Suicide and Transportation Noise: A Prospective Cohort Study from Switzerland

Mental health disorders represent a pressing public health issue. In 2019, the prevalence of mental health disorders globally was estimated to be 13% [95% confidence interval (CI): 12.1, 14.0%], which translates to almost 1 billion people affected. In Switzerland, the estimated prevalence was slightly higher with 17.3% (95% CI: 15.9, 18.8%), translating to about 1.4 million people affected.¹ Although most mental health disorders primarily lead to morbidity and decreased quality of life, a mortality outcome closely related to mental illness is suicide.^2,3 Suicide is a complex, multicausal phenomenon, involving psychological, social, biological, and environmental factors. A study on suicide in the Swiss National Cohort confirmed that mental and behavioral problems were by far the most prevalent comorbidities in suicide victims across all professions, age groups, and genders.⁴ Only recently research has started to also explore and identify possible environmental risk factors for suicide, with reported associations of an increased suicide risk with heat,⁵ air pollution.⁶ and noise.⁷ On the other hand, residential greenness and urban green space have been recognized as environmental factors with protective properties.⁸ Although suicide rates have decreased worldwide and in Switzerland in the last 20 y, the decline is not yet on course to reach the Sustainable Development Goal (SDG) aim of a reduction by one-third by 2030. The World Health Organization (WHO) report, “Suicide Worldwide in 2019,” published in 2021, estimated that 703,000 people died from suicide in 2019 worldwide, which corresponds to 1.3% of all yearly deaths.⁹ On the grounds of such numbers, reducing the occurrence of mental illnesses is a primary public health interest. Hence, understanding risk factors for the development of mental disorders and therefore for suicide is of utmost importance. In light of the growing urbanization worldwide, studying the role of urban environmental stressors as such potential risk factors can potentially yield insights of consequential importance concerning the promotion of mental health and prevention of psychiatric morbidity and mortality.

In recent years, noise has been recognized as one of the most impactful environmental stressors on health and well-being.¹⁰ Among the sources of environmental noise, transportation noise and especially road traffic noise have emerged as the most prevalent and harmful. According to a European Environmental Agency (EEA) report, 20% of the European population (139 million people) in 2017 were estimated to live in areas with transportation noise levels that are considered harmful [$>55$ dB day-evening-night level (Lden)].¹⁰ A more recent study investigating noise exposure in 700 cities in Europe estimated that 42% of the adult urban population are exposed to such harmful levels.¹¹ Due to this widespread occurrence, noise is the second most important driver of the environmental burden of disease in Europe, behind fine particulate matter (PM) air pollution. In numbers, exposure to transportation noise is estimated to be responsible for 400–1,500 disability adjusted life years lost each year per million people in Western Europe.¹² However, because more evidence including more outcomes has emerged since 2014, and, for example, effects on mental health were not included in the above-mentioned calculations, these numbers might actually represent an underestimation.

Concerning negative health effects related to noise exposure, there is growing evidence for diverse nonauditory effects, such as arterial hypertension, cardiovascular and metabolic diseases such as type 2 diabetes,^13–16 sleep disturbance,¹⁷ annoyance,¹⁸ as well as reduced quality of life and well-being¹⁹—and mental health and neurological disorders.²⁰ Although the exact pathways of the influences of noise on health remain somewhat unclear, there is evidence that noise causes physiological stress reactions involving heightened amygdalar activity,²¹ allostatic overload,^13,15 and disturbed sleep.²² Because these are all established risk factors for multiple mental health disorders, including depression,^23–27 an association of noise with poor mental health seems plausible from a pathophysiological perspective. However, the systematic review on noise-related mental health outcomes used for the 2018 WHO guidelines resulted in a judgment of very low-quality evidence for aircraft noise effects on depression and anxiety and a judgment of low-quality evidence for a null effect of road traffic or railway noise.²⁸ The review included 29 predominantly cross-sectional studies. Hence, the poor quality of evidence was attributed to a lack of robust studies investigating the mental health effects of different noise sources. A more recent meta-analysis of five studies found an increased risk of 12% (95% CI: 2%, 23%) for depression per 10 dB Lden of aircraft noise exposure.²⁹ The same study²⁹ also suggested a 2%–3% increased risk for depression per 10 dB Lden for railway noise based on three studies and for road traffic noise based on eleven studies. Another review and meta-analysis including nine studies showed an association between transportation noise and anxiety [$\text{odds\hspace{0.17em}ratio\hspace{0.17em}}\left(\text{OR}\right)=1.09$; 95% CI: 0.97, 1.23 per 10 dB], while also rating the quality of evidence as low.³⁰ Since the publication of this meta-analysis, a Swiss prospective cohort study (SAPALDIA) reported a 7% increased risk for the incidence of depression per 10 dB Lden road traffic noise [$\text{relative\hspace{0.17em}risk\hspace{0.17em}}\left(\text{RR}\right)=1.07$; 95% CI: 0.93, 1.22] and a 20% increased risk per 10 dB Lden aircraft noise ($\text{RR}=1.20$; 95% CI: 0.92, 1.55). So far, only one study investigating the associations between long-term exposure to environmental noise and suicide has been conducted.⁷ The authors examined the risk for death by suicide in relation to average nighttime noise exposure (including noise caused by transportation and industrial and recreational activities) in adults in Korea and reported an increased risk per interquartile range ($\text{IQR}=2.67\text{dB}$) of nighttime noise of 32% (95% CI: 2%, 70%) in younger adults and 43% (95% CI: 1%, 102%) in older adults. A time-series study from Spain investigating short-term effects of traffic noise exposure on suicides and emergency admission for depression and anxiety reported an increased risk for both outcomes.³¹

In comparison, the effects of air pollution on mental health have received more attention and have been studied more thoroughly. Air pollutants have been shown to cause oxidative stress and neuroinflammation and to trigger stress responses with stress hormone release, which are the major hypothesized mechanisms linking air pollution and adverse mental health outcomes.^32,33 A systematic review and meta-analysis published in 2022 including 39 studies reported significant associations between long-term exposure to various air pollutants (${\mathrm{PM}}_{2.5}$, ${\mathrm{NO}}_2$, ${\mathrm{SO}}_2$, CO) and risk of depression. The largest risk increase was observed for PM with aerodynamic diameter $\le 10\mathrm{\mu }\mathrm{m}$ (${\mathrm{PM}}_{10}$) [$\text{RR}=1.092$ (95% CI: 0.988, 1.206) per $10-\mathrm{\mu }{\mathrm{g}/\mathrm{m}}^3$ increase in exposure]. Smaller effects were also reported per $10{-\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ increase of short-term exposure to ${\mathrm{PM}}_{2.5}$ [$\text{RR}=1.009$ (95% CI: 1.007, 1.011)], ${\mathrm{PM}}_{10}$ [$\text{RR}=1.009$ (95% CI: 1.006, 1.012)], ${\mathrm{O}}_3$ [$\text{RR}=1.011$ (95% CI: 0.997, 1.026)], ${\mathrm{NO}}_2$ [$\text{RR}=1.022$ (95% CI: 1.012, 1.033)] and ${\mathrm{SO}}_2$ [$\text{RR}=1.024$ (95% CI: 1.010, 1.037)].³⁴ Another systematic review and meta-analysis from 2019 found similar results for depression, and also reported associations between short-term ${\mathrm{PM}}_{10}$ exposure and suicide risk [$\text{RR}=1.02$ (95% CI: 1.00, 1.03) per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ at lag 0–2 d, including four studies].⁶ These results were compiled in a more recent systematic review and meta-analysis from 2021 that included 10 studies, reporting a 2% (95% CI: 1%, 3%) risk increase for suicide per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ ${\mathrm{PM}}_{2.5}$ exposure.³⁵ One limitation of these reviews is that most of the included studies have not adjusted for exposure to possible confounders such as transportation noise. A recent large cross-sectional study from the UK Biobank studying ${\mathrm{PM}}_{2.5}$ and road traffic noise exposure, however, reported an increased risk for depression associated with ${\mathrm{PM}}_{2.5}$ exposure, but no association with road traffic noise exposure was found.³⁶

Residential greenness or green space is another exposure of interest in environmental epidemiology as a protective factor for health and well-being. For example, higher levels of greenness [normalized difference vegetation index (NDVI)] around people’s place of residence have been associated with a lower risk of natural-cause mortality [$\text{HR}=0.94$ (95% CI: 0.93, 0.95) per IQR (0.14 NDVI in a 500-m buffer)] in a large Swiss cohort study.³⁷ Concerning mental health, a Dutch study found a decreased risk for death by suicide in communities with high green space proportion ($>85\%$) in comparison with communities with little ($<25\%$) green space [$\text{RR}=0.879$ (95% CI: 0.779, 0.991)].³⁸ Beyond suicide, a systematic review from 2020 suggested several beneficial effects of access to green space on adolescents’ mental health, including fewer depressive symptoms and improved general mental health.³⁹ Multiple pathways are posited for this positive association, including that greener living environments or green space availability encourage healthy behavior, and that such factors can aid in stress relief.⁴⁰

This study investigates the association between exposure to road traffic, railway, and aircraft noise and the risk of death by suicide in a longitudinal, nationwide research cohort in Switzerland. We hypothesized that people exposed to higher levels of transportation noise are more likely to develop mental health disorders such as depression and therefore have a higher risk of death by suicide, independent of coexposure to air pollution and residential greenness as well as socioeconomic position. By using suicide as a surrogate, we aimed to add to the understanding of whether transportation noise exposure affects mental health.

Our findings suggest an association between exposure to transportation noise at the place of residence and the risk of death by intentional self-harm. Risk started to increase at levels of 50 dB or even lower, i.e., below the current WHO guideline levels for all noise sources. The observed associations were stronger in females than in males. We did not find any evidence for an increased risk of death by intentional self-harm due to air pollution. An inverse association with residential greenness was observed in females and in the urban setting.

There is hardly any research investigating long-term exposure to transportation noise and risk of suicide. The only previous study that investigated long-term noise exposure and suicide was conducted in Korea using environmental noise measurement data from a nationwide noise monitoring system, and not differentiating the exposure by noise source (e.g., noise caused by transportation and industrial and recreational activities). Mean monthly nighttime noise levels from the closest measurement stations were used as exposure. The authors reported a significantly increased risk for death by suicide per IQR increase of nighttime noise of 32% (95% CI: 2%, 70%) in younger adults (20–54 y, $n=\mathrm{124,994}$) and 43% (95% CI: 1%, 102%) in older adults ($\ge 55\mathrm{y}$, $n=\mathrm{30,498}$).⁷ Although these results are difficult to directly compare to ours, it is notable that both suggest a risk increase for death by suicide in relation to noise exposure.

Overall, however, we believe our results should not be interpreted as suggesting that transportation noise has a direct influence on suicide or suicidal behavior, but rather that suicide as a surrogate for underlying mental health disorders is associated with transportation noise exposure. There is conclusive evidence that mental and behavioral disorders are the predominant comorbidities in suicide victims.^2,64 Hence, we reasoned to use suicide as a surrogate for underlying mental health disorders. The advantage of this approach is that it enables the use of the extensive mortality data in the SNC to study mental disorders. However, it is clear that there are also some limitations. For example, deaths by suicide represent only the “tip of the iceberg.” Estimations suggest that worldwide, there are $\sim 20$ suicide attempts for every death by suicide,⁹ and a Swiss study even noted 32 attempts for each death.⁶⁵ Additionally, suicide is a highly complex issue with many influencing factors, however, with psychiatric diseases and especially depressive disorders representing an important factor.⁶⁴

When comparing our results on associations of transportation noise with suicide to existing literature on transportation noise and mental health, there is mixed agreement. For example, a systematic review and meta-analysis from 2020 reported an association between exposure to road traffic noise and anxiety [odds ratio $\left(\text{OR}\right)=1.08$; 95% CI: 1.01, 1.15 per 10 dB Lden], whereas no effect was found for railway and aircraft noise.³⁰ Another systematic review and meta-analysis from the same year, in contrast, reported an association of aircraft noise exposure with risk for depression [12% (95% CI: 2%, 23%) increased risk per 10 dB Lden], whereas smaller risk increases were found for road traffic [3% (95% CI: −1%, 6%) per 10 dB Lden] and railway noise [2% (95% CI: −5%, 8%) per 10 dB Lden].²⁹ A more recent longitudinal study reported an association of road traffic noise and psychological ill health.⁶⁶ A study from Switzerland, also published after the above-mentioned reviews, found an association between incidence of depression and noise annoyance, whereas no significant association was found with noise exposure of any source.⁶⁷ An interesting observation is that results from a 2022 UK Biobank study suggested the opposite of the results in our study. In their large cross-sectional study, the authors reported an increased risk for major depression associated with ${\mathrm{PM}}_{2.5}$ exposure but not an association with transportation noise exposure.³⁶ Although numerous studies report associations of transportation noise and mental health, the quality of evidence is considered low due to study design (mostly cross-sectional) and small sample sizes.^28,68,69 Additionally, heterogeneity in exposure assessments, outcome definitions and effect measures complicate a conclusive comparison of results. The consensus from the existing literature, however, is that an impact of transportation noise on mental health is highly probable,⁷⁰ which our findings further support.

One difference in our results from existing evidence is that we did not find a clear association between aircraft noise and death by intentional self-harm. This finding in particular contradicts the results from the previously mentioned meta-analysis by Hegewald et al.,²⁹ which reported a rather large increased risk of depression per 10 dB aircraft noise. These results are mainly driven by one study by Seidler et al.⁷¹ (Weight 98%), which was set around the airport of Frankfurt before the night flying ban was established in 2011. In contrast, in Zurich, the largest airport in Switzerland, a ban had already been established in 1972.⁷² Hence, an interpretation of this contrasting result could be that noise (and specifically aircraft noise) during the night is the main contributor of negative effects on mental health. This finding is in agreement with the study by Min and Min,⁷ which reported an increased risk of suicide with increasing nighttime environmental noise exposure. Additionally, this conclusion is supported by psychiatric literature judging sleep disturbances as an independent risk factor for most psychiatric disorders.⁷³ Alternatively, the absence of an association between aircraft noise and risk for death by suicide might also be due to residual confounding and exposure distribution, because aircraft noise in our study area (Switzerland) is mostly concentrated around larger cities (Basel, Geneva, Zurich), areas for which a 2016 mortality atlas of Switzerland showed average to lower suicide rates.⁷⁴ Additionally, the exposure–response curve for aircraft noise showed an inverse association in the very low exposure. Areas with low aircraft noise exposure are also more likely very rural, which might contribute to confounding. This theory is supported by results from another SNC study by Guseva Canu et al.⁷⁵ that identified men working in agriculture, hunting, and forestry, who tend to live in more rural settings, to be at an increased suicide risk in comparison with the risk found for men working in other professions.

Concerning effect modification, we consistently saw stronger associations of transportation noise and suicide risk in females than in males and also a protective effect of greenness exclusively in females in the main analysis. Because gender differences in suicide are well known, these associations are not surprising. In general, women make more suicide attempts, but suicide mortality is generally higher in men.⁷⁶ Accordingly, in our nationwide sample, 74.1% of the suicide victims were male. This discrepancy is often referred to as the “gender paradox of suicide.” Among other factors, differences in psychopathology have been proposed as possible reasons for these gender differences.⁷⁷ Already in 2004, differences in underlying psychiatric disorders in male and female suicides were reported: although diagnoses with personality, childhood, and alcohol or substance disorders were more common in males, females had more often been diagnosed with depressive or affective disorders.² This underlying psychopathology may partly explain the observed effect modification in our results. The suspected mechanisms linking transportation noise exposure with mental health (prolonged stress reactions,²¹ allostatic overload,¹⁵ and sleep disturbances²²) are thought to mostly increase the risk for affective disorders such as depression and anxiety disorders,^24,25,27 whereas effects on personality disorders and other psychiatric disorders such as schizophrenia seem less plausible. This evidence is also consistent with the existing, albeit limited, evidence investigating transportation noise exposure and mental health outcomes.²⁸ Hence, we interpret our results indicating a stronger effect of road traffic noise exposure on suicide risk in females as suggesting that such associations are mostly mediated through an increased risk for affective and anxiety disorders. Because reliable data on psychiatric diagnosis or medication prescription were not available, this hypothesis could not be tested with mediation analysis. Further studies are needed to elucidate the pathways through which transportation noise influences mental health.

Another interesting finding from our study concerns those on greenness. We observed a substantial risk reduction [$\text{HR}=0.942$ (95% CI: 0.912, 0.973) per 0.1 NDVI] in the urban setting. This finding is in line with the findings of a Dutch longitudinal case–control study,⁸ which also reported a reduced risk for suicide associated with more residential green space in urban but not rural regions. Conversely, however, we saw a strong association of higher NDVI with increased risk of suicide in the rural setting. We argue that this association might be spurious, because those areas with the highest NDVI in the rural setting are probably very rural areas, where a higher percentage of the population are agricultural workers. As mentioned above, this population is among the most at-risk professional groups in Switzerland.⁷⁵ Also, remote regions showed above-average suicide mortality in a 2016 mortality atlas.⁷⁴

Regarding air pollution, we found no association between either ${\mathrm{PM}}_{2.5}$ or ${\mathrm{NO}}_2$ and suicide mortality. The lack of association was found in models both with and without adjusting for transportation noise (Figure S6). This lack of association is in contrast to many studies reporting an association between long-term and short-term exposure to air pollution and an increased risk for depression, as well as effects on suicides.^6,34–36 Most of these studies, except for the UK Biobank study,³⁶ however, did not adjust for exposure to transportation noise. It may thus also be that some of these previous results on air pollution have been confounded by transportation noise. Future studies exploring either the effects of air pollution or transportation noise exposure on mental health should consider that both exposures may play a role.

In this nationwide administrative cohort study, we found a robust association of exposure to transportation noise and the risk for death by intentional self-harm. Though information on mental health status was not available, these findings suggest that suicide as a surrogate for mental health disorders may be related to transportation noise, adding to the growing body of evidence for such effects. Further research is needed to solidify the understanding of the complex relationship between noise exposure, other environmental stressors such as air pollution, socioeconomic factors, and mental health. However, our results emphasize the public health importance of efforts to reduce the population exposed to high levels of transportation noise.

The authors thank the Federal Statistical Office (FSO) for providing mortality and census data and for the support that made the Swiss National Cohort and this study possible. This work was supported by the Swiss National Science Foundation (grant nos. 3347CO-108806, 33CS30_134273 and 33CS30_148415). The members of the Swiss National Cohort Scientific Board are M. Zwahlen (University of Berne), M. Egger (University of Berne), V. von Wyl (University of Zurich), O. Hämmig (University of Zurich), M. Bochud (University of Lausanne), M. Röösli (University of Basel), and M. Schwyn (Federal Statistical Office).

The SNC was approved by the Ethics Committees of the Canton Bern (No KeK 153/2014, PB_2020-00050).

B.W., D.V., and M.R. worked on study concept and study design; D.V. conducted data preparation; B.W. and D.V. performed statistical modeling; and B.W. wrote and revised manuscript and performed all data interpretation, review, and commentary on the manuscript.

This work was supported by the Swiss National Science Foundation (grant nos. 3347CO-108806, 33CS30_134273 and 33CS30_148415, 324730B_201272).

The authors declare that they have no competing interests.