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.
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