Depression is a leading cause of disability worldwide, affecting an estimated 300 million individuals and contributing significantly to global disease burden and economic challenges (Frühauf et al., 2016). In the United States alone, depression impacts nearly 8.4% of adults annually, highlighting the critical need for effective prevention and treatment strategies (Owens and Bunce, 2022).

Among various interventions, outdoor activities have gained attention due to their potential to promote mental health through unique physiological and psychological mechanisms. These include enhancing physical activity, increasing vitamin D synthesis, improving sleep quality, and reducing stress (Hahn et al., 2011). Unlike pharmacological or purely psychological approaches, outdoor activities offer a holistic and accessible option for many populations, making them particularly worthy of focused research.

Previous studies have provided valuable insights into the benefits of outdoor activities. For instance, Frühauf et al. (2016) demonstrated that outdoor exercise significantly improved mood and psychological well-being, outperforming indoor exercise in boosting vitality and positivity (Buckley et al., 2018). Owens and Bunce (2022) further proposed that natural environments enhance mental health by reducing stress, decreasing rumination, and fostering positive thought patterns (Hahn et al., 2011). However, not all studies have reached consistent conclusions, with some failing to identify significant associations (Frühauf et al., 2016). These discrepancies underscore the influence of cultural, environmental, and socioeconomic factors on the effectiveness of outdoor activities (Wicks et al., 2022). Additionally, individual differences such as age and ethnicity may shape responses to outdoor activities. Despite these variations, limited research has systematically explored how these demographic factors impact the relationship between outdoor activity and depression. Addressing this gap is crucial to developing targeted, equitable public health interventions (Zhang, 2017).

This study aims to evaluate the relationship between time spent outdoors and depression among U.S. adults using NHANES 2011–2018 data. By focusing on differences across age and ethnic groups, this research seeks to provide robust evidence for integrating outdoor activities into public health strategies and tailoring interventions to meet the needs of diverse populations (Wheeler et al., 2020).

This study explored the relationship between outdoor activity duration and depression among U.S. adults using NHANES 2011–2018 data. Our findings revealed a significant negative association between outdoor activity time and depression risk. Fully adjusted models indicated that for each additional minute spent outdoors, the risk of depression decreased by 0.51-fold (Kerr et al., 2012). Subgroup analyses further demonstrated that this association was particularly strong among individuals aged 40 years and older, non-Hispanic whites, and non-Hispanic blacks.

Recent studies further substantiate the positive association between outdoor activity and mental health. For instance, Kerr et al. (2012) demonstrated that outdoor physical activities significantly alleviated depressive symptoms among middle-aged adults. Similarly, Hartig et al. (2014) highlighted the role of nature-based activities in improving mental well-being, particularly during post-pandemic recovery periods. Moreover, Pretty et al. (2005) emphasized the importance of integrating green exercise into community programs to address mental health challenges. These findings underscore the necessity of tailoring interventions to different population needs and highlight the evolving landscape of mental health research.

Mechanistically, the observed association between outdoor activity and reduced depression risk can be explained through several biological and behavioral pathways. Outdoor activity increases exposure to sunlight, which promotes vitamin D synthesis—a critical factor in mood regulation. Vitamin D deficiency has been strongly associated with depressive symptoms in numerous studies (Holick, 2007). Physical activity, often a component of outdoor engagement, elevates endorphin levels, reduces inflammation, and mitigates oxidative stress, all of which improve mood (Pearce et al., 2022; Ströhle, 2009). Additionally, time spent in natural environments has been shown to reduce cortisol levels and enhance parasympathetic activity, fostering relaxation and stress reduction (Ulrich, 1991). Regular outdoor activities also improve sleep quality, which is essential for maintaining emotional balance and mental health (Baglioni et al., 2011; Passos et al., 2011).

While most studies support the positive effects of outdoor activities, some have found no significant association. For example, a study based on Swedish adults found that, despite physical health benefits, outdoor activities did not significantly reduce depressive symptoms (Mitchell and Popham, 2008). This inconsistency could stem from cultural and environmental differences, as well as disparities in socioeconomic factors influencing both participation and effectiveness of outdoor activities (Firth et al., 2020). Individual variability, including genetic predisposition and lifestyle differences, may also play a role, requiring further research to clarify these interactions (Richardson et al., 2005).

Despite the valuable insights provided by this study, certain limitations must be acknowledged. First, the cross-sectional design precludes inference of causality. Future research should adopt longitudinal or experimental designs to establish causality. Second, self-reported measures of both outdoor activity and depressive symptoms may introduce recall bias and social desirability bias. Third, we did not account for confounders such as seasonal variation, geographic differences, or accessibility to outdoor spaces, which may have influenced the results. Furthermore, individual factors like genetic predispositions and specific health conditions were not controlled for, which may limit the generalizability of findings (Holick, 2007).

To address these limitations, future studies should focus on longitudinal designs that track outdoor activity and mental health outcomes over time. Randomized controlled trials examining specific types and intensities of outdoor activities could also help identify the most effective interventions. Moreover, studies exploring cultural and environmental factors that shape the relationship between outdoor activity and depression will enable the development of tailored public health strategies. Finally, research on population-specific responses to outdoor activity, including those influenced by genetic and socioeconomic factors, will further enhance the practical application of these findings (Richardson et al., 2005).

This study demonstrates that increased time spent outdoors is significantly associated with a reduced risk of depression, particularly among middle-aged and older adults, non-Hispanic whites, and blacks. These findings emphasize outdoor activity as a practical and accessible intervention to enhance mental health. Incorporating outdoor engagement into urban planning, community initiatives, and clinical practice could provide meaningful benefits, especially for populations at higher risk of depression.

Future research should explore causal relationships through longitudinal studies and randomized trials, while investigating the optimal types, intensities, and durations of outdoor activities to tailor interventions for diverse demographic and cultural contexts.