How the smells of nature can affect human well-being

Whether it’s a walk in the park, hike in the forest or tending to a backyard garden, there’s ample subjective and scientific evidence that being in nature can have beneficial effects for us, from relieving stress to improving our mood. But less is known about how scents of nature that are below our conscious awareness, from the unmistakable odor of a pine tree to chemicals emitted by plants, influence human health and behavior.

In a recently published paper, a team of scientists in the U.S., Europe and Asia make the case for more research to be done on the link between the rich olfactory environments of nature and human health. As air pollution and habitat loss threaten biodiversity, they also threaten olfactory diversity in the natural world.

Greg Bratman is the lead author of the paper, an assistant professor of environmental and forest sciences and the director of the Environment and Well-being Lab at the University of Washington. He joins us to share more about this effort, and how the olfactory pathway may open up new possibilities to better understand the benefits of experiencing – and smelling – nature.

Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. Whether it is a walk in the park, a hike in the forest or tending to a backyard garden, there’s plenty of subjective and scientific evidence that being in nature can have beneficial effects on us. But a lot less is known about the role that smells play in that. A team of scientists from around the world say it’s time to learn more. In a new paper, the researchers argue that we should tease out the connections between natural olfactory environments and human health. Greg Bratman is the lead author of the paper. He is the director of the Environment and Well-being Lab at the University of Washington where he is an assistant professor at the School of Environmental and Forest Sciences. He joins us now. Welcome to the show.

Greg Bratman: Thank you. Thank you for having me.

Miller: It’s great to have you on. So your work broadly is on the intersection of spending time in nature and human health. Why did you want to study smells in particular?

Bratman: Yeah, great question. So I work in this emerging field of nature and health and it takes place at the nexus of environmental psychology, ecology, public health. And a lot of this work – not all of it – has focused on the visual pathway to help explain causal mechanisms, and reasons why human beings may psychologically benefit from exposure and contact with nature. So the olfactory pathway has been quite understudied in this context. And in fact, western cultures often undervalue the sense of smell in general, but olfaction is very closely tied to the limbic system. It can have a strong influence on our behavior, formation of memories. So we could be missing something by leaving this out in the field of nature and health. We gather together this multidisciplinary group across health, social, natural sciences to form a working group on this topic of the role of the olfactory pathway in nature and human well being.

These colleagues come from a variety of aspects and have studied a variety of aspects of olfaction for many years. And that spans from the biological [and] chemical dimensions of how our olfactory systems function, to the ways in which culture and lived experiences influence human perceptions of smells around the world, to other insights that come from their expertise in exposure science or even knowledge about how animals use olfactory cues for navigation, as well as expertise in shinrin-yoku from Japan or what we call “forest bathing” – which I can talk more about as well, but has a focus on the olfactory pathway too.

Miller: So it’s not like work hasn’t been done. But as you noted, you and others say that this field, olfaction, is vastly understudied compared to visual stimuli, for example. Why is that?

Bratman: I don’t know all of the reasons for that. One might call it a bias in our approach to the scientific understanding of environmental stimuli and why they influence our well being. I think it’s rooted in many different reasons when it comes to western science. But yeah, it’s just there. And I do think that this is a world that’s now sort of opening up with the help, or at least informed by, the recent COVID-19 pandemic and the loss of sense of smell that happened for many people around the world on at least a temporary basis. I think a kind of a new appreciation for this pathway is starting to make its way into more popular conversations.

Miller: Oh, meaning when a lot of people reported that they’d lost their sense of smell, more scientists started paying attention to the effects of that?

Bratman: Yeah, I’m not sure if it’s because more scientists started paying attention to it or more popular attention came to the science that was already happening. So this science has been happening for decades, and I think it’s time that it’s integrated into the nature and health field.

Miller: Well, so you’re calling for a lot more interdisciplinary research and more focus on nature, smells in particular, and human health. But how much do we already know?

Bratman: So when it comes to the field of nature and health, there are a couple of studies that have examined the olfactory pathway specifically. For example, there was a VR  experiment by Dr. Marcus Hedblom and others from Swedish University of Agricultural Sciences. And this was in a controlled laboratory setting using virtual reality, with a stressor that all participants underwent and then different environmental settings that they were exposed to afterwards. And this included visual, auditory and olfactory stimuli.

And they found that the smells of nature were the greatest predictor of stress reduction in those participants after the stressors.

Or there’s other work from qualitative data, for example, from Dr. Phoebe Bentley and others from the University of Kent, that assessed how specific sense from a sporust experience were associated for participants with a variety of aspects of self-reported well being. And that included emotional, physical [and] cognitive dimensions, as well as a relationship to the wider world around them. And it’s also entered into thinking about designers of sensory gardens or horticultural therapy for a long time. So, there are these kinds of important studies that have taken place and we’re calling for a broadening out and increased gathering of empirical evidence. And we have sort of a conceptual framework that we’ve laid out for how we think one constructive way might be to go about that.

Miller: I was struck by one of the lines you just said there, that a lot of the subjects had self-reported well being, because one of the things that really stood out to me in the paper is the idea that there could also be compounds in the air that could act on us without our conscious knowledge. How might that work?

Bratman: Yeah, that’s another great question, and a fascinating area of this research for me and from what I learned from these colleagues in the working group. So the first thing I would say is we sort of talk about three different dimensions of the old factory pathway, when it comes to the volatile organic compounds from nature and how that might influence human well being. One is that non-conscious or sub-threshold processing of those molecules by the olfactory system that an individual may not perceive. And there are two others. The first of those two others is an initial affective response about, say, pleasantness or unpleasantness that happens in a sort of prelinguistic or precognitive, appraisal-like way. And then the third is this very important piece about subjective experience, culture, previous associations and memories with a smell that plays an important role.

But to your question about the subthreshold processing, there is a hypothesis that comes primarily from shinrin-yoku – again, that forest bathing literature and researchers – that posits that certain VOCs from trees – what are known as terpenes that are emitted from trees to repel herbivores or attract pollinators – also might have this anti-inflammatory response in the human immune system through biochemical processes, that we may or may not perceive consciously. So in addition to the aroma of pine, these volatile molecules may be moving through us in ways that have effects neuroimmunologically.

Miller: What is forest bathing? What is the Japanese theory or practice of forest bathing?

Bratman: So this is a complex practice and one that I’m not an expert in. But it involves, for instance, if one is to be a forest bathing guide here in North America, there’s a certification process that one goes through. It’s guided practices in a forest and certain ways in which one interacts with the forest. And it’s a multi-sensory experience and typically exists in a kind of therapeutic context, but we’re focusing specifically on the olfactory piece of that practice. So, that’s the way in which this feeds into the framework on the olfactory pathway. And this idea that there may be this kind of regulation of transcription factors involved in inflammatory responses that happens through the volatile organic compounds emitted from trees.

And a fascinating piece about it also is just that, what this helped open up for me anyway, is insight into the fact that the natural world is signaling and communicating within itself through volatile organic compounds. And we live embedded within that invisible context. It affects us and we affect it through our human activity; air pollution, even the way our bodies emit volatile organic compounds. So we’re constantly interacting with this world that we don’t really see, but it has an effect on us and we need to take it into account when we’re thinking about both our own well being and the actions we take as it relates to the natural world.

Miller: How universal are human responses to different smells from nature?

Bratman: That touches a bit, at least, to my knowledge on … Well, the first thing I’ll say on that is I think that’s sort of an area of investigation in this research. And there are co-authors on this paper who have expertise in that question. So one might think about that initial affective response, part of the olfactory pathway, so that sort of pre-top down subjective judgment or experience of a smell that happens sort of immediately. And we posit and we say in the paper that this might be [an] innate, possibly universal kind of initial judgment that happens through a suprathreshold, so a perceived experience of a smell, immediately following …

Miller: Like a lilac is sweet?

Bratman: Yeah, like a lilac is sweet, or this smell right here of a certain food is either healthy or perhaps not something I should ingest. So that initial response. Then there’s an additional layer that comes on after that, typically or maybe concurrently, in which culture and previous lived experiences, associations and memories play a role, and then further interpreting the meaning of that smell. In this case, from nature. And that all plays, of course, a very important role in how that would lead to different aspects of well being.

Miller: Like balsam, for one person, it smells like Christmas and family. And for another person, it smells like, I don’t know, memories of abuse or … I mean, there’s both culture and personal experience that get layered on top of olfactory memories.

Bratman: Exactly.

Miller: Let’s turn to another big part of this research which is that human activity is encroaching in various ways on a lot of the smells you’re talking about. What are those ways?

Bratman: So again, we’re still learning about this. And I’ll say this is again in the spirit of the paper, a call for more research. And so the paper fits together in so far as we call for integration of evidence and inclusion of that evidence in our decision making. So one way to think about this is, what’s happening when we decide to develop a natural area for, say, housing or other reasons that may be needed? When we do that, what are we accounting for in terms of the impacts that we’re having on the natural olfactory environments with which it was intimately connected?

And so we can think about the removal of biodiversity that happens with that development. We can think about increased air pollution that may come from roads and traffic that now take place in that area. And there’s evidence from other disciplines that sort of demonstrate how air pollution can be very disruptive to the signaling that happens in natural environments, through VOCs, for instance, through pollinators. This may also have an impact on the way in which natural olfactory environments impact human well being. And so as we sort of develop on the ground, we need to think about the repercussions of that for the air we breathe.

Miller: This made me think about a park that probably most Portlanders have been to or at least know of, Forest Park. It stretches for about eight miles on the west slope of the city. There are times when you’re there, when you can smell just wonderful mud or loamy earth or ferns and moss. But if the wind is blowing a different way, you smell petrochemicals from the tons of tanks there, from this critical energy infrastructure hub that is right below the park.

The effect that it has on me is to sort of take me out of the forest. But that is such a subjective experience. How would you go about studying that phenomenon?

Bratman: We have, in some ways, a simplified model of starting to try to think about that question as I see it and it comes about through our conceptual framework, kind of step one. In step one, we say, how do we characterize olfactory environments? How do we think about concentrations and ratios of airborne chemicals from nature and how they’re mixing and interacting with anthropogenic chemicals or ones that come about through industrialization? And how does that move through and help influence the ways in which we perceive that olfactory environment? So it’s both going to be a characterization of the environment itself, and it’s going to take into account our ability to identify or discriminate or be sensitive to the smells of nature within that environment, because it’s going to interact with our ability to perceive it.

Miller: I’m familiar with fragrance gardens that have long been a part of different cultures, botanical gardens. But what about some kind of, I don’t know, UN olfactory heritage site. What would that be like for you?

Bratman: Another great question and one in which other co-authors on this paper have expertise. In fact, in a line of work called Olfactory Heritage, this is fascinating work that focuses on both the ways in which certain smells have cultural importance through time and help contribute to sense of place and sense of identity in different communities, and how those smells are constituted chemically. And that is helping, among other sorts of movements in the conservation world, to inform this idea of sensory law. There’s recent activity to protect soundscapes and to reduce light pollution in National Parks, and we make the argument that this need not only take place in National Parks, but globally. We need to pay attention to including smellscapes in that work, in policy, and we note in the paper that this has recently been done in certain parts of France, through sensory law, it’s called. So, yeah, I think that’s an important thing for us to take into consideration as part of that practical decision making piece.

Miller: Greg Bratman, thanks very much.

Bratman: Thank you.

Miller: Greg Bratman is the director of the Environment and Well-being Lab at the University of Washington.

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