Second-Hand Smoke and Dementia

To the Editor:

China is the world’s largest producer and consumer of tobacco. In 1996, a national survey showed that among those over the age of 15, 67% of Chinese men but only 4% of Chinese women had ever smoked.¹ Among women nonsmokers, 54% were regularly exposed to second-hand smoke. More recent data indicate that these estimates have not improved; Chinese women smoke less than men but remain heavily exposed to second-hand smoke.² Previous studies have suggested possible effects of second-hand smoke on dementia.³ No study has examined this among women, who may be at higher risk of developing neurological disorders.⁴

We examined data from a large-scale population-based health survey in older people across five provinces in China to determine the association between second-hand smoke exposure and dementia in women. The methods of sampling, interviewing, and dementia diagnosis in the five-province study have been described in previous publications,⁵ and in the electronic Appendix https://links.lww.com/EDE/A688. Briefly, we carried out a third-wave interview in 2007–2009 for surviving cohort members (65 years and older) of the Anhui study in the urban and rural areas of Anhui province. In addition, we recruited a minimum of 500 people aged 60 years and older in one urban and one rural area of each of four other provinces (Guangdong, Heilongjiang, Shanghai, and Shanxi). The main interview included questions on general health and risk factors (derived partly from the Scottish Multinational Monitoring of Trends and Determinants in Cardiovascular Disease [MONICA] surveys⁶ and with detailed information on second-hand smoke exposures).⁷ We diagnosed dementia cases using the 10/66 algorithm dementia research package,⁸ which has been widely used and validated in low- and middle-incomes countries.⁸

We selected female participants aged ≥65 years who had never smoked, in order to be consistent with the 4-province data (eFigure, https://links.lww.com/EDE/A688). Among 2368 women who had never smoked, 782 (33%) were exposed to second-hand smoke (SHS) and 150 (6%) had dementia. The adjusted odds ratio for dementia was increased with SHS exposure at home (1.53 [95% confidence interval (CI) = 1.08–2.15]), at work (2.65 [1.55 – 4.52]), and elsewhere (1.98 [1.14 – 3.43]). The analysis for the combined SHS exposure from three sources showed a dose–response association with risk of dementia (Table). The data for exposure duration showed similar trends. Combining duration and level of exposure, there was a strong dose–response association (trend P < 0.001) (Table).

TABLE: Odds Ratio of Dementia in Women Who Do Not Smoke Lifelong in the Five-Province Study, China

In addition to increasing study power, the Anhui data helped us explore the possibility that onset of dementia made older women more prone to second-hand smoke exposure (reverse causality). Among the 846 never-smoking women in the Anhui cohort study, 68 were the Geriatric Mental State - Automated Geriatric Examination for Computer Assisted Taxonomy (GMS-AGECAT) “dementia” cases at baseline interview in 2001–2003, 101 were subcases, and 677 were classed as not having dementia. The prevalence of second-hand smoke exposure in 2007–2009 was 29%, 36%, and 37%, respectively. Thus, reverse causality with second-hand smoke exposure resulting from the onset of dementia seems unlikely.

The finding of a dose–response relationship between second-hand smoke exposure and dementia in women, with no evidence of reverse causality, suggests that second-hand smoke should be considered a possible risk factor for dementia in women.

ACKNOWLEDGMENTS

We thank the participants and all who were involved in the surveys. Martin Prince helped the study by providing the 10/66 dementia diagnostic algorithm, which has been validated in China.

Ruoling Chen

Division of Health and Social Care Research

King’s College London

London, United Kingdom

[email protected]

Zhi Hu

Dongmei Zhang

Ying Ma

Department of Community Health and Health Services

School of Health Administration

Anhui Medical University

Hefei, Anhui, China

Li Wei

Department of Practice and Policy

University College London

London, United Kingdom