Within 24-h post-exposure to either clean air or DE, study participants underwent a research bronchoscopy with brush biopsy to obtain bronchial epithelial cells as previously described and following a standard protocol [22 (link), 23 ]. The cytology brushes containing epithelial cells tips were placed in a 1.5 ml tube with 200 μl Trizol Buffer (ThermoFisher Scientific). DNA was extracted using the Gentra Puregene Buccal Cell Kit (Qiagen, Inc.), and samples were stored frozen at −80°C before analysis. DNA extracted from the bronchial epithelial cells was sent to a commercial laboratory (Expression Analysis, Durham, NC, USA) for DNA methylation assessment using the Illumina HumanMethylation 450K BeadChip array. The extracted DNA samples were placed on four chips. We performed background correction using noob and dye bias correction as well as corrected for probe design bias arising from Type I and Type II probes with the Beta-mixture quantile normalization (BMIQ) method [24 (link), 25 (link)]. The DNA methylation β-value, or the proportion of cytosine methylated, at 484 531 CpGs was used for analyses.