MONDAY, Aug. 1 (HealthDay News) -- A new study suggests that the children of mothers exposed to high levels of magnetic fields during pregnancy are at increased risk of developing asthma, findings that are sure to reignite the controversy over the health dangers that might be posed by exposure to power lines and electronics.

Though the study does not establish cause-and-effect, researchers found a strong association between asthma in offspring and pregnant women's exposure to magnetic fields emanating from power lines and household items such as fluorescent lights, copy machines, electric blankets, microwaves and hair dryers.

"If EMFs [electromagnetic fields] truly increase risk as we have shown here, because of the ubiquitous exposure to EMFs the public health risk is serious," said study author Dr. De-Kun Li, a reproductive and perinatal epidemiologist at the Kaiser Permanente Division of Research in Oakland, Calif.

The study is published online Aug. 1 in the Archives of Pediatrics and Adolescent Medicine.

Li and his colleagues followed the children of 626 pregnant women in Northern California for up to 13 years. During pregnancy, the women wore a meter for 24 hours that measured their average daily exposure to magnetic fields. Exposure was then divided into three groups: low, medium or high.

Nearly 21 percent of the children, or 130, developed asthma.

Children whose mothers had the highest magnetic field exposure (90th percentile or above) were 3.5 times more likely to have asthma than the kids of moms with the lowest exposure (10th percentile or less), the investigators found.

The children of mothers who were in the middle group for magnetic field exposure were 74 percent more likely to have asthma than kids of moms in the lowest group.

Put another way, about 13.6 percent of children whose mothers had the least exposure to magnetic fields had asthma, compared to 20.3 percent in the moderate group and 33.3 percent of the children of mothers with the highest magnetic field exposure.

"This is a carefully executed and analyzed study with a very provocative finding," said Dr. Jonathan Samet, a professor in the department of preventive medicine and director of the Institute for Global Health at the University of Southern California, who has studied the health impacts of electromagnetic fields. "This initial finding needs replication; the association is strong and merits follow-up."

The study also found strong associations between EMFs and asthma in women whose children had other risk factors for developing asthma, including the mother having asthma herself or being the first-born child.

The children of mothers who had asthma and were exposed to high levels of EMF had six times the chances of having asthma, while first-born children had a 40 percent increased risk of having asthma if their mothers were exposed to high levels of EMFs.

Overall, about 13 percent of U.S. children have asthma, according to background information in the article, while prevalence has risen 74 percent from 1980 to 1996. The rapid rise has led experts to search for environmental exposures that might be contributing to the increase.

Magnetic fields are ubiquitous in modern life. Anything electronic, such as appliances and power lines, generate magnetic fields. Cell phones and other wireless technology are another source of EMF, although in the study, the device worn by the women was only able to measure lower frequency magnetic fields, such as those from household appliances and power sources. It was not able to track exposure to higher-frequency electromagnetic fields coming from wireless networks and cell phones.

Concern over EMFs was first raised decades ago. Prior research has searched for a link between EMFs and cancer. But most of those studies have found no association, leading to a "dismissive attitude" about EMFs among many experts, Li said.

Most recently, a Swiss study published in the Journal of the National Cancer Institute concluded that children and teens are not at increased risk of brain cancer from cell phones.

One of the strengths of Li's study is that it is prospective -- meaning it followed a group of women over time -- and actually measured exposure levels, said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at Steven and Alexandra Cohen Children's Medical Center of New York in New Hyde Park, N.Y.

"It raises concerns and certainly prompts the need for some follow-up studies in this area," Adesman said.

In the study, moms in the highest exposure group had an average daily exposure of greater than 2 milligauss (a measure of the strength of a magnetic field), while moms in the lowest group had a daily exposure of less than 0.3 milligauss.

Typically, exposure to magnetic fields comes in bursts, such as when using a microwave. But at other times, such as during sleep, people are likely exposed to very little EMF, Li explained.

The study found every 1 milligauss increase in average daily exposure was associated with a 15 percent increased risk of having asthma.

Despite the prevalence of EMFs, it is possible to decrease exposure by standing further away from appliances when they're in use, Li said. For example, a microwave emits 300 to 500 milligauss, a measure of the strength of a magnetic field. But standing about 4 to 5 feet away from it reduces exposure to about 1 to 2 milligauss, he said.

Why EMFs might be harmful is unknown, though animal studies suggest magnetic fields may impact the developing immune system, possibly by disrupting communications between cells.

Prior research by Li found an association between high EMF exposure during pregnancy and miscarriage. The women in this study were the same as those who participated in the prior research.

Though it's too soon to make any definitive recommendations, Adesman said pregnant women might want to take some steps to limit their exposure.

"An initial finding like this may not be sufficient grounds to make public health warnings, but common sense suggests that people may need to be mindful and society may need to be mindful of the increasing amounts of magnetic field exposure that all of us are surrounded by," Adesman said.

Barbara Grajewski, a senior epidemiologist at the U.S. Centers for Disease Control and Prevention's National Institute for Occupational Safety and Health, said the study is "intriguing" and well done in that it tracked a large number of women over time.

However, measuring EMF exposure for only one day of pregnancy may not be long enough to measure true exposure levels, and there are too few animal studies that show how EMFs impact cells to provide a biological explanation for what the researchers are observing.

It's also possible that other factors known to influence asthma risk such as socioconomic status or air pollution that researchers weren't able to account for might be playing an bigger role than EMFs, she said.

"I don't think it's a definitive, final statement that asthma and EMF are linked," said Grajewski, who has studied workplace EMF exposure.

More information

The U.S. Centers for Disease and Control and Prevention has more on EMFs.