Just as asthma rates have been climbing, so has population exposure to electromagnetic fields from expanding infrastructure for wireless networks, cell phones, and electric power. Higher in-utero exposure to magnetic fields such as those emitted by power lines and cell phone towers may place children at an increased risk of asthma.  De-Kun Li, from Kaiser Permanente's Division of Research (California, USA), and colleagues examined data from a prospective cohort study involving 626 mother-child pairs. The mothers all carried a meter that measured magnetic fields -- but not electric fields -- for a single 24-hour period during the first or second trimester of pregnancy. Through a follow-up lasting as long as 13 years, 20.8% of the children developed asthma. After adjustment for age, race, education, smoking during pregnancy, and maternal history of asthma, increasing exposure to magnetic fields during pregnancy was associated in a linear, dose-response fashion with greater asthma risk.  Children whose mothers had high exposure to magnetic fields (>2.0 mG) were significantly more likely than those whose mothers had low exposure (≤0.3 mG) to develop asthma.  Although the study could not prove a causal relationship between exposure to magnetic fields and risk of asthma, the researchers submit that there is a plausible biological connection: whereby the underlying pathogenesis of asthma is likely structural and due to functional defects in epithelium and an impaired innate immune systemprenatal exposure to high magnetic field levels could interfere with the development of both epithelial cells and normal immune systems. Reporting “a statistically significant synergistic interaction was observed between the [magnetic field] effect and a maternal history of asthma and birth order (firstborn),” the researchers submit that: “Our findings provide new epidemiological evidence that high maternal [magnetic field] levels in pregnancy may increase the risk of asthma in offspring.”