The study, published in SSM – Population Health by Yeongmi Jeong, Meghan M. Skira, and Robbee Wedow, focuses on two polygenic scores, which are numerical summaries of the many small genetic differences scattered across a person’s genome. The first, called the smoking initiation score, aggregates variants associated with whether a person ever becomes a regular smoker. The second, the cigarettes-per-day or CPD score, aggregates variants associated with how heavily someone smokes once they start. Both scores were built from genome-wide association studies, or GWAS, that scanned the genomes of more than a million people of European ancestry to find single-letter DNA differences, known as single nucleotide polymorphisms, that statistically track with smoking behavior. Each person’s score is calculated by adding up the number of copies of the risk-increasing variants they carry, with each variant weighted by the strength of its estimated association.

Neither score is a crystal ball. Out-of-sample predictions indicate that the initiation score explains only about 4 percent of the variation in whether people start smoking, and the CPD score explains about 4 percent of the variation in how much they smoke. Within the study sample itself, a one standard deviation increase in the initiation score raised the probability of ever smoking by roughly 9 percentage points, while a one standard deviation increase in the CPD score was associated with about two additional cigarettes smoked per day among smokers. These are modest effects, but they are large enough, in a sample of this size, to reveal patterns that would otherwise remain invisible.

To measure health, the researchers constructed a standardized index combining thirteen distinct outcomes: self-reported general health, hospitalization, body mass index, seven doctor-diagnosed chronic conditions including high blood pressure, diabetes, cancer, lung disease, heart problems, stroke, and arthritis, and limitations in basic and instrumental activities of daily living. The analysis covered 48,444 person-wave observations from 9,515 unique participants aged 50 to 65, drawn from survey waves between 1994 and 2018. The age range was chosen deliberately, to minimize survival bias, since people with the worst health may not survive to be surveyed at older ages, and to avoid conflating genetic associations with the health effects of retirement.

The central finding is stark. A one standard deviation increase in the CPD score was associated with a 0.043 standard deviation increase in the health index, where higher values indicate worse health. Adding detailed controls for smoking behavior, including current and lifetime smoking status, the exact number of cigarettes smoked per day, the maximum ever smoked, the age of starting and quitting, and teenage smoking, attenuated the association only modestly, to 0.035 standard deviations. For context, that residual association is roughly 38 percent of the average health gap between ever smokers and never smokers in the sample. In other words, even after accounting for everything the surveys record about smoking, the genetic score still carries substantial information about health.

The most provocative result emerged when the researchers restricted the analysis to never smokers, a group of more than 20,000 observations with no reported smoking behavior to explain away. Among these individuals, a one standard deviation increase in either the CPD score or the initiation score was associated with roughly a 0.03 standard deviation increase in the health index. The CPD score also tracked with higher body mass index and obesity among never smokers, with a one standard deviation increase associated with 0.645 units of additional BMI. Because these individuals never smoked, the association cannot be attributed to their own tobacco use, although it could still reflect secondhand smoke exposure, unmeasured smoking-related environments, or something else entirely.

One obvious candidate is the family environment. Because children inherit genes from their parents, a person’s smoking-related genetic score may be correlated with parental smoking, parental substance use, parental education, and childhood socioeconomic conditions, all of which shape later health. The researchers tested this by adding controls for whether parents smoked during the respondent’s childhood, whether parents drank or used drugs heavily enough to cause family problems, parental mortality timing, parental years of schooling, and self-reported childhood financial status. These factors accounted for a modest share of the association, but more than 75 percent of the CPD score’s relationship with health remained, and about 60 percent of the initiation score’s relationship among never smokers survived the most comprehensive specifications.

The researchers then turned to psychology. Prior work has shown that smokers tend to be more impatient, risk-tolerant, and impulsive than non-smokers, and the team asked whether people with higher smoking-related genetic scores differ along similar dimensions. They found little evidence of a link with measured risk preferences, but higher scores were significantly associated with shorter financial planning horizons, greater pessimism, and lower conscientiousness, traits that economists and psychologists have long connected to lower investment in health. When these behavioral and psychological characteristics were added to the models simultaneously, they explained at most 21 percent of the remaining association between the CPD score and health, and 18 percent of the initiation score’s association among never smokers. Even with the fullest set of controls, at least 69 percent of the CPD score’s association with health persisted.

The authors are careful to emphasize what the study does not show. Polygenic scores are not clean measures of direct genetic effects. The GWAS estimates underlying them can absorb indirect genetic effects, such as parental genotypes shaping the environments children experience, as well as confounding from assortative mating, population stratification, collider bias, and case-control ascertainment. There is also the problem of pleiotropy: the same genetic variants that predict smoking may influence other biological processes and traits that affect health directly. Genetic correlation analyses have indeed documented significant overlap between smoking behaviors and a wide range of health, behavioral, and socioeconomic traits. The associations reported here are descriptive, not causal, and the study’s restriction to individuals of European ancestry, a consequence of the limited predictive power of European-derived scores in other populations, means the findings may not generalize more broadly.

Even with those caveats, the implications are significant. Economists studying smoking have long suspected that people who smoke differ from those who do not in ways that extend beyond the behavior itself, and some modeling work has speculated that genetics or correlated risk-taking might explain why smokers face elevated chronic disease and mortality risks beyond what smoking alone explains. This study provides some of the most direct evidence yet that such unobserved heterogeneity is real and partially visible in the genome. Smoking-related polygenic scores, the results suggest, contain information about health that detailed behavioral histories do not capture, pointing toward shared genetic architecture linking tobacco use, personality, and long-run wellbeing. Untangling those threads, the authors argue, will be essential for interpreting genetic risk scores in health research, and for understanding why two people with identical smoking histories can end up with very different health outcomes decades later.

Subject of Research: Associations between smoking-related polygenic scores and health outcomes in midlife

Article Title: Exploring Links Between Health and the Genetic Risk for Smoking

Article References: Jeong, Y., Skira, M. M., & Wedow, R. (2026). Exploring Links Between Health and the Genetic Risk for Smoking. SSM – Population Health, Article 101982. https://doi.org/10.1016/j.ssmph.2026.101982

Image Credits: AI Generated

DOI: 10.1016/j.ssmph.2026.101982

Keywords: polygenic scores, smoking initiation, smoking intensity, Health and Retirement Study, GWAS, pleiotropy, chronic disease, health behavior, genetic risk, conscientiousness, never smokers, population health

The research team, led by Mengkun Li and colleagues at Jilin University in Changchun, China, analyzed data from 190,403 UK Biobank participants who reported never smoking. Exposure to secondhand smoke was assessed through self-reported hours per week spent in smoky environments both at home and outside the home. Participants were categorized as exposed if they reported at least one hour of weekly exposure, and unexposed otherwise. The cohort was then followed for an average of 14.3 years, during which 7,013 individuals developed depression according to the study’s clinical criteria.

Using Cox proportional hazards models, a statistical technique that estimates how a given exposure changes the rate at which an event occurs over time, the researchers found that never smokers exposed to secondhand smoke had a 22.6 percent higher risk of developing depression than those who were not exposed. The hazard ratio of 1.226, with a 95 percent confidence interval of 1.159 to 1.296, indicates a robust and statistically significant association. Importantly, because the study was prospective, exposure was measured before depression developed, which strengthens the argument that the smoke exposure preceded the onset of the condition rather than the reverse.

The dose-response pattern was equally striking. Each additional hour of weekly secondhand smoke exposure was associated with a small but measurable increase in depression risk, reflected in a hazard ratio of 1.012 per hour. This kind of gradient, where more exposure corresponds to more risk, is a hallmark of a plausible biological relationship and is difficult to explain away by confounding alone. The consistency of the dose-response effect suggests that the relationship between tobacco smoke and mental health may operate along a continuum rather than through a simple threshold.

One of the most consequential findings emerged from the subgroup analyses. The association between secondhand smoke and depression was significantly more pronounced among individuals experiencing high socioeconomic deprivation, with a statistically significant interaction term. This pattern echoes a broader body of public health research showing that behavioral and environmental risk factors for non-communicable diseases tend to concentrate among disadvantaged populations. People living in deprivation may face both greater exposure to smoke in their homes and workplaces and fewer resources to buffer the psychological toll of that exposure, compounding existing health inequities.

To probe whether the observational link might reflect causation rather than mere correlation, the team turned to Mendelian randomization, a genetic epidemiology technique that uses naturally occurring variations in DNA as proxies for environmental exposures. Because genetic variants are randomly allocated at conception, they are largely immune to the reverse causation and confounding that plague traditional observational studies. Using the inverse variance weighted method, the researchers found a positive genetic association between secondhand smoke exposure and depression, with an odds ratio of 1.028 and a confidence interval of 1.001 to 1.056. While the effect size is modest, the fact that it crosses the threshold of statistical significance lends genetic support to the possibility that secondhand smoke exposure contributes causally to depression risk.

The biological plausibility of such a link is supported by decades of research into how tobacco smoke affects the brain. Nicotine and other smoke constituents are known to interact with nicotinic acetylcholine receptors, which play a role in mood regulation, anxiety, and reward processing. Tobacco smoke also triggers systemic inflammation and oxidative stress, processes that have been repeatedly implicated in the neurobiology of depression. Prior work has documented alterations in serotonin and dopamine signaling associated with smoke exposure, and studies of active smokers have found shared genetic liability between smoking behavior and major depressive disorder. The new study extends this framework to people who never smoke themselves but absorb the same toxic mixture involuntarily.

Secondhand smoke contains many of the same harmful chemicals as directly inhaled smoke, including nicotine, carbon monoxide, and numerous carcinogens. While the cardiovascular and respiratory harms of passive smoking are well established, its mental health consequences have received less attention. Previous studies, including cross-sectional surveys in Germany, China, Malaysia, and the Netherlands, have reported associations between secondhand smoke exposure and depressive symptoms, but those designs could not establish temporal ordering. The UK Biobank cohort, with its long follow-up period and large sample size, offers a more rigorous test of whether exposure precedes onset.

The study is not without limitations. Exposure was self-reported rather than measured with biomarkers such as cotinine, a nicotine metabolite, which introduces the possibility of misclassification. Self-reported smoking histories and exposure estimates are subject to recall limitations, though prior validation work suggests they are reasonably reliable in large cohorts. The UK Biobank population, which is predominantly of European ancestry and somewhat healthier than the general population, may also limit how broadly the findings generalize. The Mendelian randomization result, while supportive, reflects a small effect and should be interpreted as evidence of plausibility rather than definitive proof of causation.

Nevertheless, the implications are significant. Depression affects hundreds of millions of people worldwide and is a leading contributor to disability, yet modifiable environmental risk factors remain incompletely mapped. If involuntary tobacco smoke exposure is indeed contributing to depression risk, then smoke-free policies in homes, workplaces, and public spaces take on added urgency, particularly for protecting children, partners of smokers, and workers in settings where smoking persists. The findings also underscore the importance of targeting prevention efforts toward socioeconomically disadvantaged communities, where the burden of both exposure and vulnerability appears greatest. As the authors conclude, reducing involuntary tobacco smoke exposure is not only a matter of lung and heart health but potentially a matter of protecting mental well-being in people who never chose to be near a cigarette at all.

Subject of Research: The association between secondhand smoke exposure and depression risk among never smokers in a large prospective cohort study with Mendelian randomization analysis.

Article Title: Secondhand smoke exposure and depression among never smokers in a prospective cohort study

Article References: Li, M., Kou, C., He, X., Guo, X., Geng, H., Wang, Z., Su, C., Zhang, X., You, P., Sun, Y., Batebayier, B., Han, P., Bai, W., & Li, Y. (2026). Secondhand smoke exposure and depression among never smokers in a prospective cohort study. Air Quality, Atmosphere & Health, 19(10), Article 209. https://doi.org/10.1007/s11869-026-02100-7

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02100-7

Keywords: secondhand smoke, depression, never smokers, UK Biobank, prospective cohort study, Mendelian randomization, tobacco smoke exposure, mental health, dose-response, socioeconomic deprivation, public health, involuntary smoking