A US study found that maternal exposure to second-hand smoke in both the home and workplace was associated with atrial septal defects with an OR of 1.37 (95% CI: 1.09, 1.72) [19], while both maternal and paternal exposure provided an additive effect with an OR of 4.5 (95% CI: 2.5, 8.3), which was increased in offspring who carried the combined null genotype for glutathione S-transferase (GST) GSTM1 and GSTT1 enzymes [20] (genetic polymorphisms are further discussed in Potential Mechanisms of Effect). Here, GSTM1 is linked to Abnormal cardiac septum morphology.