We address each of the concerns of Brain et al regarding our article “Maternal exposure to magnetic fields during pregnancy in relation to the risk of asthma in offspring.”
First of all, we adjusted for all 19 variables listed in our Table,1 not just 3 variables as stated in the Brain et al letter. Second, as an observational epidemiological study, our findings can always be subject to speculation about potential uncontrolled and unmeasured confounders. However, the potential “confounders” suggested by Brain et al were merely potential risk factors for asthma, not necessarily confounders, since there was no evidence that these factors are related to magnetic field (MF) exposure. In fact, one of the risk factors (pediatric infections) was not related to MF exposure level in our study population and thus was not a confounder. Wire code configuration may be associated with certain types of housing, but wire code configuration is not a good measure of personal MF exposure level and was poorly related to 24-hour MF exposure level in our study population. Finally, the risk factors suggested by Brain et al are postnatal risk factors. They may have little to do with maternal MF exposure during pregnancy. Our study was about maternal MF exposure during pregnancy. Thus, those potential risk factors of asthma are not likely strong candidates for confounders.