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or more houses"), mobile home, dor- mitory, or other. For these analyses, mobile homes were combined with de- tached houses. The other 2 categories analyzed were attached houses and apartments. Subjects reported to be living in dormitory or other settings were excluded. Tobacco -Smoke Exposure: Survey The NHANES assessed household smoking with the question, "Does any- one who lives here smoke cigarettes, cigars, or pipes anywhere inside this home?" For those households in which no one was reported to smoke inside the home, no other information was available about smoking status, home or car smoking bans, other sources of exposure, or outside smoking behav- iors. Preliminary analyses on the full sample showed that children who lived in a house where anyone smoked in- side had exposure levels that over- whelmed any relationship between co- tinine level and housing type; we therefore limited the sample in this study to children who lived in a house- hold in which no member was re- ported to smoke inside the home. Like- wise, we excluded any child who admitted to smoking. Tobacco -Smoke Exposure: Biochemical Verification Serum cotinine was measured us- ing isotope dilution -high-performance liquid chromatography/atmospheric- pressure chemical ionization tandem mass spectrometry; the detectable limit in the NHANES is 0.015 ng/mL, and the coefficient of variability is 2.5%.28 Tobacco -smoke exposure was defined as a cotinine level of >_0.015 ng/mL, although comparisons also were made at cotinine cutoff levels of 0.05, 1.0, and 2.0. A sensitivity analysis was done to determine whether the results were affected by the testing variability. In addition, because plants from the nightshade family contain low levels of natural nicotine,29 we tested the model controlling for intake of tomatoes, egg- plant, and potatoes for 96.3% of the sample for whom these data were available. For this subsample, there were no significant differences in coti- nine levels when vegetable intake was included; therefore, we continued our analysis with the full sample. Analysis X2 and ttests were conducted to ana- lyze bivariate data. Analyses that in- cluded cotinine level as a continuous dependent variable used Tobit regres- sion models to account for the censor- ing of the data at the lower cutoff of 0.015 ng/mL.30 Race/ethnicity by housing -type interactions were tested using Tobit regression for cotinine lev- els as the outcome and logistic regres- sion when using tobacco exposure as the outcome. Cotinine levels were ana- lyzed using log transformations and geometric means to normalize the skewed distribution. The assumption of linearity among all continuous co - variates was checked. Stata was used to control for the complex sample weighting and design.31 This secondary analysis of NHANES data was approved as exempt by the University of Roches- ter Research Subjects Review Board. RESULTS There were 5002 children surveyed in the NHANES who were living in a home in which no one smoked inside (81%). Compared with the children who were living in a home in which someone smoked, those who were living in a home in which no one smoked were more likelyto live in a detached house (81.4% vs 73.4%) and less likely to live in an apartment (11.6% vs 16.7%; P < .02). They also were more likely to be over 12 years of age (46.9% vs 38.8%; P < .01), male (52.2% vs 46.7%; P < .01), and Hispanic (20.5% vs 8.8%) rather than black (14.0% vs 22.1 %) or white (59.1% vs 63.7%; P< .001 for all 8.A. - Page 31 comparisons). In addition, children who were living in a home in which no one smoked inside were more likely to be more than 400% of the federal poverty -level ratio (28.5% vs 11.3%) and less likely to be 100% or lower (17.8% vs 34.1%; P<.001). The remaining results pertain to those children who were living in a home in which no one smoked inside. The de- mographic characteristics ofthis sam- ple are presented in Table 1. The over- all geometric mean cotinine level amongthese children was 0.036 ng/mL (95% confidence interval: 0.030- 0.043); cotinine levels were higher among children under 12 years of age, black children, and those living below the federal poverty levels. Mean cotin- ine levels amongthose who were living in apartments (0.075 ng/mL) were higherthan in those who were living in detached houses (0.053 ng/mL; P < .01) and detached houses (0.031 ng/ mL; P< .001). Overall, using the detect- able limit of 0.015 ng/mL as the tobacco -exposure cutoff, 84.5% of chil- dren who were living in apartments had a cotinine level that indicated re- cent tobacco -smoke exposure, com- pared with 79.6% of children who were living in attached houses and 70.3% who were living in detached houses (P < .001) (Fig 1). Sensitivity analysis, using the higher cutoff of 0.05 ng/mL, showed exposure rates of 56.4% for children who were living in apart- ments, 47.0% for children who were living in attached homes, and 36.1%for children who were living in detached homes (P < .0001). Figure 2 shows the proportion of children by housingtype who were unexposed at different coti- nine levels, demonstrating the persis- tent and consistent decrease in the percentage exposed for those who live in detached homes. The percentage of children who were exposed to tobacco smoke in different housing types varied significantly by PEDIATRICS Volume 127, Number 1 January 2011 87 Downloaded from www.pediatrics.org. Provided by San Mateo Medical Center on December 13, 2010