OBJECTIVES: We investigated the association between smoke-free law coverage and cotinine among nonsmoking youth (3–19 years) with and without home secondhand smoke (SHS) exposure.
METHODS: We used data from the 1999–2006 National Health and Nutrition Examination Survey, a cross-sectional survey designed to monitor the health and nutritional status of the US population. Serum cotinine levels were available for 11 486 nonsmoking youth from 117 survey locations. Each location was categorized into 1 of 3 groups indicating extensive, limited, and no coverage by a smoke-free law. Cotinine was analyzed both as a dichotomous (≥0.05 ng/mL) and as a continuous outcome.
RESULTS: Among youth without home SHS exposure, those who were living in a county with extensive coverage of a smoke-free air law had an adjusted 0.61 times (95% confidence interval: 0.48–0.78) the prevalence of detectable cotinine and 0.57 (95% confidence interval: 0.41–0.79) times lower geometric mean compared with youth without a smoke-free air law. Among youth with home SHS exposure, youth with extensive coverage of a smoke-free air law had lower geometric mean cotinine compared with youth who were living in a county without a smoke-free air law, but these differences were no longer observed after adjustment for covariates.
CONCLUSIONS: These results suggest that smoke-free laws are an effective strategy for reducing cotinine in youth without home SHS exposure; however, among youth with home SHS exposure, no benefit was detected.
- secondhand smoke
- tobacco control policy
- children and adolescents
- National Health and Nutrition Examination Survey
WHAT'S KNOWN ON THIS SUBJECT:
There has been a large increase in the number of smoke-free air laws implemented in the past decade. In adult nonsmokers, smoke-free air laws are associated with substantial reductions in SHS exposure, as measured by cotinine levels.
WHAT THIS STUDY ADDS:
This study examined the impact that smoke-free air laws have on cotinine in children and adolescents. We studied this association in children and adolescents with and without exposure to SHS in the home.
According to the 2006 Surgeon General's Report, there is no safe level of exposure to secondhand smoke (SHS). Children are particularly vulnerable to the toxic compounds in SHS because they have higher breathing rates and their lungs are still developing. Exposure to SHS in children can irritate the lungs, resulting in coughing or wheeze, and can trigger an asthma attack among children with asthma.1 SHS also has been associated with sudden infant death syndrome, lower respiratory illnesses, and middle ear disease.1
For children, the home is the primary source of SHS exposure.2,–,7 In the United States, an estimated 20% of youth aged 3 to 19 years were exposed to SHS in the home,8 compared with only 6% of nonsmoking adults aged ≥20 years.9 Parents' smoking accounts for the majority of home exposure.10 Potential exposure sources for children outside the home include cars, private child care centers, restaurants, shopping malls, and parks.
Cotinine, the major metabolite of nicotine, is a common biomarker of SHS exposure among nonsmokers. The half-life of cotinine is 16 to 18 hours and thus represents exposure to SHS within the previous couple of days.11,12
The National Health and Nutrition Examination Survey (NHANES) has been measuring cotinine levels since 1988. Among children (4–16 years) without exposure to SHS in the home, geometric mean cotinine levels declined from 0.12 ng/mL in 1988–1994 to 0.05 ng/mL in 2003–2006. Among children with exposure to SHS in the home, geometric mean cotinine levels did not change from 1988–1994 (1.0 ng/mL) to 2003–2006 (1.09 ng/mL).8
One reason for the decline in cotinine levels among children without exposure to SHS in the home may be the increase in state and local smoke-free laws. The number of local smoke-free laws in workplaces, restaurants, and bars increased from 0 in 1988 to 175 in 2006.13 In adults, these laws have been shown to reduce exposure to SHS,14,–,25 reduce smoking prevalence and cigarette consumption,26 and increase the number of smoke-free homes, even among smokers.6,27 All of these effects would reduce the overall SHS exposure to youth in a community.
In children, studies in Scotland showed an overall 39% (from 0.36 to 0.22 ng/mL) reduction in cotinine geometric means the year after implementation of a smoke-free law compared with the year before the law,28 which was greatest for children who were living with nonsmoking parents.7 In nonsmoking adults, an analysis of NHANES data from 1999 to 2004 found lower cotinine levels in adults who were living in counties with a smoke-free law, compared with adults who were living in counties without a smoke-free law.19 We used similar methods to examine the association between smoke-free laws and NHANES measurements of cotinine in children and adolescents.
NHANES, conducted by the National Center for Health Statistics, is a series of cross-sectional surveys designed to monitor the health and nutrition status of the US population. Sample individuals are selected through a multistage, probability cluster design. Adolescents aged 12 to 19 years, adults aged ≥60 years, low-income persons, Mexican American individuals, and non-Hispanic black individuals are oversampled to improve the reliability and precision of estimates for these groups.29
The NHANES consists of a household interview and a standardized physical examination conducted in a mobile examination center. Individuals who are aged ≥16 years are interviewed directly. A responsible adult provides information for sample individuals who are younger than 16 years. For NHANES 1999–2006, 50 939 individuals were selected for the sample; 41 474 (81.4%) of these were interviewed, and 39 352 (77.3%) were examined in the mobile examination center.
We restricted our analysis to nonsmoking youth aged 3 to 19 years. Nonsmokers were defined by both cotinine level and self-reported smoking status. Youth with missing cotinine levels (n = 2479) were excluded. Youths with cotinine levels <15.0 ng/mL30 were considered nonsmokers (n = 12 411). Youth (12–19 years) who reported that they had used tobacco or nicotine in the 5 days before blood collection (n = 317) or were missing information on this variable (n = 465) were excluded. Pregnant youth also were excluded (n = 143). This resulted in a final sample size of 11 486 nonsmoking youth.
SHS Exposure Assessment
SHS exposure status was assessed by serum cotinine measurements. Laboratory methods for analyzing serum cotinine have been described previously.30 Blood was obtained by venipuncture during the physical examination for survey participants who were aged ≥1 year, and cotinine was analyzed for survey participants who were aged ≥3 years.
We defined exposure to SHS as a detectable cotinine level. The cotinine limit of detection (LOD) was improved from 0.05 ng/mL in 1999–2000 to 0.015 ng/mL in 2001–2002 with the introduction of a newer, more sensitive mass spectrometer. For comparability and consistency, we defined detectable cotinine as ≥0.05 ng/mL for all survey cycles, although the limit of detection was 0.015 ng/mL in the later examinations.
We also analyzed levels of continuous cotinine using data from 2003–2006, in which the LOD was 0.015 ng/mL, corresponding to the lower LOD. Cotinine levels that were less than the LOD were assigned a value equal to the LOD divided by the square root of 2. Cotinine was natural log–transformed and analyzed as a continuous outcome.
For assessment of home SHS exposure, 1 member of each household was asked, “Does anyone who lives here smoke cigarettes, cigars, or pipes anywhere inside this home?” When at least 1 person smoked inside the home, all members of that house were classified as having home SHS exposure.
Exposure to Smoke-free Laws
NHANES participants were classified into 3 smoke-free law coverage categories by their county and state of residence. From 1999 to 2006, NHANES sampled youth from 117 survey locations, or counties. Information on state and local smoke-free laws was obtained for each county from a database of indoor air ordinances maintained by the American Nonsmoker's Rights Foundation.31 Locations that were classified as having a smoke-free law completely banned smoking and did not allow for separately ventilated smoking rooms, size exemptions, or smoking in bars attached to restaurants. Laws were included only when they were enacted before the examination portion of the survey was administered.
The American Nonsmoker's Rights Foundation list indicated presence of smoke-free laws for workplaces, restaurants, and bars at the city, county, and state levels. Each county was categorized into 3 smoke-free law coverage groups:
extensive coverage (26 counties): at least 1 smoke-free workplace, restaurant, or bar law at the county or state level that covered the entire county population;
limited coverage (11 counties): No county or state smoke-free law but at least 1 city within the county with a smoke-free workplace, restaurant, or bar law; or
no coverage (80 counties): No smoke-free law at the state, county, or city level.
The following variables were included in each model because they have been shown to be associated with smoke-free law enactment and/or cotinine: 2-year survey cycle, age (3–5, 6–11, 12–15, and 16–19 years), gender, race/ethnicity (non-Hispanic white, non-Hispanic black, Mexican American, and other), region of the country, number of meals eaten outside the home, and socioeconomic status as measured by the ratio of family income to poverty (above versus below poverty threshold).
The adult community smoking prevalence may affect SHS exposure in youth. The county-specific prevalence of smoking was defined as the mean prevalence of current adult smokers in each county in NHANES 1999–2006. Models were presented without (model 1) and with (model 2) adjustment for this variable because smoke-free laws may be more likely to be adopted in areas with a low smoking prevalence (confounder), or smoke-free laws may lower the smoking prevalence in a community (intermediate). It is important to adjust for confounders to remove bias, but adjusting for intermediate variables can remove some of the association between the exposure and outcome.32
Data management was conducted by using SAS 9.1 (SAS Institute Inc, Cary, NC) and data analysis in SUDAAN 9.0 (Research Triangle Institute, Research Triangle Park, NC), which accounts for the multistage, probability cluster design. To account for differential probabilities of selection and for nonresponse, we used examination sample weights. Variance estimates were calculated by using the Taylor linearization with replacement method. T tests were calculated to test the null hypothesis of no difference in proportions by using a significance level of P < .05.
As explained in the NHANES analytic guidelines, publicly released data files provide masked variance units to estimate sampling errors.29 Masked variance units were created to comply with disclosure avoidance principals that prohibit the public release of the primary sampling units. The exposure of interest, the smoke-free law coverage category, was based on the true primary sampling units, and we used these variables, which are available through the National Center for Health Statistics Research Data Center (www.cdc.gov/rdc) for calculating SEs for all estimates.
Cotinine was analyzed as both a dichotomous (<0.05 vs ≥0.05 ng/mL) and a continuous outcome. In the dichotomous analysis, the association between smoke-free laws and detectable cotinine (≥0.05 ng/mL) was analyzed. Adjusted prevalence ratios and 95% confidence intervals (CIs) were calculated from the predicted marginals of a logit model.32,–,34
To examine the association between smoke-free laws and level of cotinine, we calculated geometric means and 95% CIs. Linear regression was conducted by using the natural log-transformed cotinine as the outcome. We limited the data on the continuous analysis to the years 2003–2006, in which the limit of detection was consistently 0.015 ng/mL. Because smoke-free laws are primarily designed to reduce exposure to SHS in places outside the home, results were stratified by home SHS exposure status.
Detectable Cotinine (1999–2006)
The majority of nonsmoking youth (69.7%) lived in counties without a smoke-free law (Table 1), 8.6% lived in counties with limited coverage, and 21.7% lived in counties with extensive coverage. More than half (56.4%) of all nonsmoking youth had detectable levels of cotinine (Table 1). Youth from a county with extensive coverage of a smoke-free law had a lower percentage with detectable cotinine (32.7%), compared with youth from a county with limited coverage (49.6%) or no coverage (64.6%). Approximately 21% of youth reported exposure to SHS in the home. Almost all (98%) of these youth had detectable levels of cotinine. Among youth without reported exposure to SHS in the home (79%), 45.4% had detectable levels of cotinine.
We observed a higher frequency of detectable cotinine in earlier survey cycles (1999–2000 and 2003–2004); younger children (aged 3–5 years); non-Hispanic black individuals; low-income families (below poverty threshold); and the Northeast, South, or Midwest compared with the West (Table 1). Some of the same groups with a higher percentage of detectable cotinine were likely to live in counties with no or limited coverage of a smoke-free law. Groups with a lower percentage with extensive coverage included youth with home SHS exposure, those from the 1999–2000 survey, non-Hispanic black individuals, and those from the South and Midwest (Fig 1).
Youth in the extensive and limited smoke-free coverage groups had a lower average NHANES adult county smoking prevalence, 19.0% (95% CI: 15.0%–23.1%) and 19.2% (95% CI: 13.2%–25.2%), compared with the no coverage group, 26.4% (95% CI: 25.2%–27.6%). The prevalence and prevalence ratios for detectable cotinine (Table 2), unadjusted and adjusted for survey cycle, gender, age, race, ratio of family income to poverty, region, and restaurant visits per week (model 1) and, in addition, by adult county smoking prevalence (model 2) showed reduced detectable cotinine associated with smoke-free law coverage.
Unadjusted prevalence ratios for all youth and youth without home SHS exposure were similar and significant for both extensive and limited coverage of a smoke-free law. Among youth without home SHS exposure, living in a county with extensive coverage was associated with an adjusted 0.51 times (95% CI: 0.39–0.68) the prevalence of detectable cotinine, compared with living in a county without a smoke-free law. After additional adjustment for adult county smoking prevalence, this ratio attenuated to 0.61, although still significant. Limited coverage of a smoke-free law was associated with an adjusted 0.78 times (95% CI: 0.65–0.95) the prevalence of detectable cotinine. After adjustment for adult smoking prevalence, this prevalence ratio was no longer significant (0.91 [95% CI: 0.76–1.08]). Among youth with home SHS exposure, no difference was seen in the adjusted prevalence ratios by smoke-free law coverage group.
Continuous Cotinine (2003–2006)
The association between smoke-free law coverage and cotinine was also analyzed on the continuous scale for survey years 2003–2006. Figure 2 shows the distribution of log cotinine by smoke-free law coverage category among nonsmoking (Fig 2A) and smoking (Fig 2B) homes. Among nonsmoking homes, the distribution of log cotinine shifted toward higher cotinine moving from extensive to no smoke-free law coverage. For smoking homes, the distribution was similar for the 3 groups.
Among all nonsmoking youth, the geometric mean was lower in the extensive (0.051 ng/mL) and limited coverage groups (0.067 ng/mL), compared with the no coverage group (0.128 ng/mL; Table 3). This reduction associated with extensive coverage was statistically significant after adjustment. Among youth without home SHS exposure, extensive coverage of a smoke-free law was associated with 0.50 times (95% CI: 0.37–0.67) lower geometric mean compared with the no coverage category (Table 3). After adjustment for adult county smoking prevalence, this ratio attenuated to 0.57 (Table 3). The adjusted association between limited coverage of a smoke-free law and cotinine was not significant.
Among youth with home SHS exposure, unadjusted cotinine geometric means were lower in the extensive (0.835 ng/mL) and limited (0.903 ng/mL) coverage categories compared with the no coverage category (1.132 ng/mL), but these differences were not statistically significant. After adjustment for individual and county covariates, there was no significant difference in geometric mean cotinine by extent of smoke-free law coverage (Table 3).
We examined the association between smoke-free law coverage and SHS exposure as measured by cotinine among children and adolescents in the United States. Our results indicate a significant decrease in SHS exposure among youth with extensive coverage of a smoke-free law compared with youth with no law. Limited coverage of a smoke-free law was not associated with SHS exposure.
Almost 80% of youth in this sample were not exposed to SHS in the home. Among these youth, living in a county with extensive coverage of a smoke-free law was associated with 39% lower prevalence of detectable cotinine and 43% lower geometric mean cotinine level. Living in a county with limited coverage of a smoke-free law was associated with a nonsignificant 9% lower prevalence of detectable cotinine and a nonsignificant 5% lower geometric mean cotinine level.
The estimates substantially attenuated after adjustment of adult county smoking prevalence. There are 2 possible reasons for this attenuation. Smoke-free laws may be more likely to be adopted in areas with a low adult smoking prevalence (confounding). Alternatively, smoke-free laws may reduce adult smoking prevalence and thus reduce exposure to SHS in youth (intermediate). In this analysis, it is likely that both mechanisms were operating. Although it is difficult to determine the contribution that each scenario had on the results, there was a significant association between extensive coverage of a smoke-free law and cotinine in models with and without adjustment for adult county smoking prevalence.
Among the 20% of youth with self-reported SHS exposure at home, almost all (98%) had cotinine levels above the limit of detection (≥0.05 ng/mL). Youth with extensive and limited coverage of a smoke-free law had lower geometric mean cotinine compared with youth living in a county without a smoke-free law, but these differences were no longer observed after adjustment for individual and county covariates.
Misclassification of exposure to smoke-free laws was possible because county smoke-free laws may not capture individual exposure to these laws. Specific restaurants or workplaces may have more restrictive voluntary policies than their local or state laws. Alternatively, youth residing in a county with no smoke-free law coverage may preferentially eat in restaurants where smoking is prohibited or may spend time outside their county of residence, where they are exposed to a different set of smoke-free laws. In the limited coverage group, youth may have lived in the city covered by the smoke-free law, or they could have lived outside that city and thus not have been covered by the city smoke-free law. These potential misclassifications of smoke-free law coverage were likely not associated with cotinine, thereby biasing the results toward the null.
Despite these limitations, our results were consistent with 2 Scottish studies that examined the association between smoke-free laws and cotinine levels among children.7,28 These studies examined changes in salivary cotinine among school-aged children after implementation of smoke-free legislation that prohibited smoking in most enclosed public places. Among children from nonsmoking homes, adjusted for age and family affluence, there was a 51% decrease in geometric mean cotinine the year after the ban, compared with the year before the ban.28 Our study found a very similar decrease of 50% in geometric mean cotinine among youth from nonsmoking homes (not adjusted for adult county smoking prevalence). Among homes where the mother or both parents smoked, the Scottish study did not find an association.28 Our study also did not find a significant association between smoke-free air laws and cotinine among youth with exposure to SHS in the home.
Smoke-free laws have been shown to reduce exposure to SHS among adults. Our results show a similar association in children and adolescents who did not live with a smoker in the home; therefore, it is important to continue to enact smoke-free laws to protect both children and adults from exposure to SHS.
Approximately 20% of youth lived with a smoker in the home. These children had the highest cotinine levels and could benefit the most from an intervention to reduce exposure; however, there was no impact of smoke-free laws on cotinine levels among children with home SHS exposure. Interventions designed to reduce or prevent adults from smoking around children are needed, such as parental counseling by physicians to stop smoking.35
This research was funded by the Flight Attendants Medical Research Institute Clinical Innovator Award. Mrs Dove was supported in part by a National Institute of Environmental Health Sciences Training Program in Environmental Epidemiology (grant 2 T32 ES07069-26).
We gratefully acknowledge the assistance with data management of Susan Schober, Debra Brody, and Robert Krasowski from the National Center for Health Statistics, Centers for Disease Control, and Jim Davis from the Center for Economic Studies.
- Accepted March 23, 2010.
- Address correspondence to Melanie Dove, MPH, Exposure, Epidemiology and Risk Program, Harvard School of Public Health, Landmark Center, Room 405 West, 401 Park Dr, PO Box 15677, Boston, MA 02215. E-mail:
FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.
- SHS =
- secondhand smoke •
- NHANES =
- National Health and Nutrition Examination Survey •
- LOD =
- limit of detection •
- CI =
- confidence interval
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