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Nature and Public Health

To set the stage for the workshop’s discussions, Taylor Ricketts, from the University of Vermont, summarized ways in which people and nature interact and how they affect one another. Humans exert changes on the environment by the way they modify natural systems and harvest their resources, by using these natural systems for waste disposal, and through a variety of other activities. “For the [p]ast century or two, the story has been about exploiting nature,” said Ricketts. “People have exploited nature and liquidated natural capital to develop and expand the human enterprise.”

At the same time, he added, humans rely on natural ecosystems for their livelihoods and well-being, so the effects that people have on natural ecosystems come back to affect people and their well-being. What will be emerging in the next two decades, Ricketts said, will be an understanding of the limits and dangers of exploiting nature and liquidating natural capital and the clear roles that natural ecosystems play in supporting human well-being and development. This understanding could lead to a shift to managing nature to the benefit of both the biosphere and people rather than depleting nature through human-centric activities.

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¹ This list is the rapporteurs’ summary of the main points made by the speaker, and the statements have not been endorsed or verified by the National Academies of Sciences, Engineering, and Medicine.

Over the past 60 years or so, the framing for conservation has evolved and expanded from conserving nature for nature’s sake to focusing on people and nature together (Figure 2-1; Mace, 2014). For the past 25 years, said Ricketts, the ecosystem services framework has been a dominant approach for thinking about the relationships between nature and humans, with much of the research conducted on the ways that nature supports humanity and focused on some of the biophysical outcomes—carbon emissions and pollution, for example—or economic outcomes. In contrast, a broader approach that connects human health and ecosystems could focus on the well-being of human communities around the world, rather than on financial incentives, while still providing quantifiable results of use to decision makers. One important aspect of the environment-associated burden of disease that would be captured in this broader approach, Ricketts noted, is that it is not distributed equally around the world. In fact, the connection between environmental degradation and detrimental effects on health is likely to be most pronounced in less developed and less wealthy nations (Myers et al., 2013).

There are several frameworks for thinking about the connections between human and ecosystem health (e.g., Planetary Health, One Health, ecosystem services, and others), and Ricketts believes they share more commonalities than differences, given that they are all centered on the question of how the biosphere and human health are intricately linked and how people can manage the biosphere to benefit both. Through that perspective, he proposed the following key questions for consideration:

• For any policy or decision, what change in ecosystems occurs because of that policy decision?
• What are the effects of any changes on the dynamics of species and their environment?
• How do these translate into specific risk factors for human health and into measured health outcomes?
• Are there mediating factors that might insulate people from a risk and help them avoid an adverse health outcome?

Ricketts illustrated how these key questions help to understand the relationship between the biosphere and human health through an example of how an ecosystem change resulting from a policy decision—such as deforestation—can lead to human health consequences. In this example, deforestation creates microhabitats at the edge of deforested areas that are prime breeding ground for mosquitos. Increasing mosquito populations serve as vectors that spread malaria to people living near those deforested areas. This human health outcome might be mediated through distribution of bed nets, known to be an effective measure to prevent malaria.

One project that Ricketts has worked on examined whether nearby nature improves children’s health. Using data from the National Science Foundation’s National Socio-Environmental Synthesis Center and the U.S. Agency for International Development that provided information on health outcomes, social factors, and household factors for some 800,000 children in 49 countries, Ricketts and his collaborators merged those data with information on natural features such as climate, land use, and parks to show that proximity to forests helps reduce some health risks in children, though the effects are not observed uniformly for all children (Herrera et al., 2017; Naidoo et al., 2019; Rasolofoson et al., 2018). Ricketts then described some examples of health risk reduction that this study found. For instance, the presence and density of forests in upstream watersheds are associated with reduced incidence of diarrheal disease in children, but this effect was only observed for rural households. Proximity and exposure to forests increase diet diversity in children by 25 percent and reduce the incidence of malaria in children, but the effect was more pronounced in poorer households than in wealthy ones. Finally, forests in the form of protected areas or national parks reduces incidence of stunting, but only if they are accessible (i.e., by tourists and by local households). “We are finding again and again that nearby forest[s], upstream forest[s], or national parks are improving the health [of] or [are] associated with improved health in children, but not for everyone equally, which is probably the most important finding of all,” said Ricketts. The take-home message, he said, is that forests and conservation can improve children’s health, with the effect being the strongest for the most vulnerable children. These effects are general enough to detect at broad scales that cover a variety of sites.

Ricketts and his collaborators have also shown that they can detect, at broad scales, positive effects on mental health from exposure to nature. For this study, they used social media to follow people as they moved in and out of green spaces in cities and quantified people’s mood or happiness distilled from their Twitter messages using a well-established metric (Schwartz et al., 2019). The research team found a positive effect on mood and happiness that lasts as long as four hours after spending time in a park, with no such effect from spending time in a civic plaza. He noted that the change in measured happiness resulting from spending time in nature was about as large as that observed for Christmas Day, which is typically the biggest happiness bump each year.

A subsequent analysis was conducted for the 25 most populous U.S. cities. Except for Baltimore, there was a significant increase in happiness across these cities, with the magnitude of the increase

depending on the city (Figure 2-2; Schwartz et al., 2022). The take-home lesson, he stated, is that a policy decision—creating urban green spaces—allowed urban ecosystems to exist, which can improve mood and increase happiness among the residents of those cities.

Ricketts offered three suggestions for research directions:

1. Linking broad and mechanistic studies (Kweka et al., 2016; South et al., 2018).
2. Providing policy context by comparing the results of a nature-based approach to alternatives (Herrera et al., 2017) and comparing the costs of a nature-based approach to alternatives (Das and Vincent, 2009).
3. Focusing on who benefits and who suffers from changes in ecosystems with an emphasis on equity and the distribution of benefits (Hoffman et al., 2020; Marlier et al., 2021).

Ricketts concluded his remarks by reiterating that nature affects human health in diverse ways, from physical health to mental health. Consequently, investments in nature can be public health investments, which is a powerful and exciting idea given that it is possible to conserve nature and save lives simultaneously. He also noted that targeting research in some of the ways he outlined could increase the insights, impact, and equity gained from research.

When asked if the characteristics of a forest, and not just deforestation, affect human health, Ricketts replied that the current studies appear to indicate an association between human health and the

general presence of a forest, but cautioned that there is a lack of detailed data to delineate contributions from specific characteristics of the forest toward this effect. He surmised two characteristics of forests—that they tend to be cooler and have less standing water—may reduce the incidence of vector-borne diseases, but there needs to be more research that involves measuring the microclimates around forests.

To better share the results of this type of research both across academic disciplines and between academics and implementors, Ricketts said that workshops such as this one, societies such as the EcoHealth Alliance and EcoHealth International, and organizations such as the Planetary Health Alliance provide important venues for sharing information. Given that researchers respond to incentives, research network grants can play an important role in supporting interdisciplinary research. He noted that one of the most powerful ways to connect with implementors is to start projects with them as colleagues and co-owners of the research rather than as consumers of research results. That approach has produced the biggest impacts for his organization’s research.

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