Kids are more likely to develop asthma if their mothers are frequently exposed to electromagnetic fields when pregnant, U.S. researchers said in a study in news reports Tuesday. 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, said Dr. De-Kun Li, a senior research scientist at the Kaiser Permanente in Oakland, California. Dr. Li designed a prospective study in which 801 pregnant women wore monitors to measure 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, and transformer stations, but excluding higher frequency electromagnetic fields generated by mobile phones. Researchers used medical records to follow the women\'s children for 13 years. They found 20.8 percent of study participants developed asthma. And a majority of these cases were diagnosed before age 5. And mothers whose exposure was in the highest range would have about a 33 percent risk of having children who developed asthma.