How might contact with nature promote human health? Promising mechanisms and a possible central pathway

Abstract

How might contact with nature promote human health? Myriad studies have linked the two; at this time the task of identifying the mechanisms underlying this link is paramount. This article offers: (1) a compilation of plausible pathways between nature and health; (2) criteria for identifying a possible central pathway; and (3) one promising candidate for a central pathway. The 21 pathways identified here include environmental factors, physiological and psychological states, and behaviors or conditions, each of which has been empirically tied to nature and has implications for specific physical and mental health outcomes. While each is likely to contribute to nature's impacts on health to some degree and under some circumstances, this paper explores the possibility of a central pathway by proposing criteria for identifying such a pathway and illustrating their use. A particular pathway is more likely to be central if it can account for the size of nature's impacts on health, account for nature's specific health outcomes, and subsume other pathways. By these criteria, enhanced immune functioning emerges as one promising candidate for a central pathway between nature and health. There may be others.

Keywords: greenspace; immune; literature review; mechanism; mental health; natural environment.

FIGURE 1

The nature-health link: filling in the details. This Figure summarizes the state of the scientific literature on nature and health, listing (1) the “active ingredients” in nature that have been identified as having impacts on health or health antecedents; (2) physiological/psychological states, behaviors, and conditions tied to both nature and health; and (3) specific health outcomes that have been tied to nature (controlling for socioeconomic variables). Note that physical activity (in brackets) is only sometimes tied to nature; and that allergies, asthma, and eczema are sometimes positively and sometimes negatively tied to nature. DHEA: didehydroepiandrosterone; acute UTI: acute urinary tract infection; ADHD: attention deficit hyperactivity disorder; IDIC: infectious disease of the intestinal canal; MUPS: medically unexplained physical symptoms; URTI: upper respiratory tract infection.

FIGURE 2

Enhanced immune function as one possible central pathway. All items in white text are currently known or have been proposed to be causally tied to immune function. As the Figure shows, enhanced immune function can account, at least partially, for each of the specific health outcomes currently tied to nature and may subsume, at least partially, all but two of the other pathways. Other pathways may also contribute the nature-health link, and other central pathways may exist.