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The hydrogen cyanide (HCN) absorbed from tobacco smoke is detoxified in the liver, yielding thiocyanate (SCN). However, SCN in serum and other biological fluids does not exclusively originate from inhaled tobacco smoke. Thiocyanate also can be derived from the diet (Haley et al., 1983; Jarvis, 1985).

Before 1975, primarily two colorimetric methods were used for the manual determination of thiocyanate in biological fluids (Aldridge, 1944; Bowler, 1944). Subsequently, the automatic method by Butts et al. (1974) has found wide application in comparing physiological fluids from smokers and nonsmokers. It entails determination of thiocyanate by its reaction with ferric ions, which yield a color complex with maximal absorbance at 460 nm, the intensity of which can be measured in an autoanalyzer. In sera of nonsmokers, Butts et al. (1974) determined up to 95 µmol/L of SCN. The critical value in differentiating between smokers and nonsmokers was 85 µmol/L of SCN. In other investigations, 100 µmol/L of SCN was found to be the critical level for serum (Junge et al., 1978) and for saliva (Luepker et al., 1981). This fact and the low concentrations of HCN in ETS (Hoffmann et al., 1984) explain why some investigators were unable to distinguish between nonsmokers exposed to ETS and those without any exposure to tobacco smoke (Hoffmann et al., 1984; Jarvis, 1985).

Similarly, the mean serum level of SCN in healthy pregnant women at term who were exposed to ETS (35.9 µmol/L) was not distinctly different from that in those without ETS exposure (32.3 µmol/L), nor was there a measureable difference in SCN levels in the umbilical cords of the neonates (26 versus 23 µmol/L) (Hauth et al., 1984).

In one study, it appeared that there was a trend toward higher thiocyanate levels in the saliva of nonsmoking children residing with smokers compared to the SCN levels in saliva of children without ETS exposure, yet this trend was insignificant (Gillies et al., 1982). In a study of six volunteer nonsmokers exposed to a smoke-filled room for 4 hours, there was a significant increase in salivary SCN. However, the SCN values of the nonsmokers

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