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Serum cotinine and urine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) concurred in the classification of cigarette smoke exposure in most subjects (n = 59). The dots represent individual subjects. The dashed lines represent the cutoff between active and secondhand smoke exposure (serum cotinine = 3.1 ng/mL; urine NNAL = 64 pg/mg creatinine [Cr]). One subject was not included because of urine NNAL measurement interference (serum cotinine = 286 ng/mL).
