Int. J. Environ. Res. Public Health 2009, 6 1638 characteristics and SHS exposure, the four waves of data were combined. Differences in self-reported exposure among population subgroups were evaluated using multivariate logistic regression models which adjust for gender, race/ethnicity, country of birth, education level, and poverty income ratio. Adjusted odds ratios and 95% confidence intervals (CIs) were estimated. Differences in geometric means of cotinine levels among sub-populations were evaluated using multivariate linear regression models of log-transformed cotinine levels by adjusting for sociodemographic covariates We considered estimates to be statistically significant if the two-sided P-value < 0.05. 3. Results 3.1. SHS Exposure over Time Table 1 shows the prevalence of self-reported exposure, prevalence of detectable serum cotinine, and geometric mean of cotinine levels among nonsmokers for the four waves of data and the three age groups. Consistent with earlier studies, the serum cotinine measure indicates a much higher prevalence of SHS exposure than self-reports. For all the three age groups, the difference in exposure rates is nearly threefold. Over the four time periods from 1999 to 2006, children and adolescents were found to have self-reported home exposure rates in the range of 14.6-23.7% and 15.2-23.4% respectively, compared to cotinine measured exposure rates of 50.9-65.2% and 47.0-63.2% respectively. Adult total self-reported exposure includes both exposure status in the home and exposure status at work. Work only exposure rates exceeded home only exposure rates. Very few people, 1.1 percent or fewer of the adult nonsmokers, were exposed in both settings. During the three time periods from 1999 to 2004 for which work and home exposure data are available, adults’ total self-reported rates were in the range of 13.1-15.7% compared to cotinine measured exposure rates of 37.3-47.4%. For both self-reported and cotinine measured prevalence over all periods, children had the highest SHS exposure rates. Adolescents’ rates were a bit lower except for the self-reported rates during 1999-2000 and 2005-2006, and adults had the lowest rates. The geometric mean of cotinine levels for children ranged from 0.11 to 0.18 ng/mL during the 8-year period. For adolescents the range was 0.09 to 0.14 ng/mL and for adults the range was 0.06 to 0.08 ng/mL. During every time period, children had the highest mean cotinine levels, followed closely by adolescents, and adults had the lowest levels. While all three measures of SHS exposure were lower in the fourth time period than in 1999-2000, they did not consistently decrease over time. In particular, there was an increase noted for 2003-2004 over the previous period for all measures and all age groups except children measured through self- reports. The multivariate regression of trend analyses indicates that the prevalence of self-reported exposure decreased significantly over time only for children. However, for adolescents and adults, both the prevalence of detectable cotinine and the geometric mean of cotinine levels proved to be statistically significantly decreasing over time. We further explored whether there might be trends for specific race/ethnic subgroups within an age group in a separate analysis. We found that none of the racial/ethnic subgroup for adolescents and adults exhibited any significant trend in self-reported exposure over time.