Int. J. Environ. Res. Public Health 2009, 6 1644 predictor of cotinine-indicated exposure with odds ratios of 43.5 and 46.0 for children and adolescents. For adults, the odds ratios were 90.0, 27.6 and 2.7 for exposure both in the home and at work, home only, and work only, respectively. Children and adolescents living in households headed by someone with education beyond high school, living in households above the poverty threshold, or who were Mexican American, were less likely to have detectable cotinine level after controlling for self-reported SHS exposure. Adults with education beyond high school or in the mid or high income group were less likely to have detectable cotinine level. Table 4. Comparison of self-reported secondhand smoke exposure and cotinine measured exposure for nonsmokers by age group1: U.S., 1999-2006. Sample Size (unweighted) % with Detectable Cotinine Level ≥ 0.05 ng/mL (95% CI) Geometric Mean Cotinine, ng/mL Children (3-11) Self-Reported Home Exposure No Self-Reported Home Exposure Adolescents (12-19) Self-Reported Home Exposure No Self-Reported Home Exposure Adults (20+) 2 Self-Reported Home or Work Exposure Self-Reported Home Exposure Only Self-Reported Work Exposure Only Self-Reported Home and Work Exposure No Self-Reported Home or Work Exposure 1060 4053 1101 5069 1237 527 640 70 8083 98.2 (97.2-99.3) 47.6 (43.6-51.6) 97.4 (96.4-98.4) 42.0 (37.7-46.2) 77.2 (73.1-81.3) 94.2 (91.9-96.5) 63.5 (57.1-69.8) 98.4 (95.9-100.0) 36.1 (32.7-39.5) 1.20 0.08 0.75 0.07 0.22 0.52 0.11 0.65 0.06 1 Data source: National Health and Nutrition Examination Survey. 2 For adults, exposure rates are analyzed and compared based on the NHANES data in 1999-2004. Note: All the estimates are based on weighted analyses accounting for complex survey design. 4. Discussion Self-reported SHS exposure rates for children fell significantly between 1999 and 2006; however the prevalence rates of detectable cotinine and the mean cotinine levels did not show statistically significant trends. For adolescents and adults, the opposite occurred, with no significant trends in self- reported exposure, but a decrease in both the rate of people with detectable cotinine and the mean cotinine levels. All three SHS exposure measures are lower in the final time period compared to the first time period. However, the data for all measures show an increase during 2003-2004, and this might have contributed to a lack of statistically significant trends in exposure for some groups. Over the eight years we studied, serum cotinine concentration declined by 31% in children, 36% in adolescents, and 25% in adults. In contrast, Pirkle and colleagues [10] reported a 70% decline in serum cotinine concentration over the 14 years from 1988-2002. It is encouraging that the level of exposure continues to decline in more recent years.