Since Janssen et al. and Abrams et al. have found that air pollutants exposure such as NO2 was related to global hypomethylation of DNA as well as methylation of ADORA2B of which a gene signify for preeclampsia, epigenetic methylation alterations of placenta caused by air pollutants would be one possible pathway accounting for our findings that exposed to the highest concentrations of nitrogenous compounds was related to higher risks of preeclampsia (34, 35). The gene discussed is ADORA2B; the disease is preeclampsia.