Here, we evaluated the direct effects of benzo[a]pyrene (BaP), a representative PAH, on the methylation status of the 5 ́-flanking region and gene expression of IFNγ and IL4 in Jurkat T cells and two adenocarcinoma cell lines (A549 and H1793) as in vitro models of epigenetic responses of immune or airway cells to PAH exposure. The gene discussed is IL4; the disease is pulmonary arterial hypertension.