What Spending Time Outside Does to Your Microbiome

Written by Emma Loewe: Writer, author, and editor of Cultured. Her writing explores the intersection of nature, climate, and human health. Emma is the author of “Return to Nature” and “The Spirit Almanac.”
Reviewed by Jennie O’Grady: Senior SciComms Specialist at Seed Health

Nature excels at leaving its mark on us: it fills our lungs, scatters freckles on our shoulders, and shifts our perspective. We walk away not just with memories, but with invisible souvenirs: bacteria, viruses, and fungi of every shape and size.

When we sprawl in the grass, amble through the woods, or dip in the ocean, these organisms nestle themselves into our holobiont—the microbial counterpart to our human bodies.¹ In this way, it’s impossible to tell where we end and the natural world begins; we are ever-evolving maps of the places we’ve traveled.

Clearly, the human body is not just a visitor in nature, but an ecological unit all its own. So how might this knowledge help us solve some of today’s most looming threats to human and environmental health—from disease to climate change—and strengthen our relationships with nature along the way?

Why We’re Thinking About Dirt All Wrong

More than half of our cells are not actually ours; they belong to trillions of microbes that help protect us from intruders and keep vital biological processes running smoothly.² Most of these bacteria, fungi, and other microbial specimens reside in our gut microbiome, one of the densest microbial habitats on Earth, but they also inhabit and exert influence over our skin, mouth, eyes, nose, lungs, and more.³ 

Part of the human microbiome forms as we are born, but it progresses with our interactions with the outside world: the 100 million bacteria we inhale each day, the dozens of bacterial species on the fruits and vegetables we eat, etc.^4-6 This environmental influence explains why people who share a household tend to have similar gut microbiomes (even if they’re not related) and why our microbiomes undergo noticeable shifts when we move.^7,8

According to the “Old Friends” theory, since we co-evolved with certain microbial hitchhikers, we’ve grown to form commensal (mutually beneficial) relationships with them.^9,10 As such, the microbes in our environment have played a major role in shaping the human immune system and inflammatory response over time.¹⁰

These days, however, our contact with these microbes (in the Western world, at least) is steadily decreasing. The average American now spends 87% of their time indoors and an additional 6% in an enclosed vehicle like a car or bus.¹¹

This move indoors has correlated with a surge in inflammatory diseases from asthma to allergies, autoimmune disorders to anxiety (and that’s just the As).¹²  In microbe-rich environments, such as Amish farm communities and Indigenous villages, rates of these diseases are lower.^13,14 This has led researchers to speculate that a growing disconnection from the natural environment may be contributing to the rise of some diseases.¹⁵ 

If shutting ourselves inside strips us of beneficial microbes, can getting outside support our microbial health? It certainly seems that way.

Microscopic Organisms, Macroscopic Health Benefits

Microbioscape researchers analyze how the microbes in the natural world shape human health.¹⁵ They’ve discovered that the various microbiomes of the body do “pick up” some of the microbes we encounter outdoors. The microbial composition of the skin becomes more similar to the soil microbiome when we dig in the dirt, for example, while our nasal microbiome shows parallels to the airborne microbiome after we take a big whiff of fresh air.¹⁶ 

When we adopt these natural microbial assemblies, they quickly get to work on us—often in ways that benefit our health and combat dysbiosis (microbial imbalance). When certain microbes from the environment make their way to our digestive tract (via hand-to-mouth transfer, the food we eat, etc.), for example, they seem to diversify the gut microbiome, helping it outcompete pathogenic microbes that carry disease.¹⁷ 

Or consider the research that shows that adults who live surrounded by vegetation tend to have a more stable balance of gut bacteria in their stool than those who don’t.¹⁸ Separate research with preschool children found that those who took a 10-week outdoor school program experienced changes to their gut microbiome that correlated with feeling less stress (potentially due in part to the gut-brain axis).¹⁹

When we brush against a plant or dip into ocean water, it can also alter our skin microbiome in ways that restrict the growth of pathogens.²⁰ In a study out of Finland, adding sod and vegetation to children’s play areas was enough to increase the richness and diversity of beneficial bacteria on children’s skin while lowering pathogenic bacteria and positively impacting immune function.²¹ 

Beyond exposing us to a tapestry of commensal bacteria, engaging with familiar natural environments is known to reduce psychological stress, further supporting microbiome health.²² (The inflammation that frequently co-occurs with stress harms our microbial defenses and can trigger infection.)²³ 

Clearly, spending time outdoors is enriching, invigorating, and utterly essential—for us, and our microbes. Unfortunately, it’s getting harder to do. Within the next few decades, an estimated two-thirds of the world’s population will live in urban areas, while biodiversity loss will continue to threaten microbial populations around the world.^24,25 As we disturb and develop rural landscapes to make way for cities, we make them (and, by extension, ourselves) more susceptible to pathogenic bacteria and fungi.²⁶ At the same time, increasing heat days and extreme weather events are poised to continue to limit our access to the outdoors.²⁷

Climate change, the extinction crisis, chronic disease—these are interconnected problems that require interconnected solutions. As a team of microbial ecologists wrote in a 2018 research paper, we are now faced with “the global challenge of halting and reversing dysbiosis in all its manifestations.”²⁸

How to Embrace Nature’s Microbes (Even If You Live in a City)

The odds may be stacked against us, but it’s totally possible (not to mention fun) to adopt habits that strengthen our microbial connections outdoors. Here are a few ways to get dirty for the sake of your health—whether you’re surrounded by towering trees or towering skyscrapers.  

1. Take a break in greenspace: Try to split up long periods indoors with outdoor breaks. This doesn’t need to mean heading to wild, untouched frontiers: your local park or patch of grass will be perfect. Research shows that city parks are still rich reservoirs of biodiverse soil microbes, and clusters of street trees are teeming with fungal and bacterial diversity.^29,30 Communities of plants attract insects, birds, and mammals—fellow holobionts that carry commensal bacterial species of their own—so make a game out of visiting any patch of green you can spot on the map.³¹ Once you get there, sit in the grass, touch the ground (bonus points if your shoes are off), and take a few deep breaths to invigorate your system with new (but actually very old) microbial friends.³² Resist the urge to look at your phone and give the stress-relieving power of the environment your full attention. 
2. Move some workouts outdoors: We all know that exercise is good for us, and doing it outside seems to be even more beneficial. According to the green exercise hypothesis, outdoor fitness can reduce levels of cortisol (a stress hormone) and have more positive impacts on blood pressure than doing the same exercise inside.³³ When the weather and air quality permit, try doing one workout a week outside—hike the hills of your city, do a bodyweight routine in the grass—and see if any positive changes ensue. 
3. Write your own “green prescription”: Green prescribing is the budding movement of physicians “prescribing” time outdoors to patients as a preventative health measure.³⁴ You don’t necessarily need a medical degree to bridge the language of healthcare and ecology and write your own green Rx.³⁵ Try getting specific about your nature goals, be it committing to a standing Sunday park date or a weekly gardening club, and see if taking a prescriptive approach makes them easier to stick to.
4. Bring home a pet (or just a pet plant): Tempted to adopt a pet? Your microbiome will definitely be in favor of it. Research shows that having an outdoor cat or dog as a pet is beneficial for microbiome diversity and protective against gut issues like IBD.³⁶ But if you’re not quite ready for the added responsibility, how about starting with a plant? Bringing the outdoors in with a structural cactus, a dynamic prayer plant, or an intricate fern is a quick way to expand your home’s microflora. Living green walls have even been shown to alter the microbiome and modulate the immune system among office workers.³⁷ These examples show that (despite society’s obsessions with sanitizing and disinfecting), germs aren’t the enemy, and instead of shutting our homes off from the outside world, we’re better off letting the outdoors in. 
5. Do a microbe-inspired meditation: The next time you could use a perspective shift, plop down in a comfortable spot outdoors near a tree, a plant, or any type of greenery. Give it your full attention, focusing on a smaller and smaller area until you get to the tiniest detail you can spot. Then, think about the microbial world that composes an even smaller part of the plant—too tiny to see with the naked eye. Observe the plant for a few minutes before pulling your attention outwards, gradually zooming out to take in the rest of the landscape around you. Close your eyes and consider the vast sense of scale that exists in the natural world—from bacteria that are measured in nanometers (that’s one billionth of a meter) to grasslands that span millions of acres. Let this be your reminder of the expansiveness that is always around and within you.
6. Advocate for greenspace where it’s needed most: It’s no surprise that greenspace is not evenly distributed, and those who live in less affluent neighborhoods too often lack access to it. On a micro-scale, this has led to a “disparity of microbiota” (also known as dysbiotic drift) that is exacerbating health challenges among the socioeconomically vulnerable.³⁸ Analyses show that those who live far from greenspace also tend to lack fresh food options and are more likely to have poor dietary habits and a higher risk of insulin resistance.³⁹ Access to nature is a human right. If you’re lucky enough to have it, consider supporting those who don’t by advocating for clean, well-maintained parks for all.

Connecting to Our Human Nature 

Humans are not indoor creatures. Emerging research reveals just how much our health hinges on the microbial landscapes of the natural world.⁴⁰ In order to thrive, we must protect remaining wild lands and ensure that our built environments still allow for barefoot walks in the grass, brushes with wildflowers, and all the other chance encounters that are so vital to our human nature.