Over a 5-year period, ETS exposure declined in this population of older adults. Decreases in exposure were observed for all settings examined (home, work, and social settings). Controlling for several factors including age, later time period was associated with reduced odds of exposure at home and in social settings, but was not significantly associated with odds of exposure at work, although temporal shifts in workplace ETS exposure may have occurred prior to 1998 as some employers instituted smoke-free policies. Older age and female sex were each associated with decreased odds of exposure at work and in social settings, but not associated with home exposure. College education, compared to less than high school education, was associated with reduced odds of exposure in all three settings.
In this cohort of older participants, we expected retirement and death among smoking spouses would explain some of the decreased ETS exposure. Among those working at both time points, the percentage reporting no workplace exposure was similar, but slightly greater at the later time period (78% vs. 80% at the earlier time period) and later time period was not associated with reduced odds of exposure at work in the multivariate GEE model. Thus, among this older population, retirement and reduced work hours appear to account for a portion of the small decrease in prevalence of workplace exposure that was found in the whole sample. Regarding exposure in the home, we observed the same decrease as in the whole sample when we looked at the subset with no change in marital status, suggesting that death of smoking spouses was not a major reason for the small decrease noted in the prevalence of ETS exposure at home.
These results are consistent with reports of decreased ETS exposure among nonsmokers in the U.S. population, based on several analyses using National Health and Nutrition Examination Survey (NHANES) cotinine data.7–8
Pirkle et al. reported that serum cotinine levels in nonsmokers decreased by 70% between 1988 and 2002.8
Chen et al. analyzed NHANES cotinine data collected from 2001 – 2006 and concluded that the previously observed declining trend in ETS exposure has leveled off.7
A Centers for Disease Control report stated the prevalence among nonsmokers of serum cotinine levels ≥ 0.05 ng/mL decreased from 52.5% in 1999–2000 to 40.1% in 2007–2008, and that the greatest change was observed between the periods 1999–2000 and 2001–2002.14
The current study included this time period in which greatest change was observed in the NHANES data.
In general, reductions in ETS exposure are thought to be due to decreases in the prevalence of smoking, implementation of non-smoking policies and laws for workplaces and public indoor spaces, individual household bans on indoor smoking, and a general shift in social norms towards the unacceptability of exposing nonsmokers, especially children or frail individuals, to ETS.15
In the present study, the decreased odds of exposure at the later time period, independent of age and other covariates, likely reflects such changes in cultural norms in this population from south central Wisconsin, although we had no data to test this directly. It is likely that ETS exposure levels, especially in public places and workplaces, have decreased after this period due to a Wisconsin statewide ban on smoking in enclosed public places and workplaces, which took effect on July 5, 2010. Supporting this possibility, a recent study from the Survey of the Health of Wisconsin reported that participants interviewed after July 5, 2010 were less likely to report ETS exposure and more likely to report smoking bans in their households, compared to those interviewed before July 5, 2010.16
In particular, they observed greater differences among older participants, for both home and work ETS, suggesting that policy changes may have led to subsequent reductions in ETS exposure among older Wisconsin adults. Our longitudinal data, with repeated assessments in the same people, are consistent with their findings.
Interpretation of the current results should take into account some strengths and limitations of the study. A potential limitation is that exposure data was self-reported. Cotinine measures would have provided an objective measure. However, previous analyses comparing self-reported exposure and cotinine levels in a subset of this cohort indicated that self-reported ETS exposure was useful for categorizing relative exposure.13
In addition, cotinine level represents exposure over only the previous 2 to 3 days, whereas the participants in this study self-reported on their usual exposure, which is more relevant for long-term health outcomes such as cancer and heart disease. Cotinine levels are also unable to provide information about the specific settings in which ETS exposure occurs, which is important for potential interventions, and is attainable through self-reported questionnaires. Our cotinine-validated questionnaire and classification scheme for ETS exposure accounted for exposure in each of three specific settings, which is important because neglecting exposure outside the home may lead to underestimation of total ETS exposure.17
This is a large, prospective, population-based cohort with consistently high participation rates. The current analyses were limited to those with data at both time points in order to examine change in ETS among the same group of individuals, unlike the NHANES design which utilizes serial cross-sectional samples. However, the vast majority of this population is non-Hispanic white, and results may not be generalizable to minority groups.