Experimental investigation of the influence of acute nature exposure on psychological and physiological health

While cities provide economic and social opportunities, they are also rich in environmental pollutants, such as sound, light and air pollution, and higher levels of stress compared to rural areas. Higher prevalence of chronic stress in cities may be related to a mismatch between our evolutionary habitat (nature) and the recent transition to built city environments. The natural environments inhabited throughout the human evolutionary journey have set the environmental parameters within which natural selection has acted to shape human evolution. Human physiology is, therefore, considered to be adapted for optimal function in these natural ancestral habitats. The field of nature and human health investigates the health effects of spending time in nature. Epidemiological and experimental data has revealed that nature can have a positive effect on mental, cognitive, immune and physical health; however, the application of nature interventions in public health and preventative medicine requires more holistic multisystem evidence to understand the wider effects of nature on health.

This thesis aims to address this by using crossover experimental designs to measure the psychological, physiological, and functional effects of acute nature immersion. Study 1 investigated the effect of a 70-minute walk in a forest trail (nature) and a tree-lined dual carriageway (green urban) on mood, physiological stress, and eating behaviour in healthy females. It was demonstrated that walking in both wild nature and green urban environments improved mood (all Profile of Mood States [POMS] scores), and physiological stress (resting heart rate, blood pressure, and salivary cortisol), while wild nature elicited greater changes in positive affect (esteem related affect) and had better overall mood, depression, confusion, and esteem related affect throughout the intervention visit. There were no differences in energy intake or salivary ghrelin between the environments.

Study 2 used a multisystem approach to investigate the effects of 80-minute physically inactive nature immersion on stress, appetite, and function (cognitive, immune, and physical) in healthy young adults. This study found psychological improvements (improved mood, reduced psychological stress, and increased optimism) with limited differences in appetite, cognitive, and immune function, potentially as physiological stress was not significantly different. For physical function, higher ratings of perceived exertion at maximal endurance performance and

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improved submaximal exercise economy indicated that some psychologically-mediated differences in endurance exercise emerged following the physically inactive nature intervention, with overall endurance performance (time to exhaustion) trending towards significance.

Study 3 considered the effects of a 45-minute physically inactive lunchtime nature immersion on mood, work motivation, physiological stress (salivary cortisol), and innate immune (secretory IgA) in an office working population. Similar to Study 2, mood improved in nature and worsened in the urban environment. Work motivation and preparedness also improved in the nature but not urban immersion. Furthermore, salivary cortisol recovered from baseline stress in nature; however, this was not replicated in the urban group. There were no changes in secretory IgA as a result of the acute lunchtime intervention.

In summary, this thesis has revealed that an acute nature immersion can improve psychological outcomes with limited effects on functional outcomes. This work represents a step toward a holistic multisystem understanding of the influence of natural environments on human biology. Future work aiming to increase the prescription of nature and population-level access to the benefits of natural environments should seek to elucidate the mechanisms linking environment to human biology.