Features with a relative standard deviation (RSD) > 50 % in the QC-pooled samples were discarded. Since different cheminformatic approaches and software were employed to analyze the non-target and suspect screening LC-HRMS data, four different types of criteria were used to annotate confidence levels to the features (Table ). Note that only high confidence features, Level 1 to Level 3 (matching various criteria shown in Table ), were considered in this study to ensure the quality of the biological interpretation. Features were annotated as Level 1 when the match between the chemical standard (Table ) and tentative candidate (in the dust or serum) yielded a SpectrumSimilarity score ≥ 0.7 and the retention time (RT) shift was <1 min. OrgMassSpecR , was used to calculate spectral similarity. Xcalibur Qual Browser (version 4.1.31.9) was used to check the RT and to extract the MS/MS information. The annotated chemicals were classified using the MetOrigin web server and the PubChem Classification Browser , using both the PubChem Compound TOC and the NORMAN-SLE browsers. Chemical structures generated using CDK Depict .