in part, because there are no known biological effects that could explain how these exposures might increase risk of leukemia in children.
Some investigators have reported that children living in homes close to high tension power lines have a 2- to 3-fold significantly increased risk of ALL, while other studies have found no evidence of an elevated risk. In each study where findings indicated statistically significant excess risk, researchers used surrogate measures of magnetic field exposures such as "wire codes" to characterize the thickness, configuration, and distance between the child's residence and nearby power lines. Researchers who have actually measured magnetic fields in homes have found little or no evidence of a significant increase in risk.
Many previous studies had one or more shortcomings such as small numbers of children with leukemia, magnetic field measurements restricted to a single home regardless of the number of homes each child had resided in, a long interval between cancer diagnosis and residential magnetic field measurements, data collectors aware of which children had leukemia and which did not, and differential residential mobility between cases and controls. These limitations have made it difficult to interpret results. The NCI/CCG study was done with the aim of overcoming some of the problems of earlier studies and providing more definitive answers.
3. Who conducted the NCI/CCG study? How was it done? How many children were involved?
The National Cancer Institute (NCI) and the Children's Cancer Group (CCG), a multicenter network of pediatric oncologists and other researchers from 38 institutions and affiliated hospitals in the United States, collaborated on the study, which was directed by Martha Linet, M.D., of NCI's Radiation Epidemiology Branch in the Division of Cancer Epidemiology and Genetics.
The NCI/CCG study was a case-control study: The researchers calculated risk of ALL among 638 children with leukemia (cases) and 620 healthy children (controls). Eligible participants for the residential magnetic field exposure assessment were subjects who resided in nine states: Illinois, Indiana, Iowa, Michigan, Minnesota, New Jersey, Ohio, Pennsylvania, and Wisconsin. Children who participated as controls were matched to the children with leukemia for age, race, and telephone area code and exchange. About
58 percent of the children were under age 5.
A detailed description of the study's methods will be published in the September 1997 issue of the journal Epidemiology.
4. How were magnetic fields assessed in the study?
The researchers used an electronic meter and sought to measure magnetic field levels in four rooms in each current and former home of the case and control children. The meter took readings in the child's bedroom every 30 seconds for 24 hours. In addition,
30-second "spot" measurements were made in the bedroom, family room, kitchen, and the room the mother slept in when pregnant with the child. Results of earlier studies by NCI researchers** showed that combining the measurements from these three to four rooms gives results similar to those obtained by having children wear a portable meter, particularly for children less than 9 years old, who made up 84 percent of those in the study. The researchers also obtained a measurement immediately outside the front door of each home to be used if the family did not allow measurements to be taken inside the home. The earlier studies also revealed that the front-door measurement correlated well with in-home measurements, and thus could be used if in-home measurements could not be taken because access within the home was not permitted.
Eligible subjects were included in the study if magnetic field measurements were obtained in all homes in which the subject had lived for at least 70 percent of his or her lifetime for children under age 5, or at least 70 percent of the five years immediately preceding diagnosis for subjects age 5 and older. The overall measurement determined for each home was assigned a numerical weight that corresponded to the duration of time the subject lived in each home. The individual home measurements were then combined to provide a summary "time-weighted average" magnetic field exposure over each child's lifetime (or the five years prior to diagnosis for children age 5 and older).
In analyzing the results, the researchers split the data into four groups based on time-weighted average exposure to magnetic fields, expressed as microtesla (µT).*** Then they compared the least-exposed group with more highly exposed groups to determine whether risk rises with increasing level of exposure.
The researchers also diagramed the thickness, configuration and distance from the home of nearby power lines, and a computer algorithm used this information to assign a wire code category to each home (see Question 2). Two types of wire code classifications were assigned. Similar to the results of previous U.S. studies, the measured magnetic field levels in the NCI/CCG study increased with increasing wire code categories for both wire code classifications. Although less accurate than in-home measurements as an estimate of a child's personal exposure, wire codes have been associated more strongly with childhood cancer than in-home measurements have been in earlier studies.
5. How does the NCI/CCG study differ from previous studies?
Most earlier studies, particularly those with in-home measurements, included relatively small numbers of leukemia cases. Small numbers reduce the reliability of results. The NCI/CCG study included four times as many children as the next-largest comparable study. In addition, prior studies often had long intervals, sometimes as long as two or three decades, between leukemia diagnosis and magnetic field measurement, and lacked measurements for homes the children lived in for substantial periods of their lives. In the NCI/CCG study, most measurements were taken within two years of diagnosis, and measurements covered residences the children lived in for at least 70 percent of their lives (or 70 percent of the five years prior to diagnosis for children older than 5). In addition, most earlier studies have been done in a single city or other small geographic area. The NCI/CCG study was conducted in homes in both urban and rural areas across nine states, making it less likely that factors specific to one geographic area will unduly affect the results.
6. What were the results of the study?
For children living in homes with magnetic fields measured at 0.2 µT or above, the researchers calculated a non-significant relative risk for ALL (estimated as an odds ratio) of 1.24 compared with children living in homes with magnetic fields below 0.065 µT. In other words, children who lived in homes with magnetic fields levels of 0.2 µT or greater were estimated to have a slightly (24 percent) but non-significantly higher probability of developing ALL than children living in homes with levels below 0.065 µT. The tendency for risk to be slightly higher among children residing in homes with high levels was based on small numbers and was not characterized by a consistent pattern of increasing risk with increasing exposure level.
Similarly, children who lived in high wire-code homes had no higher risk of ALL compared with those who lived in low wire-code homes (relative risk of highest to lowest wire code category estimated as odds ratio of 0.88). This means that those living in homes classified as very high wire code category had a slightly (12 percent) but non-significantly lower risk than those living in homes with nearby power lines that were underground or very low wire code category.
7. What conclusions can be drawn from the results?
The researchers conclude that their results do not support the theory that residential magnetic fields cause childhood leukemia, particularly at the levels found in most homes. If high magnetic fields increase risk for ALL, researchers would expect that the higher the measured levels and wire codes in homes, the greater the risk of developing ALL. But in general, they did not see this trend. While the risk of ALL appeared to be slightly higher among children residing in homes with high measured magnetic field levels, the absence of a statistically significant and consistent pattern of increasing risk with increasing exposure level suggests that the excess could be due to chance. The possibility of an increased risk at high levels (greater than 0.3 µT) cannot be entirely ruled out, however. But if this risk is real, it could explain only a small proportion of ALL cases.
Background information on electric and magnetic fields (EMFs) and on childhood leukemia:
8. What are EMFs?
Power lines, electrical wiring, and appliances all produce electric and magnetic fields. EMFs are invisible lines of force that surround any electrical device. Electric and magnetic fields have different properties and possibly different ways of causing biological effects. Electric fields are easily shielded or weakened by conducting objects (for example, trees, buildings, and human skin), but magnetic fields are not. The strength of both electric and magnetic fields drops off very sharply within a few feet or inches of a source, such as an electrical appliance.
The earth itself produces EMFs, mainly in the form of direct current (DC) static fields. Electric fields are produced by thunderstorm activity in the atmosphere. Magnetic fields are thought to be produced by electric currents flowing deep within the earth's molten core.
9. What is power-frequency EMF and how does it compare with other types of fields?
The electromagnetic spectrum covers an enormous range of frequencies. These frequencies are expressed in cycles per second (hertz). Electric power (60 hertz in North America, 50 hertz in most other places) is in the extremely-low-frequency range, which includes frequencies below 3,000 hertz.
The higher the frequency, the shorter the distance between one wave and the next, and the greater the amount of energy in the field. Microwave frequency fields, with wavelengths of several inches, have enough energy to cause heating in conducting material. Still higher frequencies like X-rays cause ionization or the breaking of molecular bonds, which damages genetic material. In comparison, power frequency fields have wavelengths of more than 3,100 miles (5,000 km) and consequently have very low energy levels that do not cause heating or ionization. However, AC fields do create weak electric currents in conducting objects, including people and animals.
10. What happens when a person is exposed to EMFs? Why do scientists disagree about whether EMFs could cause cancer?
EMFs can create weak electric currents in the bodies of people and animals. This is one reason why there is a potential for EMFs to cause biological effects. But the amount of this current, even directly beneath a large transmission line, is extremely small—millionths of an ampere. (An ampere is a unit of electrical current.) The current is not only too weak to damage DNA, but is too weak even to penetrate cell membranes and cause damage inside cells. It is present mostly between the cells.
Currents from 60-hertz EMFs are weaker than natural currents in the body, such as those from the electrical activity of the brain and heart. Some scientists argue that it is therefore impossible for EMFs to have any important effects. Other scientists argue that, just as a trained ear can pick up a familiar voice or cry in a crowd, so a cell may respond to an induced current of low intensity even through the background "noise" of the body's natural currents. Numerous laboratory studies have shown that biological effects can be caused by exposure to EMFs. In most cases, however, it is not clear how EMFs produce these effects.
Because 60-hertz EMFs are too weak to damage the DNA of cells, scientists believe that if EMFs are associated with cancer at all, they must work as cancer promoters. Promoters are agents that can push a cell with DNA or genetic damage closer to the uncontrolled cell growth and division that characterizes cancer.
11. How common is acute lymphoblastic leukemia? Is it treatable?
Of every 1 million children under age 15 in the United States, about 30 are diagnosed with ALL and about five die from ALL each year. About 1,600 children are expected to be diagnosed with ALL this year. The disease is most commonly diagnosed in white children under 5 years old. It is twice as common in white children as in black children, and is slightly more common in boys than in girls.
ALL is much more treatable now than in the past. Most children with ALL can now be cured, and about 80 percent of patients under age 15 survive at least five years after diagnosis, compared with about 1 percent in the 1950s.
12. What are the known risk factors for ALL? What others have been proposed?
Only a few risk factors are known, although many have been proposed and studied. Children with Down syndrome have a greatly increased risk of ALL, reported to be 10 to 40 times the risk of other children. Other, rarer chromosomal and genetic abnormalities may also increase risk for ALL. Children whose mothers had diagnostic X-rays during pregnancy are about one-and-a-half times more likely to have ALL compared with children whose mothers had no X-rays. Risk factors that have been proposed, but not proven, include certain birth characteristics such as high birth weight; medical conditions or drugs affecting delivery; mothers' prior reproductive problems such as repeated miscarriages; pesticides and other chemicals; certain viruses; and natural background ionizing radiation.
13. Are other studies of magnetic fields and childhood leukemia under way? Are studies under way of other risk factors for childhood leukemia?
Population-based studies of residential magnetic fields and childhood leukemia are under way in Canada and the United Kingdom. The results are expected within one to two years.
The NCI/CCG investigators also collected information on use of electrical appliances by mothers during pregnancy and children after birth. The data are still being analyzed, with results expected to be published in 1998. The NCI/CCG magnetic field study was part of a larger CCG study of more than 1,900 children diagnosed with ALL between 1989 and 1993, and 1,900 controls. This ongoing study is designed to evaluate the risk of ALL associated with a wide range of factors, including maternal diseases and medication use during pregnancy, childhood infectious and other diseases, parental occupational exposures, prenatal and postnatal environmental exposures, parental smoking and alcohol use, lifestyle, and genetic factors. Results are expected in about two years.
*Both electric and magnetic fields are present around appliances and power lines. But recent interest and research have focused on potential health effects of magnetic fields. This is because studies have found associations between increased cancer risk and power-line configurations which are more closely related to magnetic than to electric fields.
**Friedman, D.R. et al. "Childhood Exposures to Magnetic Fields: Residential Area Measurements Compared to Personal Dosimetry." Epidemiology, March 1996
Kaune, W.T. et al. "Development of a protocol for assessing time-weighted average exposures of young children to power-frequency magnetic fields." Bioelectromagnetics, January 1994.
***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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