Exposure to BC was positively associated with FeNO, and negatively associated with DNA methylation in NOS3. We found strongest association between FeNO and BC at lag 0–6 h while strongest associations between methylation at positions 1 and 2 in NOS3 and BC were at lag 13–24 h and lag 0–24 h, respectively. The gene discussed is NOS3; the disease is breast cancer.