Cotinine is a metabolite only of nicotine. Among nonsmokers,the presence of cotinine in serum indicates exposure to secondhand tobacco smoke. From 1988-1991 through 2001-2002, median serum cotinine levels decreased by 74% in children aged 4-11 years, or 79% in persons aged 12-19 years,and 83% in persons aged >20 years, suggesting a substantial reduction in the exposure of the U.
S. population to secondhand tobacco smoke.
Source: cdc.gov