Author Affiliations: Harvard School of Public Health, Boston (Drs Brain and Kavet), and Gradient, Cambridge (Dr Valberg), Massachusetts; and Electric Power Research Institute, Palo Alto, California (Dr Kavet).
Li et al1 report an association between childhood asthma and maternal magnetic field exposure of 2 mG or more during pregnancy, based on a single day of personal magnetic field (40-800 Hz) measurement. While we have a common interest in understanding the causes and prevention of asthma, we offer the following concerns.
Though Li et al acknowledged a “short list” of potential confounders (such as maternal asthma and smoking), a far greater number of risk factors for asthma have been identified.2 Evidence suggests that exposure to endotoxin, microbes, and sunshine are protective. Also, magnetic field exposure may serve as a proxy for time spent indoors—a known risk factor for asthma. Time indoors (higher magnetic fields) would predict less endogenous vitamin D and greater asthma risk. Jackson et al3 report a 26-fold increased risk of asthma by age 6 years in children who experienced rhinovirus infection with wheezing in the third year of life. Before one can have an accurate grasp of a new potential risk factor for asthma, such documented factors should be included in the analysis.
Brain JD, Kavet R, Valberg PA. Observations on Power-Line Magnetic Fields Associated With Asthma in Children. Arch Pediatr Adolesc Med. 2012;166(1):97-99. doi:10.1001/archpedi.166.1.97-b