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What Spending Time Outside Does to Your Microbiome

It’s impossible to tell where you end and the natural world begins; you are an ever-evolving map of the places you’ve traveled. Here’s how to use this knowledge to support your health—and the health of the planet.

9 minutes

40 Citations

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 

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

Microscopic Nature, 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.

Citations

  1. Robinson, J., Mills, J., & Breed, M. (2018). Walking ecosystems in microbiome-inspired green infrastructure: An ecological perspective on enhancing personal and planetary health. Challenges, 9(2), 40. https://doi.org/10.3390/challe9020040
  2. Sender, R., Fuchs, S., & Milo, R. (2016). Revised estimates for the number of human and bacteria cells in the body. PLoS Biology, 14(8), e1002533. https://doi.org/10.1371/journal.pbio.1002533
  3. Mirzaei, M. K., & Maurice, C. F. (2017). The mammalian gut as a matchmaker. Cell Host & Microbe, 22(6), 726–727. https://doi.org/10.1016/j.chom.2017.11.015
  4. Gaufin, T., Tobin, N. H., & Aldrovandi, G. M. (2018). The importance of the microbiome in pediatrics and pediatric infectious diseases. Current Opinion in Pediatrics, 30(1), 117–124. https://doi.org/10.1097/mop.0000000000000576
  5. Flies, E. J., Clarke, L. J., Brook, B. W., & Jones, P. (2020). Urbanisation reduces the abundance and diversity of airborne microbes – But what does that mean for our health? A systematic review. Science of the Total Environment, 738, 140337. https://doi.org/10.1016/j.scitotenv.2020.140337
  6. Leff, J. W., & Fierer, N. (2013). Bacterial communities associated with the surfaces of fresh fruits and vegetables. PloS One, 8(3), e59310. https://doi.org/10.1371/journal.pone.0059310
  7. Rothschild, D., Weissbrod, O., Barkan, E., Kurilshikov, A., Korem, T., Zeevi, D., Costea, P. I., Godneva, A., Kalka, I. N., Bar, N., Shilo, S., Lador, D., Vila, A. V., Zmora, N., Pevsner-Fischer, M., Israeli, D., Kosower, N., Malka, G., Wolf, B. C., . . . Segal, E. (2018). Environment dominates over host genetics in shaping human gut microbiota. Nature, 555(7695), 210–215. https://doi.org/10.1038/nature25973
  8. Shad, N. S., Shaikh, N. I., & Cunningham, S. A. (2023). Migration spurs changes in the human microbiome: A review. Journal of Racial and Ethnic Health Disparities. https://doi.org/10.1007/s40615-023-01813-0
  9. Garn, H., Potaczek, D. P., & Pfefferle, P. I. (2021). The hygiene hypothesis and new perspectives—Current challenges meeting an old postulate. Frontiers In Immunology, 12. https://doi.org/10.3389/fimmu.2021.637087
  10. Rook, G. a. W. (2023). The old friends hypothesis: Evolution, immunoregulation and essential microbial inputs. Frontiers in Allergy, 4. https://doi.org/10.3389/falgy.2023.1220481
  11. Klepeis, N. E., Nelson, W. C., Ott, W. R., Robinson, J. P., Tsang, A. M., Switzer, P., Behar, J. V., Hern, S. C., & Engelmann, W. H. (2001). The National Human Activity Pattern Survey (NHAPS): A resource for assessing exposure to environmental pollutants. Journal of Exposure Science and Environmental Epidemiology, 11(3), 231–252. https://doi.org/10.1038/sj.jea.7500165
  12. Von Hertzen, L., Hanski, I., & Haahtela, T. (2011). Natural immunity. EMBO Reports, 12(11), 1089–1093. https://doi.org/10.1038/embor.2011.195
  13. Frei, R., Heye, K., & Roduit, C. (2022). Environmental influences on childhood allergies and asthma — The farm effect. Pediatric Allergy and Immunology, 33(6). https://doi.org/10.1111/pai.13807
  14. Clemente, J. C., Pehrsson, E. C., Blaser, M. J., Sandhu, K., Gao, Z., Wang, B., Magris, M., Hidalgo, G., Contreras, M., Noya-Alarcón, Ó., Lander, O., McDonald, J., Cox, M., Walter, J., Oh, P. L., Ruiz, J. F., Rodriguez, S., Shen, N., Song, S. J., . . . Dominguez-Bello, M. G. (2015). The microbiome of uncontacted Amerindians. Science Advances, 1(3). https://doi.org/10.1126/sciadv.1500183
  15. Robinson, J. M., & Jorgensen, A. (2020). Rekindling old friendships in new landscapes: The environment–microbiome–health axis in the realms of landscape research. People and Nature, 2(2), 339–349. https://doi.org/10.1002/pan3.10082
  16. Selway, C. A., Mills, J. G., Weinstein, P., Skelly, C., Yadav, S., Lowe, A., Breed, M. F., & Weyrich, L. S. (2020). Transfer of environmental microbes to the skin and respiratory tract of humans after urban green space exposure. Environment International, 145, 106084. https://doi.org/10.1016/j.envint.2020.106084
  17. Tasnim, N., Abulizi, N., Pither, J., Hart, M. M., & Gibson, D. L. (2017). Linking the gut microbial ecosystem with the environment: Does gut health depend on where we live? Frontiers in Microbiology, 8. https://doi.org/10.3389/fmicb.2017.01935
  18. Parajuli, A., Hui, N., Puhakka, R., Oikarinen, S., Grönroos, M., Selonen, V. A., Siter, N., Kramna, L., Roslund, M. I., Vari, H. K., Nurminen, N., Honkanen, H., Hintikka, J., Sarkkinen, H., Romantschuk, M., Kauppi, M., Valve, R., Cinek, O., Laitinen, O. H., . . . Sinkkonen, A. (2020). Yard vegetation is associated with gut microbiota composition. Science of the Total Environment, 713, 136707. https://doi.org/10.1016/j.scitotenv.2020.136707
  19. Sobko, T., Liang, S., Cheng, W. H. G., & Tun, H. M. (2020). Impact of outdoor nature-related activities on gut microbiota, fecal serotonin, and perceived stress in preschool children: The Play & Grow randomized controlled trial. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-78642-2
  20. Grönroos, M., Parajuli, A., Laitinen, O. H., Roslund, M. I., Vari, H. K., Hyöty, H., Puhakka, R., & Sinkkonen, A. (2018). Short‐term direct contact with soil and plant materials leads to an immediate increase in diversity of skin microbiota. MicrobiologyOpen, 8(3). https://doi.org/10.1002/mbo3.645
  21. Roslund, M. I., Puhakka, R., Grönroos, M., Nurminen, N., Oikarinen, S., Gazali, A. M., Cinek, O., Kramná, L., Siter, N., Vari, H. K., Soininen, L., Parajuli, A., Rajaniemi, J., Kinnunen, T., Laitinen, O. H., Hyöty, H., & Sinkkonen, A. (2020). Biodiversity intervention enhances immune regulation and health-associated commensal microbiota among daycare children. Science Advances, 6(42). https://doi.org/10.1126/sciadv.aba2578
  22. Pasanen, T. P., White, M. P., Elliott, L. R., Van Den Bosch, M., Bratman, G. N., Ojala, A., Korpela, K., & Fleming, L. E. (2023). Urban green space and mental health among people living alone: The mediating roles of relational and collective restoration in an 18-country sample. Environmental Research, 232, 116324. https://doi.org/10.1016/j.envres.2023.116324
  23. Madison, A. A., & Bailey, M. T. (2024). Link stress-related gut microbiota shifts to mental health outcomes. Biological Psychiatry, 95(4), 339–347. https://doi.org/10.1016/j.biopsych.2023.10.014
  24. United Nations. (n.d.). 2018 Revision of World Urbanization Prospects | United Nations. https://www.un.org/en/desa/2018-revision-world-urbanization-prospects
  25. Haahtela, T., Holgate, S., Pawankar, R., Akdis, C. A., Benjaponpitak, S., Caraballo, L., Demain, J., Portnoy, J., & Von Hertzen, L. (2013). The biodiversity hypothesis and allergic disease: World allergy organization position statement. The World Allergy Organization Journal, 6, 3. https://doi.org/10.1186/1939-4551-6-3
  26. Liddicoat, C., Weinstein, P., Bissett, A., Gellie, N. J., Mills, J. G., Waycott, M., & Breed, M. F. (2019). Can bacterial indicators of a grassy woodland restoration inform ecosystem assessment and microbiota-mediated human health? Environment International, 129, 105–117. https://doi.org/10.1016/j.envint.2019.05.011
  27. Summary for policymakers — Global warming of 1.5 OC. (n.d.). Global Warming of 1.5 oC. https://www.ipcc.ch/sr15/chapter/spm/
  28. Robinson, J., Mills, J., & Breed, M. (2018). Walking ecosystems in microbiome-inspired green infrastructure: An ecological perspective on enhancing personal and planetary health. Challenges, 9(2), 40. https://doi.org/10.3390/challe9020040
  29. Charlop-Powers, Z., Pregitzer, C. C., Lemetre, C., Ternei, M. A., Maniko, J., Hover, B. M., Calle, P. Y., McGuire, K. L., Garbarino, J., Forgione, H. M., Charlop-Powers, S., & Brady, S. F. (2016). Urban park soil microbiomes are a rich reservoir of natural product biosynthetic diversity. Proceedings of the National Academy of Sciences of the United States of America, 113(51), 14811–14816. https://doi.org/10.1073/pnas.1615581113
  30. Robinson, J. M., Cando-Dumancela, C., Antwis, R. E., Cameron, R., Liddicoat, C., Poudel, R., Weinstein, P., & Breed, M. F. (2021). Exposure to airborne bacteria depends upon vertical stratification and vegetation complexity. Scientific Reports, 11(1). https://doi.org/10.1038/s41598-021-89065-y
  31. Mills, J. G., Brookes, J. D., Gellie, N. J. C., Liddicoat, C., Lowe, A. J., Sydnor, H. R., Thomas, T., Weinstein, P., Weyrich, L. S., & Breed, M. F. (2019). Relating urban biodiversity to human health with the ‘Holobiont’ concept. Frontiers in Microbiology, 10. https://doi.org/10.3389/fmicb.2019.00550
  32. Sinatra, S. T., Sinatra, D. S., Sinatra, S. W., & Chevalier, G. (2023). Grounding – The universal anti-inflammatory remedy. Biomedical Journal, 46(1), 11–16. https://doi.org/10.1016/j.bj.2022.12.002
  33. Loureiro, A., & Veloso, S. (2016). Green exercise, health and well-being. In International Handbooks of Quality-Of-Life (pp. 149–169). https://doi.org/10.1007/978-3-319-31416-7_8
  34. Stanhope, J., & Weinstein, P. (2023). What are green prescriptions? A scoping review. Journal of Primary Health Care, 15(2), 155–161. https://doi.org/10.1071/hc23007
  35. Robinson, J., & Breed, M. (2019). Green prescriptions and their co-benefits: Integrative strategies for public and environmental health. Challenges, 10(1), 9. https://doi.org/10.3390/challe10010009
  36. Cholapranee, A., & Ananthakrishnan, A. N. (2016). Environmental hygiene and risk of inflammatory bowel diseases. Inflammatory Bowel Diseases, 22(9), 2191–2199. https://doi.org/10.1097/mib.0000000000000852
  37. Soininen, L., Roslund, M. I., Nurminen, N., Puhakka, R., Laitinen, O. H., Hyöty, H., Sinkkonen, A., Cerrone, D., Grönroos, M., Hui, N., Luukkonen, A., Mäkelä, I., Nurminen, N., Oikarinen, S., Parajuli, A., Puhakka, R., Roslund, M. I., Saarenpää, M., Soininen, L., . . . Sinkkonen, A. (2022). Indoor green wall affects health-associated commensal skin microbiota and enhances immune regulation: A randomized trial among urban office workers. Scientific Reports, 12(1). https://doi.org/10.1038/s41598-022-10432-4
  38. Prescott, S. L., Wegienka, G., Logan, A. C., & Katz, D. L. (2018). Dysbiotic drift and biopsychosocial medicine: How the microbiome links personal, public and planetary health. Biopsychosocial Medicine, 12(1). https://doi.org/10.1186/s13030-018-0126-z
  39. Logan, A. C. (2015). Dysbiotic drift: Mental health, environmental grey space, and microbiota. Journal of Physiological Anthropology, 34(1). https://doi.org/10.1186/s40101-015-0061-7
  40. Robinson, J. M., & Jorgensen, A. (2020). Rekindling old friendships in new landscapes: The environment–microbiome–health axis in the realms of landscape research. People and Nature, 2(2), 339–349. https://doi.org/10.1002/pan3.10082