Int. J. Environ. Res. Public Health 2009, 6 1645 Exposure patterns by demographic characteristics were similar when measured by either self- reports or serum cotinine. We found SHS exposure to be highest in children and also high in adolescents, consistent with earlier studies [10,11,13]. Among racial/ethnic groups, exposure rates and mean cotinine levels were highest in non-Hispanic Blacks across all age groups as documented in other studies [14-16], and lowest in Mexican American children [16] and adolescents. However, our findings for black children raise some interesting questions. While black children had higher rates of self-reported exposure, the adjusted odds ratio was less than one. This implies that after adjusting for the other demographic variables in the model, the exposure rates might have been lower. That would be consistent with previous findings indicating that blacks smoke fewer cigarettes per day, and lower intensity of smoking is associated with lower SHS exposure [12,24]. In fact, we observed that black children were more likely to be living in poor or low income households compared to white children (67% vs. 37%). Furthermore, black children’s exposure rates when stratified by poverty level were much lower than white counterparts for the poor group (36% vs. 51%) and low income group (25% vs. 34%) but were higher only for the high income group (12% vs. 7%). Thus, the inter-relationship among race/ethnicity, socioeconomic status, and SHS exposure merits further research. Our findings indicate that poor children have the highest exposure rates and level of exposure. While were encouraged to find that both self-reported and cotinine-indicated exposure rates showed a significant decline over the 8 years for poor children, the rates are still unacceptably high and there remains a need to focus more effort on reducing the health risk for this particularly vulnerable group. We found that the prevalence rate of self-reported SHS exposure was much higher than the prevalence rate of cotinine measured exposure, as shown in other studies [9,24-26]. This could be explained by several factors. It is possible that people underreport their SHS exposure or that the survey designs lead to underreporting, particularly when self-reported SHS exposure is measured by the presence of a self-identified smoker in the household. It is also possible that the large proportion of the population with detectable cotinine is reflective of the widespread ubiquity of SHS in our environment and the challenges of controlling exposure in multiple settings. The self-reported exposure measure is based on questions pertaining to two environments only – the home and the workplace. However, serum cotinine is not limited to those settings, and might also reflect exposure in restaurants, other public and private places, and outdoors. There are also dietary sources of nicotine, and while these are unlikely to have a major impact on cotinine levels in the population as a whole, they may have an impact on some individuals. Our findings suggest that the lower self-reported exposure levels underestimate the extent of the continuing public health issue. A number of limitations of this study need to be acknowledged. We did not have self-reported data on exposure for anyone in other indoor or outdoor settings. We did not account for the self-reported exposure in the workplace for adolescents. Data on workplace exposure for adults for 2005-2006 were not available at the time of our analyses. Serum cotinine is a good measure of SHS exposure during the previous 3-4 days. However, it has a short half-life and thus would not measure exposure that had occurred more than a few days ago. However, cotinine is still considered the biomarker of choice for SHS exposure assessment [1]. Despite many years of tobacco control efforts aimed at reducing SHS exposure in the home and workplace, exposure to SHS in the home and in the workplace continues to be an important public health problem. While the prevalence of exposure as well as the mean cotinine concentration are lower