Int. J. Environ. Res. Public Health 2009, 6 1636 19), and adults (age 20 and older). We excluded anyone less than three years old because serum cotinine was not measured in this group. The age cutoffs for adolescents and adults were used to be consistent with two sets of NHANES questionnaires about cigarette smoking and tobacco use. As part of the home interview, respondents aged 20 and older were asked about their history of cigarette smoking and other tobacco use. These individuals (i.e., adults aged 20 and older) were defined as “current smokers” if they answered that they have smoked at least 100 cigarettes in their lifetime and currently smoke cigarettes everyday or some day, or that they have used other tobacco products including pipe, cigar, snuff, or chewing tobacco at least 20 times in their entire life and now use these tobacco products. As part of the audio computer-assisted self interview during the physical examination in the MEC, respondents aged 12 to 19 years, who answered that they have tried cigarette smoking, were asked how many days they smoked cigarettes in the past 30 days. These individuals (i.e., adolescents aged 12-19) were defined as “current smokers” if they answered that they had smoked cigarettes at least one day in the past 30 days. In addition to these two sets of questionnaires, all respondents aged 12 and older were also asked about their use of tobacco products (cigarettes, pipes, cigars, chewing tobacco, snuff, nicotine patches, nicotine gum, or any other product containing nicotine) in the past five days during the physical examination in the MEC. We considered all children to be nonsmokers. For adolescents and adults, nonsmokers were those who were not “current smokers” as defined above, had not used any tobacco products in the past five days, and had serum cotinine levels ≤ 10.0 ng/mL. While the choice of an arbitrary serum cotinine cutoff may lead to classifying some light smokers as nonsmokers, or excluding some nonsmokers who are heavily exposed to SHS, others have reported that using a cutoff of 10 or 15 mg/mL has little impact on results [9,10]. For the analyses of self-reported SHS exposure, individuals with responses to home exposure questions of missing, “don’t know”, or “refused” were excluded. The final sample size for nonsmokers was 25,329 for 1999-2006. For the analyses of serum cotinine measured exposure, individuals with missing serum cotinine data were excluded; the final sample size for nonsmokers was 24,001 for 1999-2006. 2.3. Measures of SHS Exposure We analyzed exposure to SHS using two types of measures: self-reported exposure and cotinine measured exposure. Self-reported exposure to SHS was defined on the basis of the following NHANES questions: for each household participating in the survey, one family member was selected to answer whether any of the household members smoked cigarettes, cigars, or pipes anywhere inside the home. If any household member smoked, then all members of that household were classified as being exposed to SHS in the home. Respondents aged 16 and older who reported that they had a job or business in last week were asked: “At this job or business, how many hours per day can you smell the smoke from other people’s cigarettes, cigars, and/or pipes?’’ Those who reported one hour or more were defined as being exposed to workplace SHS. For children and adolescents, we only examined their SHS exposure status in the home. For adults, we examined both exposure status in the home and exposure status at work. Serum cotinine, a metabolite of nicotine, was assessed in the NHANES using an isotope dilution, liquid chromatography tandem mass spectrometry method [9]. Analyses were conducted at the Centers