Children whose mothers had high exposure to electromagnetic fields while pregnant may have an increased risk of developing asthma, U.S. researchers said on Monday in a study that adds to an ongoing debate.

Many prior studies have failed to consistently show that chronic exposure to electromagnetic fields -- from power lines and appliances such as microwaves ovens, hair dryers and vacuum cleaners -- are harmful to human health.

But many of these studies required people to estimate their exposure levels over several years, says Dr. De-Kun Li, senior research scientist at the Kaiser Permanente in Oakland, California, whose study appears in Archives of Pediatrics & Adolescent Medicine.

To get a more conclusive answer, Li designed a so-called prospective study in which 801 pregnant women wore monitors that measured their exposure to magnetic fields for 24 hours.

These monitors measured their exposure to low-frequency magnetic fields from electronics such as microwaves, hair dryers, fans, coffee grinders and fluorescent light bulbs, power lines, and transformer stations.

It did not monitor exposure to higher frequency electromagnetic fields generated by cellphones or cellphone towers.

The team used medical records to follow the women's children for 13 years. During the follow-up, 130 children, or 20.8 percent of study participants, developed asthma.

Most of these cases were diagnosed before age 5.

They then compared exposure levels during pregnancy to rates of asthma and found that children whose mothers had the highest exposure levels -- within the top 10 percent of women in the study -- were 3.5 times more likely to develop asthma than those who were in the bottom 10 percent.

The risk for children whose exposure was somewhere in the middle -- between 10 percent and 90 percent -- was 75 percent higher than for those in the lowest exposure group.

For the average population, Li said, children of women whose exposure levels were in the range of the bottom 10 percent in the study would have about a 13.6 percent absolute risk of developing asthma. Women whose exposure was in the highest range would have about a 33 percent risk of having children who developed asthma over the 13 year study period.

Some 13 percent of children under age 18 have asthma, which is caused by malfunction of the respiratory organs and the immune system.

Li said it is not clear why exposure to power lines might increase the risk of asthma, but he said there are several possibilities.

He said a prior study by his team found high exposure to electromagnetic fields increased the risk of miscarriages. And some animal studies have suggested that electromagnetic field exposure can affect immune response, which could increase the risk of asthma.

Exposure to power lines has been fodder for significant debate, and while many studies have found an effect of some sort -- ranging from immune disorders and poor semen quality to certain types of cancers -- Li said his study offers a stronger argument that concerns about magnetic fields may affect human health.

"This really needs to be studied," Li said. He said there have been a lot of dismissive attitudes about the health effects of exposure to magnetic fields, and he hopes his study -- which measured exposure levels ahead of time -- will encourage others to look further.

Still, he concedes that his findings need to be replicated by different scientists.

Li said if the findings are confirmed, it may offer new strategies for preventing the chronic disease in children.

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