2.2. Urinary Polycyclic Aromatic Hydrocarbon (PAH) Metabolites

The methods used to quantify urinary PAH metabolites have been described previously (34,35). Briefly, spot urine samples were analyzed for 7 metabolites by gas-chromatography/isotope dilution high-resolution mass spectrometry. Quality control procedures included isotope dilution with internal standards labeled with carbon-13. A total of 6 PAH metabolites were detected in >30% of the population and subsequently included in this analysis (36), specifically: 1- Napthol (CAS 90–15-3), 2-Napthol (CAS 135–19-3), 3-Fluorene (CAS 6344–67-8), 2-Fluorene (CAS 2443–58-5), 1-Phenanthrene (CAS ‎85–01-8), and 1-Pyrene (CAS 5315–79-7). Additionally, we generated a composite measure of total PAH, which was a summation of all 6 PAH metabolites. The limit of detection (LOD) for each metabolite varied between NHANES cycle. Subsequently, the maximal LOD of each metabolite across all 6 NHANES cycles was assigned to standardize LODs (37)(See Appendix A - Table S1). Concentrations below the LOD were replaced with the LOD divided by the square root of 2 for each metabolite. Only 0.05–2% of samples were below the LOD for each metabolite, except for 1-pyrene which had 31% of samples were below the LOD. Intra-individual variations in urine dilution were accounted for by adjusting for urinary creatinine in regression models (38).