Cigarette smoking is the most significant alterable factor contributing to both morbidity and mortality in humans. Reference Rigotti, Kruse, Livingstone-Banks and Hartmann-Boyce1 Smoking is particularly perilous among older adults, since it is associated with an elevated risk of various health issues in this demographic group, including dental caries, periodontitis, various forms of cancer, frailty, cognitive impairment and an increased likelihood of mortality. Reference Chu, Nishita and Tange2 Tobacco smoking is responsible for approximately 7.69 million fatalities and 200 million disability-adjusted life-years (DALYs), accounting for 13.6% of total global mortality and 7.89% of all DALYs.
Second- and third-hand smoking have recently emerged as important areas of interest for contemporary research. Evidence has shown that exposure to second-hand smoke is linked to an increased risk of respiratory symptoms, asthma, chronic obstructive pulmonary disease, cognitive impairment and susceptibility to tuberculosis infection. Reference Korsbæk, Landt and Dahl3 In 2017, home-based second-hand smoke exposure was responsible for 526 000 DALYs, equivalent to 0.36% of total DALYs, and 24 000 deaths, accounting for 0.46% of total deaths, across 28 countries in the European Union. Reference Carreras, Lachi and Cortini4
Mental health conditions are a significant concern in the healthcare sector, particularly in the older adult population; depression is one of the most prevalent mental health conditions in this population. A report from the UK highlighted that the prevalence of depression among older adults was 8.7%, which increased to 9.7% when considering individuals with concurrent dementia. Reference McDougall, Kvaal and Matthews5 Depression in older adults is correlated with a decline in physical care, reduced social participation and increased incidence of suicidal ideation and behaviour. Reference Kim, Kim, Lee, Choi and Roh6
Recent reports have shown an association between second-hand smoke exposure and depressive symptoms, although most of these reports were cross-sectional studies. Reference Patten, Williams and Lavorato7 A 10-year prospective study in the USA revealed that exposure to second-hand smoke in childhood and adulthood was associated with increased risks of depression and panic attacks in adulthood. Reference Taha and Goodwin8 A systematic review that included eight studies described positive associations between depression and second-hand smoke exposure, in both private and public places, among non-smoking children and adolescents in the majority of the studies. Reference Christian and Kim9
However, other studies have reported contradictory results. A recent cross-sectional study using data from the Korean National Health and Nutrition Examination Survey reported no significant association between self-reported exposure to second-hand smoke and self-reported depressive symptoms. Reference Gim, Yoo, Shin and Goo10 A prospective cohort study in the UK revealed no discernible association between second-hand smoke exposure and the prevalence of mental health problems. Reference Lam, Kvaavik, Hamer and Batty11 Similarly, a study conducted in The Netherlands that analysed data from two cohort studies found no significant link between exposure to second-hand smoke and depression. Reference Bot, Vink and Willemsen12 Erdsiek and Brzoska conducted a large survey in Germany and found no association between second-hand smoke exposure and depression; their findings even showed a protective effect among men. Reference Erdsiek and Brzoska13
Several biological mechanisms may explain the associations among smoking, second-hand smoke exposure and depressive symptoms. Nicotine-induced neurotransmitter dysregulation is a key factor, because chronic nicotine exposure alters the dopamine and serotonin systems, which play crucial roles in mood regulation. Reference Laviolette14 Chronic inflammation is another proposed mechanism, because tobacco smoke contains numerous pro-inflammatory chemicals that can trigger systemic inflammation. Elevated inflammatory markers, such as C-reactive protein and interleukins, have been linked to an increased risk of depression. Reference Glaus, Vandeleur and von Känel15 Additionally, oxidative stress, caused by an imbalance between free radicals and antioxidants, has been associated with both smoking and mood disorders. Smoking generates oxidative damage in neural tissues, potentially contributing to neurodegeneration and mood disturbances. Reference Moylan, Jacka, Pasco and Berk16 These mechanisms highlight the potential physiological pathways through which smoking and second-hand smoke exposure may contribute to the development of depressive symptoms.
Thus, the association of second-hand smoke exposure with depressive symptoms requires elucidation. Therefore, we conducted this 20-year longitudinal prospective cohort study, first to evaluate the longitudinal relationship of cigarette smoking and second-hand smoke exposure with the subsequent incidence of depressive symptoms among community-dwelling middle-aged and older adults in Japan, and second to answer the following important questions: (a) Is cigarette smoking associated with future depressive symptoms? (b) Are cigarette smoking and exposure to second-hand smoke associated with depressive symptoms? (c) Does second-hand smoke exposure increase the risk of depressive symptoms among smokers?
Method
Participants
The data for this study were sourced from the National Institute for Longevity Sciences – Longitudinal Study of Aging (NILS-LSA), a prospective cohort study based in Japan that was established by the National Center for Geriatrics and Gerontology (NCGG) in 1997. The primary aim of this study was to meticulously observe the natural ageing process and identify the factors contributing to longevity. Initially, the study recruited 2267 randomly selected men and women aged 40–79 years from the towns of Obu and Higashiura in Aichi Prefecture, Japan. From the first to the seventh waves (1997–2012), participants underwent biennial follow-up assessments, which included a range of examinations encompassing collection of general information, medical assessments, physical fitness evaluations, nutritional surveys and psychological tests. When participants aged ≤79 years were unable to attend these follow-up surveys, new age- and gender-matched participants were recruited from the same residential areas. Additionally, to maintain the age diversity of the cohort, new participants aged 40 years were included in each wave from the second to the seventh. The eighth wave (2013–2016) and ninth wave (2018–2022) of the study featured examinations of a more limited scope, conducted among all participants from the first to the seventh waves. All participants provided written informed consent, and the study protocol was approved by the Committee on Ethics of Human Research of the NCGG (approval no. 1665-3). Comprehensive details of this study have previously been documented. Reference Shimokata, Ando and Niino17
Participant data for this study were collected from the third to the ninth waves (2002–2022). Of the 2378 initial participants (third wave, baseline), those who met any of the following criteria were excluded: (a) participants lacking information on depressive symptoms and those already exhibiting depressive symptoms at baseline (n = 292); (b) participants without data from the fourth to the ninth wave (n = 254); and (c) participants with missing data regarding smoking habits, second-hand smoke exposure and other covariates from the third to the ninth wave (n = 135). On the basis of these inclusion and exclusion criteria, the study ultimately included 1697 participants. The mean (s.d.) age of these participants was 58.7 (11.2) years (Fig. 1).
Fig. 1 Flow chart of the study.
Measurements
Cigarette smoking and second-hand smoke exposure
Data on cigarette smoking habits and exposure to second-hand smoke were collected using a self-administered questionnaire in wave 3. Participants were asked to respond to the question: ‘Do you currently smoke cigarettes?’, with one of three possible answers: ‘never’, ‘quit’ or ‘currently smoking’. For this analysis, responses were consolidated by grouping non-smokers and those who had quit smoking into a single, non-current smoker category. To assess second-hand smoke exposure, participants were asked two specific questions; indoor exposure was determined by asking: ‘Does anyone in your family currently smoke?’, while outdoor exposure was assessed with the question: ‘Does anyone in the office, transportation or conference room currently smoke?’ The response options provided were ‘no’, ‘sometimes’ and ‘almost every day’. Participants who answered ‘no’ to both queries were classified as not having been exposed to second-hand smoke. Conversely, participants who answered ‘sometimes’ or ‘almost every day’ to either question were categorised as having been exposed to second-hand smoke.
Depressive symptoms
Depressive symptoms were assessed using the Japanese version Reference Shima18 of the Center for Epidemiologic Studies Depression (CES-D) scale. Reference Radloff19 This scale is known for its credibility and dependability in gauging symptoms of depression, especially in the elderly population. Reference Berkman, Berkman and Kasl20 The questions are introspective in nature, with examples such as ‘I felt depressed’ and ‘I thought my life had been a failure’. Participants answered the questionnaire while evaluating their emotional states over the previous week. The CES-D includes 20 items with total scores ranging from 0 to 60, with higher scores indicating more severe depressive symptoms. A cut-off score of ≥16 was used to identify participants with relevant depressive symptoms. Reference Radloff19,Reference Hann, Winter and Jacobsen21 Incident depressive symptoms in during waves four to nine were documented.
Other variables
At baseline, data on the level of education (years), marital status (married, others), history of chronic disease (hypertension, hyperlipidaemia, heart disease and diabetes; yes or no) and participation in social activities were collected using a self-administered questionnaire. Social activity participation was defined by asking: ‘How often do you participate in social activity in the community or voluntary work?’ The answers included ‘rarely’, ‘several times a year’, ‘two or three times a month’ and ‘once a week’. Participants who reported engaging in social activities ‘two or three times a month’ or ‘once a week’ were categorised as participating in social activities.
Alcohol consumption was quantified in grams per day using a semi-quantitative food frequency questionnaire. Reference Sasaki, Kobayashi, Ishihara and Tsugane22 Additionally, the level of leisure-time physical activity over the preceding year was evaluated through interviews. Leisure-time physical activity was assessed based on a modified version of the Minnesota Leisure – Time Physical Activity Questionnaire, Reference Kozakai, Doyo and Tsuzuku23 and metabolic equivalents (METs; METs × h/h) were then calculated.
Statistical analyses
The distributions of gender, smoking status, marital status, history of chronic diseases and social activity participation are represented as N (percentage). Variables including age, level of education, alcohol consumption and leisure-time physical activity are reported as mean values with s.d. In this study, baseline smoking and second-hand smoke exposure data were collected during the third wave while depressive symptoms were assessed at each follow-up wave from the fourth to the ninth. This longitudinal design allowed for repeated assessments of depressive symptoms over time, providing a comprehensive evaluation of their association with smoking exposure. Generalised estimating equations (GEEs) were used to examine the longitudinal relationships between cigarette smoking and second-hand smoke exposure at baseline in wave three, and subsequent incident depressive symptoms in waves four to nine. These analyses were adjusted for a range of variables, including baseline information on age and gender (model 1); age, gender, years of education, marital status and alcohol consumption (model 2); age, gender, years of education, marital status, leisure-time activity and history of chronic disease (model 3); and age, gender, years of education, marital status, leisure-time activity, social activity participation, CES-D score at baseline and history of chronic disease (model 4). GEE models are particularly advantageous in this context because they facilitate the analysis of cumulative longitudinal data considering repeated observations and follow-up years within the same participants. This approach was critical in identifying general patterns across the entire study period. The GEE model is a well-established method in this field of research, especially in studies with longitudinal designs involving repeated measurements. Reference Chu, Tange and Nishita24
Statistical analyses were performed using SAS System version 9.3 for Windows (SAS Institute Inc., Cary, NC, USA). GEE models were fitted using the GENMOD procedure in SAS software. Statistical significance was set at a two-tailed P-value of <0.05.
Results
The final analysis included 1697 participants (mean (s.d.) age, 58.7 (11.2) years; mean follow-up period, 12.9 years). Table 1 shows the basic characteristics of all participants at baseline: men (51.6%), married (88.6%) and current smokers (18.7 %). The mean number of years of education was 12.4, and mean leisure time was 1.68 METs × h/h. The incidence of depressive symptoms ranged from 8.0% (wave four) to 6.5% (wave nine), and that of depressive symptoms in each wave is shown in Supplementary Table 1 available at https://doi.org/10.1192/bjp.2025.83.
Table 1 Demographic and clinical characteristics of participants of varying smoking status at baseline
CES-D, Center for Epidemiologic Studies Depression Scale; METs, metabolic equivalents.
Table 1 also shows the demographic and clinical characteristics of participants of varying smoking status. The distributions of age, gender, leisure-time activity, marital status, social activity participation, alcohol consumption and chronic diseases such as hypertension and hyperlipidemia were significantly different among participants with varying smoking status at baseline; 64.4% of participants reported to have second-hand smoke exposure, which is shown in Supplementary Table 2. Demographic and clinical characteristics between participants with or without depressive symptoms at wave three are given in Supplementary Table 3.
Table 2 shows the associations of cigarette smoking and second-hand smoke exposure with incident depressive symptoms using a multivariate GEE analysis. After adjusting for age, gender, years of education, marital status, social activity participation, leisure-time activity, hypertension, stroke, heart disease, diabetes, hyperlipidaemia and depressive symptoms at baseline (model 4), smoking was not significantly associated with a higher risk of depressive symptoms (odds ratio: 1.27; 95% CI: 0.96–1.68). However, cigarette smokers with second-hand smoke exposure showed a significantly (P < 0.05) higher risk of depressive symptoms (odds ratio: 1.50; 95% CI: 1.05–2.14) than non-smokers without second-hand smoke exposure.
Table 2 Multivariate generalised estimating equations analysis of the association of cigarette smoking and second-hand smoke exposure with incident depressive symptoms
OR, odds ratio; Ref, reference (odds ratio 1).
a. Adjusted for age and gender.
b. Adjusted for age, gender, years of education, marital status and alcohol consumption.
c. Adjusted for age, gender, years of education, marital status, leisure-time activity, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
d. Adjusted for age, gender, years of education, marital status, social activity, leisure-time activity, Center for Epidemiologic Studies Depression Scale score at baseline, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
Table 3 shows the negative association of second-hand smoke exposure for cigarette smokers. In comparison with smokers who were not exposed to second-hand smoke, smokers with both indoor and outdoor second-hand smoke exposure showed a higher risk of developing depressive symptoms (odds ratio: 1.39; 95% CI: 1.00–1.94 in multivariate analysis (model 4)).
Table 3 Multivariate generalised estimating equations analysis of the association of indoor and outdoor second-hand smoke exposure with incident depressive symptoms among cigarette smokers (N = 318)
OR, odds ratio; Ref, reference (odds ratio 1).
a. Adjusted for age, gender, and smoking status.
b. Adjusted for age, gender, years of education, marital status and status of both alcohol consumption and smoking.
c. Adjusted for age, gender, years of education, marital status, leisure-time activity, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
d. Adjusted for age, gender, years of education, marital status, social activity participation, leisure-time activity, Center for Epidemiologic Studies Depression Scale score at baseline, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
Table 4 shows the results of subgroup multivariate GEE analysis of the association of cigarette smoking with depressive symptoms among participants of varying age (<65/≥ 65 years) and gender. The association of cigarette smoking with the incidence of depressive symptoms was observed only among male participants, with an odds ratio and 95% CI of 1.40 and 1.00–1.94, respectively, in multivariate analysis (model 4). In regard to the analysis among different age groups, participants ≥65 years of age showed a greater risk of developing depressive symptoms if they were cigarette smokers (odds ratio: 1.62; 95% CI: 1.00–2.63; model 4).
Table 4 Subgroup analysis of association with cigarette smoking and depressive symptoms among participants of varying age and gender by multivariate generalised estimating equations analysis
OR, odds ratio; Ref, reference (odds ratio 1).
a. Adjusted for age and gender.
b. Adjusted for age, gender, years of education, marital status and alcohol consumption.
c. Adjusted for age, gender, years of education, marital status, leisure-time activity, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
d. Adjusted for age, gender, years of education, marital status, social activity, leisure-time activity, depressive symptoms at baseline, hypertension, stroke, heart disease, diabetes and hyperlipidaemia.
Discussion
To the best of our knowledge, this is the first study to use longitudinal data to explore the relationship of cigarette smoking and second-hand smoke exposure with depressive symptoms in middle-aged and older Japanese adults. Our findings indicate that the combination of cigarette smoking and second-hand smoke exposure is associated with a higher incidence of depressive symptoms, and that second-hand smoke exposure is also associated with incidence of depressive symptoms among smokers.
The results of this study are partially consistent with those of previous studies. Patten et al reported that the risk of major depressive episodes among non-smokers with second-hand smoke exposure was 1.4 times that in non-smokers without second-hand smoke exposure. Reference Patten, Williams and Lavorato7 A Korean study with a large sample showed that household second-hand smoke exposure was significantly associated with depression among male and female participants. Reference Park25 A 10-year longitudinal study in the USA demonstrated that exposure to second-hand smoke in both childhood and adulthood was associated with an increased incidence of major depressive disorder in adulthood, with an odds ratio of 1.8 after adjustment for confounding factors. Reference Taha and Goodwin8 However, another study in Korea found no significant association between second-hand smoke exposure and depressive symptoms. Reference Gim, Yoo, Shin and Goo10 Lam et al observed a non-significant association between objectively measured second-hand smoke exposure and poor mental health, with only smokers showing an increased risk of psychological distress, Reference Lam, Kvaavik, Hamer and Batty11 which is different from our study results. In comparison with previous studies, our study highlights the combined influence of cigarette smoking and second-hand smoke exposure, and we also analysed the association between second-hand smoke exposure and depressive symptoms among smokers. We believe that our results provide a broader understanding of the association between second-hand smoke exposure and depressive symptoms.
The mechanisms underlying the association between exposure to second-hand smoke and depressive symptoms may be multifactorial. First, people who are exposed to second-hand smoke may have lower socioeconomic status, Reference Gagné, Lapalme, Ghenadenik, Jl and Frohlich26 which could lead to depressive symptoms. A previous systematic review suggested that poor socioeconomic conditions are associated with higher odds of depression. Reference Richardson, Westley, Gariépy, Austin and Nandi27 Another study also illustrated that low socioeconomic groups were more affected by anxiety and depressive symptoms. Reference Azizabadi, Aminisani and Emamian28 In our study, although we adjusted for the level of education and social activity participation, which could be partially related to socioeconomic status, future studies are warranted to explore the mediating role of socioeconomic status on the relationship between second-hand smoke exposure and depressive symptoms. Second, second-hand smoke contains numerous chemicals, including nicotine, which can disrupt the central nervous system and cause chronic inflammation of the vascular system. Reference Adams, Wan and Wei29 These disruptions can lead to depressive symptoms. Nicotine exposure may lead to persistent alterations in the dopamine system, potentially causing a long-term imbalance in dopamine transport that can increase the risk of experiencing negative mood or depression. Reference Danielson, Putt, Truman and Kivell30 Third, second-hand smoke exposure is associated with a range of physical health issues, including respiratory and cardiovascular diseases. These health complications, along with an overall decline in well-being, may play a role in increasing psychological distress and depressive symptoms. Reference Han, Liu, Gong, Ye and Zhou31
Our findings suggest that older adults (≥65 years) who smoke are at a higher risk of developing depressive symptoms, which is consistent with previous research highlighting the vulnerability of ageing populations to smoking-related health effects. The negative association of cigarette smoking with second-hand smoke exposure in older adults is an important factor in ageing societies. A recent publication in China showed that second-hand smoke exposure is associated with higher odds of depressive symptoms in older adults, and the effect was seemingly additive. Reference Yang, Yang, Sun, Zhao, Magnussen and Xi32 As the global population ages rapidly, despite the gradual decrease in the prevalence of smoking, addressing the problems associated with second-hand smoke exposure among older adults is more important than ever. Exposure to second-hand smoke domestically varies significantly internationally, with data from the Global Adult Tobacco Survey indicating that this prevalence ranges from 17.3% in Mexico to 73.1% in Vietnam. Reference King, Mirza, Babb and GATS Collaborating Group33 Another study, in India, showed that current tobacco use among older adults was 44.6%, and that the rates of second-hand smoke exposure at home and in public places were 20.0 and 30.0%, respectively. Reference Mbulo, Murty, Zhao, Smith and Palipudi34 This increased susceptibility may be due to age-related physiological changes, including decreased nicotine metabolism, Reference Molander, Hansson and Lunell35 greater inflammatory responses Reference Hou, Chai and Zhang36 and heightened vulnerability to oxidative stress, Reference Liguori, Russo and Curcio37 all of which have been implicated in smoking-related health deterioration and depression. Additionally, older adults who smoke are more likely to experience chronic health conditions, reduced physical activity and social isolation, further compounding their risk of depression. Because the older population is at a higher risk of developing depressive symptoms, we believe that relevant public service organisations should pay more attention to investigating the prevalence of second-hand smoke exposure among older adults, and implement effective measures to mitigate the subsequent risk. Future research should explore whether interventions targeting both smoking cessation and second-hand smoke reduction in older populations could help mitigate these mental health risks.
Furthermore, our results reveal that second-hand smoke exposure is associated with an increased likelihood of depressive symptoms among smokers, a relationship that has not been extensively explored. Multiple studies have evaluated the interaction between active smoking and second-hand smoke exposure in regard to the development of lung cancer, Reference Li, Tse, Au, Wang, Qiu and Yu38 and previous reports suggest that second-hand smoke exposure, combined with the tobacco smoke that active smokers directly inhale from burning cigarettes, may work in tandem. This dual exposure has the potential to interact with, and contribute to, DNA damage and play a role in sustained inflammatory processes. Reference Schick and Glantz39,Reference Schick and Glantz40 The combined association of smoking and second-hand smoke exposure may exacerbate the risk of depressive symptoms through multiple physiological and psychological mechanisms. Lin et al demonstrated a possible synergistic interaction between ambient fine particulate matter (PM2.5) levels and smoking on depression among adults in six countries, but the interaction was not statistically significant in the multiplicative model. Reference Lin, Guo and Kowal41 Smokers exposed to second-hand smoke experience increased nicotine intake, leading to higher nicotine dependence and withdrawal symptoms, which are closely linked to mood disturbances and depression. Reference Wang, Ho, Lo and Lam42,Reference Wilson-Frederick, Williams and Garza43 Research indicates that second-hand smoke exposure is associated with higher cotinine levels among smokers, reinforcing nicotine addiction and making cessation more difficult, potentially worsening depressive symptoms. Reference Lindsay, Tsoh, Sung and Max44 Additionally, second-hand smoke exposure contributes to increased respiratory symptoms such as persistent cough, wheezing and airway inflammation, which can lead to chronic discomfort and mental distress. Reference Lam, Ho and Hedley45,Reference Lai, Ho, Wang and Lam46 These health burdens may elevate stress levels and contribute to the development of depressive symptoms, particularly in smokers who already face increased risks due to tobacco use. Furthermore, adolescent smokers exposed to second-hand smoke have a higher likelihood of medical consultations and hospitalisations, suggesting a link between second-hand smoke-related health complications and psychological distress. Reference Wang, Ho, Lo and Lam47 Our results contribute to an understanding of the combined associations of cigarette smoking and second-hand smoke exposure with depressive symptoms. Further research is required to elucidate the mechanisms underlying this association.
Gender differences in smoking behaviours, nicotine dependence and psychosocial factors may explain why male smokers have a higher risk of depressive symptoms than female smokers. Men tend to smoke more frequently and consume more cigarettes per day, leading to greater nicotine exposure and a higher likelihood of withdrawal-related depressive symptoms; Reference Chinwong, Mookmanee, Chongpornchai and Chinwong48 in contrast, women metabolise nicotine more slowly and tend to use smoking for emotional regulation, which may reduce their risk of depression. Reference Benowitz and Hatsukami49 Men’s nicotine dependence is influenced by multiple factors, including work stress and social environments, whereas women’s dependence is more closely tied to subjective feelings of addiction and environmental cues. Reference Torchalla, Okoli, Malchy and Johnson50 Additionally, women are more likely to intend to quit smoking, often due to health concerns, which may lower their long-term risk of depression. These factors suggest that higher nicotine exposure and withdrawal effects in men may contribute to their increased risk of depressive symptoms.
Strengths and limitations
This study has several strengths. First, because NILS-LSA is a regional, population-based cohort study, the study sample is representative of the broader Japanese population, enhancing the generalisability of the findings. Second, the longitudinal nature of the study, which included a 20-year follow-up period, allowed for comprehensive adjustments for a wide array of major confounding factors. This extended duration was instrumental in understanding the long-term associations and trends related to the focus areas of this study. Third, the inclusion of middle-aged participants in the analysis marks a significant departure from many previous studies that have often overlooked this demographic group. The inclusion of this group broadens the scope of this study and provides valuable insights into a population segment that is frequently underrepresented in similar research contexts.
Although this study had several strengths, its limitations also require consideration. First, the external validity of our findings should be approached with caution. This research was conducted solely among healthy, physically mobile older adults in Japan, and the findings may not reflect broader, diverse populations, particularly those with health issues or mobility constraints. Second, the lack of objective data, such as blood testing or imaging results, could be a limitation because such information often provides deeper insights into participants’ health status and could enhance the findings. Third, the study failed to accurately measure the exact cigarette consumption or cumulative pack-years of each participant. The absence of precise data on smoking intensity and duration restricted our understanding of how smoking habits correlate with the onset of depressive symptoms. Future studies exploring the dose–response relationship between cigarette smoking and depressive symptoms in older adults are essential. Such research could offer more detailed insights into the effects of different levels of smoking exposure on the mental health outcomes in this group. Fourth, we used a complete-case analysis approach to handle missing data, following the standard methodology of NILS-LSA. Nonetheless, we acknowledge that missing data may introduce selection bias, and future studies should consider alternative approaches, such as multiple imputation, to further assess the robustness of these findings.
Implications and future directions
This study highlights the significance of understanding the relationship between cigarette smoking and second-hand smoke exposure, particularly in relation to the development of depressive symptoms. These findings suggest that combined direct and indirect exposure to cigarette smoke is a pertinent health concern, especially in the context of mental health, among older adults. These risks can be mitigated by enhanced tobacco cessation programmes, public education initiatives and comprehensive public health measures aimed at reducing active smoking and exposure to second-hand smoke, especially among middle-aged and older adults. These strategies could significantly lower the prevalence of smoking-related health issues, including depressive symptoms. Furthermore, this study highlights the critical role of healthcare professionals in identifying older adults at risk of developing depressive symptoms, potentially due to cigarette smoke exposure. Therefore, early diagnosis and intervention are crucial. Healthcare providers should be proactive in advising at-risk individuals and implementing preventive measures to reduce cigarette smoke exposure. This approach will address the immediate concerns related to smoking and second-hand smoke exposure, and contributes to the broader goal of promoting mental health and well-being among older adults.
In conclusion cigarette smoking combined with second-hand smoke exposure was found to be associated with a higher incidence of depressive symptoms among middle-aged and older adults in Japan, and second-hand smoke exposure was linked to an increased likelihood of depressive symptoms among smokers.
Supplementary material
The supplementary material is available online at https://doi.org/10.1192/bjp.2025.83
Data availability
The data that support the findings of this study are available from the corresponding author, Y.N., upon reasonable request.
Acknowledgement
The authors thank the National Center for Geriatrics and Gerontology, Japan, for funding support through the Research Funding for Longevity Sciences.
Author contributions
Conceptualisation was the responsibility of W.-M.C. and Y.N. Data curation was undertaken by C.T., Y.N., S.Z., R.O. and H.S. Formal analysis was carried out by W.-M.C. Funding acquisition was undertaken by by R.O. Investigation was performed by W.-M.C. Methodology was the responsibility of W.-M.C. and Y.N. H.A. undertook project administration and resources. S.Z. and Y.N. were responsible for software. Supervision was performed by H.A. Validation was carried out by R.O. and Y.N. Visualisation and roles/writing of the original draft were carried out by W.-M.C. M.-C.L. and H.A. undertook writing review and editing. All authors have read and agreed to the submission of this version of the manuscript for publication.
Funding
This work was supported by Research Funding for Longevity Sciences from the National Center for Geriatrics and Gerontology, Japan (grant nos 20-1, 21-18).
Declaration of interest
The authors declare no conflicts of interest.
Ethical standards
This study was approved by the Institutional Review Board of the National Center for Geriatrics and Gerontology, Japan (case no. 899-6).
Consent to participate
Informed consent was obtained from all individual participants included in the study.
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