Differential Immune Modulation Following Short-Term Exposure to Two Forest Types Compared to an Urban Environment

The prevalence of non-communicable diseases (NCDs), including immune disorders such as allergies and autoimmunity has risen since the start of industrialization and is now the leading cause of mortality worldwide. The evolutionary mismatch hypothesis argues that the rise in NCDs is a consequence of humans evolving in habitats vastly different from the highly industrialised environments we live in today. While prior research has examined the effects of diet, antibiotics and chronic stress on immune function, the role of different environments remains understudied. Forest exposure is associated with increased natural killer (NK) cell levels, higher cytotoxic cytokine expression (granzyme and perforin) and lower levels of pro-inflammatory cytokines. However, previous studies showing these relationships have been limited in scope, focused on isolated immune markers, were constrained by small sample sizes and a took a homogeneous view of nature that did not consider ecological differences between environments. This randomized-controlled study (N=77) investigates how immune function is affected by a 3-hour exposure to one of two forest types (deciduous: Sihlwald, Zurich and coniferous: Parc Jura Vaudois, Lausanne) or an industrialised urban centre (Zurich City). Venous blood was collected pre- and post-intervention and analyzed using flow cytometry. We characterized 18 immune cell populations, with 4 cytokines assessed in a subset of 8 functionally relevant populations. Our findings suggest that short-term exposure to different environments can modulate innate and adaptive immunity. Forest exposure was linked to reduced antigen-presenting cells, increased naïve T cells and elevated CD8+ and CD4+ effector T cells. Cytokine changes were limited. Previously reported differences in NK cells and granzyme levels were not replicated. While some effects were subtle, the overall pattern may reflect environmentally driven modulation, potentially involving circadian rhythm alignment.