Association of Passive Smoking by Husbands with Prevalence of Stroke among Chinese Women Nonsmokers

Xianglan Zhang1, Xiao Ou Shu1 , Gong Yang1, Hong Lan Li2, Yong Bing Xiang2, Yu-Tang Gao2, Qi Li2 and Wei Zheng1

1 Department of Medicine and Vanderbilt-Ingram Cancer Center, Vanderbilt University, Nashville, TN.
2 Department of Epidemiology, Shanghai Cancer Institute, Shanghai, China.

Received for publication May 12, 2004; accepted for publication August 16, 2004.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Growing evidence suggests that exposure to environmental tobacco smoke may have deleterious cardiovascular effects. Few studies have investigated environmental tobacco smoke exposure in relation to stroke. The authors examined the association between smoking by husbands and stroke prevalence among women nonsmokers in Shanghai, China, where two thirds of men but few women smoke. They analyzed baseline survey data (1997–2000) from a population-based cohort study, the Shanghai Women’s Health Study. A total of 60,377 eligible women (aged 40–70 years) were included in the analysis. Information on husbands’ smoking status and history of physician-diagnosed stroke was obtained through in-person interviews. Logistic regression was used to compute odds ratios for the associations. There were 526 prevalent cases of stroke reported. The adjusted odds ratios of stroke in women associated with husbands’ current smoking of 1–9, 10–19, and ≥20 cigarettes per day were 1.28 (95% confidence interval: 0.92, 1.77), 1.32 (95% confidence interval: 1.01, 1.72), and 1.62 (95% confidence interval: 1.28, 2.05), respectively (p for trend = 0.0002). Prevalence of stroke also increased with increasing duration of husbands’ smoking. The authors found that women nonsmokers who lived with husbands who smoked had an elevated prevalence of stroke, and prevalence increased with increasing intensity and duration of husbands’ smoking.

cerebrovascular accident; tobacco smoke pollution; women


Abbreviations: CI, confidence interval; ETS, environmental tobacco smoke; OR, odds ratio.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Active cigarette smoking has been established as a major risk factor for coronary heart disease and stroke (both ischemic and hemorrhagic stroke) (15). More recently, a growing body of evidence suggests that exposure to environmental tobacco smoke (ETS), or passive smoking, may also have deleterious cardiovascular effects (612). Many epidemiologic studies have linked ETS exposure to an increased risk of coronary heart disease (13). Few studies, however, have investigated the association between ETS exposure and stroke. Moreover, many published studies that have addressed this issue included current or/and former active smokers and thus could not rule out the potential confounding effect due to active smoking (1416).

Stroke is the leading cause of death and disability in China, where active smoking and thus exposure to ETS are widespread (1719). About two of every three Chinese adult men are regular smokers, whereas very few Chinese women smoke (19). Studies of the major health consequences of both active and passive smoking in China, a country where the population smokes the most cigarettes in the world and where the tobacco epidemic is worsening (19), have important implications. By using the comprehensive baseline survey data collected in the Shanghai Women’s Health Study, we examined the association between husbands’ cigarette smoking status and stroke prevalence among middle-aged and older Chinese women who had never smoked.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Study population and procedures
The Shanghai Women’s Health Study, initiated in March 1997, is a population-based cohort study of women who were 40–70 years of age and living in seven urban communities of Shanghai, China. The study was approved by relevant institutional review boards for human research in Shanghai and the United States. Of the 81,170 eligible women identified from the Shanghai Resident Registry, 2,407 (3.0 percent) refused to participate in the study, 2,073 (2.6 percent) were not available during the baseline recruitment period, and 1,469 (1.8 percent) were not enrolled for other miscellaneous reasons such as mental disorder. The remaining 75,221 women were recruited and completed the baseline survey during 1997–2000, yielding a participation rate of 92.7 percent. After exclusion of those who were found to be outside of the study’s age range at the time of interview, the final cohort consisted of 74,943 women.

The baseline survey was conducted at participants’ homes by trained interviewers using a structured questionnaire designed to collect information on demographic characteristics, lifestyle habits, and medical history. Anthropometric measurements, including current weight, height, and circumferences of the waist and hips, were also taken by using a standard protocol. All interviews were tape recorded and were selectively checked by quality control staff to monitor the quality of the interview data.

The questionnaire included, among other items, questions on participants’ marital status, smoking history, alcohol consumption, medical history, and use of medications such as aspirin and antihypertensives. For married women, the husbands’ demographic information, lifestyle (including smoking and drinking habits), and medical conditions were also recorded, and husbands were asked to help answer these questions whenever possible.

Smoking status of the husband was assessed by asking the following five questions: 1) "Has your husband ever smoked regularly at least one cigarette per day for at least 6 months?" 2) "At what age did your husband start smoking?" 3) "Usually, how many cigarettes a day does your husband smoke?" 4) "Does your husband smoke regularly now?" and 5) "At what age did your husband stop smoking?" A history of stroke was ascertained by asking this question: "Have you ever had a stroke diagnosed by a physician?"

A set of questions similar to those used to assess husbands’ smoking history was used to obtain information regarding study subjects’ own smoking habits. Only 2,113 (2.8 percent) women had ever smoked. Of the 97.2 percent of women who had never smoked, 654 (0.9 percent) had never been married, 5,233 (7.2 percent) were widowed, 774 (1.1 percent) were living separately from their husbands, and 1,287 (1.8 percent) were divorced. For 50 women nonsmokers who were married and were currently living with their husbands, data on the husbands’ smoking status were missing. All of these women were excluded from the present study. Because coronary heart disease is associated with an increased risk of stroke (20), and a diagnosis of coronary heart disease is likely to affect smoking status and passive exposure to smoking, we further excluded 4,455 women who reported a history of coronary heart disease. Therefore, the final study population comprised 60,377 women.

Statistical analysis
Women were classified into three categories according to their husbands’ smoking status: never smokers, former smokers, and current smokers. Those married to a husband who had never smoked were treated as the reference group. Logistic regression was used to calculate the odds ratios of stroke and the 95 percent confidence intervals associated with husbands’ passive smoking. Potential confounding factors adjusted for in multivariate analyses included age, level of education, usual occupation, family income, regular alcohol consumption (ever drank at least three times a week for at least 6 months), exercise (at least once a week for at least 3 months), body mass index (weight in kilograms divided by the square of height in meters), menopausal status, hormone therapy, oral contraceptive use, history of hypertension and diabetes mellitus, and use of antihypertensive medication and aspirin. We further evaluated the dose-response relation between husbands’ current smoking and stroke prevalence based on the intensity (number of cigarettes smoked per day) and the duration (number of years of smoking) as well as the measure of both—pack-years of smoking (number of packs of cigarettes per day multiplied by the number of years of smoking)—since they were married. The significance of the linear trend was tested by treating a categorical variable as an ordinal variable in the regression model. All p values are two sided. Statistical analyses were performed by using SAS version 8.2 software (SAS Institute, Inc., Cary, North Carolina).


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Among 60,377 lifelong women nonsmokers included in this study, 53.5 percent were living with a husband who was a current smoker and 8.5 percent with a former smoker. Demographic characteristics, lifestyles, and cardiovascular risk factors for these women according to their husbands’ smoking status are presented in table 1. Compared with those married to a never smoker, women living with a current smoker were younger; were less educated; were less likely to have a professional, technical, or administrative position; and had a lower family income. They were less likely to be postmenopausal or use oral contraceptives and hormones. In addition, they were more likely to ever have consumed alcohol but less likely to exercise or have a history of hypertension or diabetes. Women married to a former smoker had characteristics similar to those of women married to a current smoker except that they were older and were more likely to have hypertension or diabetes and to exercise.


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TABLE 1. Characteristics of women nonsmokers according to their husbands’ cigarette smoking status, Shanghai Women’s Health Study, China, 1997–2000
 
There were 526 prevalent cases of stroke reported by the study subjects. After we adjusted for age, the odds ratio of stroke was significantly higher for women living with a husband who was a current smoker (odds ratio (OR) = 1.47, 95 percent confidence interval (CI): 1.22, 1.78) but not for women whose husband was a former smoker (OR = 1.03, 95 percent CI: 0.79, 1.35) compared with women married to a never smoker (table 2). Further adjustment for potential confounders, including socioeconomic and lifestyle factors as well as cardiovascular risk factors, led to little change in the results. The odds of stroke increased with increasing number of cigarettes the husbands smoked per day (table 3). Compared with women whose husbands had never smoked, those whose husbands were current smokers of 1–9, 10–19, and 20 or more cigarettes per day had odds ratios of stroke of 1.28 (95 percent CI: 0.92, 1.77), 1.32 (95 percent CI: 1.01, 1.72), and 1.62 (95 percent CI: 1.28, 2.05), respectively (p for trend = 0.0002), after we controlled for potential confounding factors. Similarly, the odds increased with increasing duration of husbands’ smoking (OR = 1.13, 95 percent CI: 0.70, 1.82 for ≤17 years and OR = 1.47, 95 percent CI: 1.22, 1.78 for >17 years; p for trend = 0.0004) and pack-years of smoking (OR = 1.12, 95 percent CI: 0.82, 1.54 for ≤13 pack-years and OR = 1.55. 95 percent CI: 1.27, 1.90 for >13 pack-years; p for trend < 0.0001) (table 3).


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TABLE 2. Odds ratios of stroke for women nonsmokers according to their husbands’ smoking status, Shanghai Women’s Health Study, China, 1997–2000
 

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TABLE 3. Odds ratios of stroke for women nonsmokers according to the intensity and duration of husbands’ smoking,* Shanghai Women’s Health Study, China, 1997–2000
 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In this large population-based study, we found that husbands’ current smoking was significantly associated with increased prevalence of stroke among Chinese women nonsmokers. The association was independent of socioeconomic and lifestyle factors as well as established risk factors for stroke. The more cigarettes and the longer that husbands smoked, the higher the prevalence of stroke among women. Women never smokers living with a husband who was a current heavy smoker (≥20 cigarettes per day) had a 62 percent greater probability of suffering a stroke than women never smokers living with a husband who never smoked.

The potentially harmful health effects of ETS have received considerable attention in recent years. However, few studies have investigated exposure to ETS in association with stroke, and those that have have yielded inconsistent results (1416, 2123). Despite different study designs, one cohort study (21) and one case-control study (16) found a positive association of stroke with ETS among women nonsmokers, and the strength of the association was comparable to our findings. In contrast, a recent case-control study showed no association (22), while a cross-sectional study reported an inverse association (23). One study of spousal smoking showed a positive association, but the association was weakened and no longer statistically significant after the analysis was confined to subjects who had never smoked, suggesting the possibility of confounding by the subjects’ own active smoking (14). Misclassification is thought to be a key issue in studies of ETS and may explain some of these discrepancies (24).

Exposure to ETS may increase the risk of stroke through several mechanisms similar to active smoking. Both active smoking and ETS exposure have been associated with increased carotid artery intimal-medial thickness, intimal-medial-thickness progression, and arterial wall stiffness (8, 9, 25). They were also found to be associated with elevated levels of C-reactive protein, fibrinogen, homocysteine, and oxidized low density lipoprotein cholesterol, increased platelet aggregability, and impaired endothelial function (6, 7, 1012, 2628).

Some methodological limitations need to be kept in mind when interpreting these study results. Since exposure and outcome were ascertained at one time point, the temporal sequence could not be determined. As with many epidemiologic studies, we used the presence of a husband who smoked as a surrogate for passive smoking among women, which might have led to underestimation of the strength of the association between ETS and stroke (24). Solely relying on self-reporting of husbands’ smoking status is subject to misclassification. Nevertheless, the proportion of never, current, and former smokers reported in this study was similar to that reported previously in a cohort study of men in Shanghai (17), indicating that the degree of misclassification should not be a major concern. Moreover, a national survey showed that awareness of the hazards of smoking and, in particular, ETS was low in the Chinese population (19); thus, differential reporting, if any, would be expected to be minor.

Another limitation of this study is that stroke history was self-reported. However, since stroke is a medical emergency and likely causes disability, the potential error on reporting a physician-diagnosed, broadly defined stroke (without classification of subtypes) should not be large. In addition, stroke was ascertained in this study through an in-person interview conducted by retired health professionals, thus minimizing outcome misclassification. Although we could not completely exclude the possibility that some unknown sociocultural factors might have influenced reporting of stroke outcomes, we are not aware of any social stigma related to the diagnosis of stroke.

Failure to investigate the subtypes of stroke is a major limitation. It has been shown that ischemic stroke is the predominant subtype (62.4 percent) in the Chinese population, as it is in Western populations, although hemorrhagic stroke was found to be more frequent in Chinese than in Western populations (18). Hemorrhagic stroke is more likely than ischemic stroke to be underrepresented in the study population because of the higher fatality rate of the former. It is possible that ETS may have different effects depending on the type of stroke and the different mechanisms through which each occurs. Prospective studies with specific information on types of stroke are needed to test this hypothesis.

Another concern is that other sources of household air pollutants may confound the association under study. However, additional adjustment in the analysis for the types of cooking fuel and cooking oil as well as the ventilation conditions of the kitchen did not change the results materially (data not shown). Unmeasured or incompletely controlled confounding is also a concern, although careful adjustment for socioeconomic status and other potential confounding factors led to no major changes in the results.

Finally, the findings of this study may not be directly extrapolated to women other than those in the population studied. Nevertheless, the knowledge gained is very relevant to many developing countries, where little effort has been expended on protecting women from passive smoking.

The strengths of this study include a population-based design with a high participation rate, a very large sample size (the largest known survey to date on passive smoking and stroke), and the use of in-person interviews. Another important advantage is that we evaluated the dose-related associations with both the intensity and the duration of exposure. In addition, low active smoking rates among women and high active smoking rates among men enabled us to evaluate ETS without confounding from active smoking. The high rate of stable marriage facilitated exposure assessment and investigation of long-term exposure.

In conclusion, we found in this large population-based study that women nonsmokers who lived with a husband who smoked had an elevated prevalence of stroke, and prevalence increased with the number of cigarettes and the duration of time that their husbands smoked. Education about the health consequences of tobacco use and exposure to ETS is urgently needed in China, a country with more than 300 million smokers (19).


    ACKNOWLEDGMENTS
 
This research was supported by National Institutes of Health grant R01CA70867.


    NOTES
 
Correspondence to Dr. Xiao Ou Shu, Center for Health Services Research, 6009 Medical Center East, Vanderbilt University, 1215 21st Avenue South, Nashville, TN 37232-8300 (e-mail: Xiao-Ou.Shu{at}Vanderbilt.edu). Back


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 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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