Good Things Come In Trees : Short Wave Do you ever feel better after walking down a street that's lined with lush, green trees? You're not alone! For decades, researchers have been studying the effects of nature on human health and the verdict is clear: time spent among the trees seems to make us less prone to disease, more resistant to infection and happier overall.

Aaron Scott talks with environmental psychologist Ming Kuo about why we need greenery and how you can bring more of it into your life.

Good Things Come In Trees

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EMILY KWONG, HOST:

You're listening to SHORT WAVE from NPR.

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AARON SCOTT, HOST:

Hi, SHORT WAVE. Aaron Scott here. I want you to close your eyes and take a deep breath. Now imagine the place where you feel most at ease. What does it look like? What does it sound like? Is it indoors or outdoors? Maybe it's a beach or a forest. For me, it's this grassy knoll surrounded by a lake at the Ridgefield Wildlife Refuge. There are turtles basking on a log, great blue heron walking through the shallows, clouds floating up overhead. For Ming Kuo, it's up in the tree canopy.

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MING KUO: We have a tree-lined neighborhood, and I walk to campus through the trees. And I - as I go, I kind of have my chin tilted up, and I'm mentally flying through the canopy all the way to school (laughter). And I think it looks pretty silly probably to passersby. But it's actually quite a lovely experience. I really like it.

SCOTT: Ming is an environmental psychologist at the University of Illinois, Urbana-Champaign, and her decision to take the scenic route is actually a scientific one. A walk under the canopy has been scientifically proven to refresh you.

KUO: There are specific signs that tell you it's working. One of the signs is a sense of awe. Experiencing a sense of awe on a regular basis is very much tied to inflammatory cytokines, for example. Feeling genuinely relaxed is a big signal - noticing good smells because good smells are often associated with changes in serotonin metabolism.

SCOTT: Today on the show, we're going to look up to that tree canopy to explore the different ways that urban nature makes for healthier humans and how you can maximize the time spent in greenery through some small changes. I'm Aaron Scott, and this is SHORT WAVE, the daily science plant-cast from NPR.

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SCOTT: Before we get specific, I'd love it if you'd be willing to give a quick overview of, more generally, what we know about the relationship between nature and urban green spaces on the one hand and human health and well-being on the other.

KUO: What we know is that nature seems to be tied to a huge array of health outcomes. So if you live in a greener area, you're less likely to have diabetes or heart disease or mental health disorders or muscular issues or skeletal issues.

SCOTT: Wow. So just everything. But it does beg the question because we talk a lot about causation versus correlation - how can you know that these people who live in greener spaces have better health outcomes because of that access to greenery, as opposed to something like, they just have more money and better access to health care?

KUO: There's a whole bunch of work where we're actually doing experiments. So I might take you on a walk through an urban area with trees or an urban area without. After the walk, I pull out my syringe (laughter), and I jab you, and I see that, oh, look; your blood chemistry is different after the green city walk versus the not-green city walk. And so we can actually trace the whole sort of causal sequence.

SCOTT: You are looking at actual biological processes that are taking place in these spaces.

KUO: Right. There are specific studies tying nature to our defenses against viruses. The natural killer cell is - they're in charge of both cancer and viruses. And it turns out that taking a couple days in a green area can boost your natural killer cells by roughly 50%. And then even a month later, they're still 25% above baseline. So this is huge (laughter). It's like giving the United States a big boost in their defense budget. Suddenly, we have 50% more troops. Their job is to run around the body and look for intruders and take care of them. And so it'd be pretty surprising if we had more troops and weren't more effective in handling intruders.

SCOTT: Wow. I mean, that's remarkable to me. And it makes me think of back in olden days when somebody was ill and it'd be like, oh, we're going to send them to the country. And, you know, we now think like, oh, that's silly, but it's not silly.

KUO: No. Yeah. There's a huge amount of biology behind it, actually.

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KUO: One of the fundamental things the body does is it manages resources in a kind of smart way. When you are being chased by a lion (laughter), your body puts all the resources into your muscles and into thinking about escape routes, right? That is not the time to make long-term investments in strengthening the immune system; that's the time to put everything into running (laughter).

But when we feel truly safe and comfortable and like nothing threatening is coming over the horizon, that's when the body invests in long-term outcomes. And so what you see is natural killer cells are going to be boosted when you're in a calm, safe situation for a long time. Phytoncides are the nice aromas that come from pine or lavender. Those systematically help us switch into that calm state. Water does that. Trees do that. In fact, holding somebody's hand does that. So it's not just trees. But everything that helps us feel truly safe and calm helps the body know, OK, it's safe to put investments into immune functioning.

SCOTT: You're giving me a checklist for my next vacation.

KUO: (Laughter).

SCOTT: On a dock, holding a hand, surrounded by trees with some flowers to smell.

KUO: There you go (laughter).

SCOTT: We'd love to talk about a study that you published last year that was looking at the relationship between green spaces and park access and COVID-19 rates. Can you tell us a little bit about that research?

KUO: Right, right, right. We compared areas that have the same income, racial composition, urbanicity, the same proportion of older folks. And what we see is, even when all of those things are the same, if the greenness differs, then the greener place will tend to have lower rates of COVID. A 0.1 difference in our measure of greenness corresponds to a 4% difference in COVID.

SCOTT: That's fascinating and also maybe revealing about how nature can influence so many other health factors, which actually brings us to some of your other recent research. You explored how greenness affects health care costs. Can you tell us a little bit more about that research?

KUO: Yeah, that's a new study. We looked at 5 million Kaiser Permanente members. And we're saying, OK, who are you similar to in our group of 5 million, in terms of race and income? So an individual who lives in a tree-poor neighborhood is going to spend hundreds of dollars more per year than their counterpart living in a tree-rich neighborhood. It's looking like $374, on average, per person, per year, which kind of dwarfs the amount we spend on trees.

So that suggests that there's the potential of relatively small investments in greening could have huge health impacts for the population. And I think one finding that's been really well known in the public health area for forever is that low-income areas often have very poor or very little tree cover. In fact, it's so obvious that you can look at cities from space - you can tell which neighborhoods are wealthy and which ones are not. It's just, like, jump-out, punch-you-in-the-face obvious. So those differences in tree cover may be contributing to huge differences in health outcomes and in health care spending.

SCOTT: And when we think about investing in green space, I imagine it's important to think about what that looks like. I mean, is all green space equal? Is having access to, you know, wilderness and the mountains the same as having access to a park with trees? And is that the same as having access to a park with just grass in it?

KUO: Right. The reason the scientists who study this stuff talk about nature, which sounds like wilderness and, you know, Yosemite - right? - is that's where we started. But as we became more and more desperate to run studies close to home (laughter), we started to use forms of nature that are outside our door or on a university campus. What we've discovered is those kinds of everyday, almost banal forms of nature - a view of trees outside your window, a bouquet of flowers - all of those things have these same systematic effects. They don't have as large effects, but they definitely have them. So you just need to use the nature you have access to.

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KUO: Whatever you can inject into your daily routine is great - if that's looking out at a green view from your window as you wash dishes or having your chair or your desk face a green view. If you have a yard, then gardening has kind of amazing effects. If you have a tree-lined street or a park nearby that's safe and nice, then make a point of taking a walk after dinner through there, ideally holding hands.

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SCOTT: What are your takeaways in thinking about public health?

KUO: I genuinely think we're at the point where greening or tree planting should be one of the next big public health interventions, you know, like, just like we decided having indoor plumbing was a good idea. I think having trees for everyone is a good idea. Trees seem to make us our better selves, not only in terms of our physical well-being and functioning but also our social and cognitive well-being and functioning. Trees are part of a necessary part of a healthy human habitat. Where there are trees, we see people smarter, kinder, healthier, stronger in kind of every imaginable dimension.

SCOTT: Thank you so much for sharing your research with us.

KUO: Thank you for having me.

SCOTT: For a closer look at Ming's research on COVID rates, health spending and green space, head to our show notes. This episode was produced by Margaret Cirino, edited by Gabriel Spitzer and fact-checked by Rachel Carlson. The audio engineer with Stu Rushfield. Gisele Grayson is our senior supervising editor. Beth Donovan is our senior director. And Anya Grundmann is our senior vice president of programming. I'm Aaron Scott. Thank you for listening to SHORT WAVE from NPR.

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SCOTT: Could I just have you introduce your favorite houseplant?

(LAUGHTER)

KUO: I don't have a favorite houseplant.

SCOTT: No? That's totally OK.

KUO: Because all I do with them is kill them.

(LAUGHTER)

SCOTT: Wait. You study the positive effects of nature and yet have the exact...

KUO: No houseplants.

SCOTT: ...opposite effect on the nature that you try to foster yourself?

KUO: That - yeah, that's right.

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