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

Many prior studies have failed to show convincingly that chronic exposure to electromagnetic fields – from power lines and appliances such as microwave 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, a scientist at the Kaiser Permanente in Oakland, California.

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

Measurements were taken from 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 five.

They then compared exposure levels during pregnancy to rates of asthma and found that children whose mothers had the highest exposure levels 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 – 10 percent to 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.

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

Li suggested reasons why exposure to power lines might increase the risk of asthma.

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.

Li said there have been a lot of dismissive attitudes about the health effects of exposure to magnetic fields, and he hopes his study will encourage others to look further. – Reuters