Science Friday - A Little Grime Can Boost Kids’ Health. But What Kind?

Episode Date: February 16, 2026

You may have heard that a little dirt is good for kids. It helps them build up their immune systems, and sets them on a path to future health. But what kind of filth does the trick? Producer Kathleen Davis digs into the latest science on the benefits of exposing kids to the outdoors with microbiologist Jack Gilbert and pediatric epidemiologist Amber Fyfe-Johnson.Guests:Dr. Jack Gilbert is a microbiologist and professor at the Scripps Institution of Oceanography and in the department of pediatrics in UC San Diego School of Medicine.Dr. Amber Fyfe-Johnson is an associate professor and pediatric epidemiologist at Institute for Research and Education to Advance Community Health at Washington State University.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey, it's Kathleen Davis, and you're listening to Science Friday. Today on the show is Grime Good for Kids. We asked you, our listeners, to tell us the grossest thing your kids have done. Hi, my name is Cheryl. I'm calling from Bozeman. My son, I one time caught him out at the barn. He was about three years old, and he was eating fresh llama poop, putting them in, like one pellet at a time like they were raisins. And he had, his mouth was covered with this like green black goop.

Cheryl, thank you so much for selling your kid out for the story. You may have heard that a little dirt is good for kids. It helps them build up their immune systems and it sets them on a path to future health. But what type of filth does the trick? Is a little llama poop good for the gut? Here to answer those pressing questions and dig into the latest science into the benefits. of exposing your kids to the outdoors are my guests. Dr. Jack Gilbert, microbiologist and professor at the Scripps Institution of Oceanography

and in the Department of Pediatrics in UC San Diego School of Medicine, and Dr. Amber Fife Johnson, Associate Professor and Pediatric Epidemiologist at the Institute for Research and Education to Advanced Community Health at Washington State University based in Spokane. Thank you both so much for being here. My pleasure. It's exciting to be here. Thank you so much. What a treat.

Okay, before we really jump in, I want to ask you to, what are your thoughts on Cheryl's call? Are you as horrified as I was when I first listened to that? I'll just start by saying, filth, I don't know, man. We probably shouldn't be exposing the kids to filth,

but that sounds really bad. And I wouldn't necessarily want my kids eating llama poop, purely because their breath would smell really bad. But on the whole, you know, if what doesn't kill them might make them stronger. What about you?

Amber. I have to second, Jack's opinion here. I'm all for dirt and microbes. Poop might be taking a little far. Lama poop in particular. Okay. So Jack, when we're talking about letting kids play in the dirt, you know, what kind of benefits to dirt are there? How do these different parts of the dirt fit into the equation? Really, it's all about immune stimulation. I mean, you know, what we're dealing with are ape creatures ourselves, who evolved. involved in a world that's seething with microbes, microbial stimulants, all the little bacterial cells, viral cells, fungi floating around, and they're stimulating our immune systems. So all of our ancestors got this monstrously large microbial exposure, and we're talking about trying to recapture that

microbial exposure for children, because their immune systems are expecting to see it. I think that it's important to remember that the childhood immune system development is, is fundamentally foundational across the health of the lifespan. It's not just immediate. And so from birth through adolescence, the immune system really is learning how to defend against infections while also avoiding harmful overreactions. And early disruptions in this process,

such as limited microbial exposure or things like chronic inflammation, can really bias an immune system and immune system responses towards allergy or onimmunity or chronic disease in childhood. And the gut microbiome plays a central role. I know about 70% of immune cells interact with the gut. So, for example, children with reduced gut microbial diversity in early life have higher rates of asthma and allergic disease later in childhood. Childhood is just really critical intervention window because immune system development is really highly plastic in early life.

And intervening later when we're adults is far less effective than supporting immune development early on. And so what happens in early life does not stay just in early life. It programs immune function for health across the lifespan. What are the environmental exposures that make the biggest difference for kids? We found that really it's exposure to animals and dirt. So soil, I should say, for my soil scientist friends, is key, right? The soil is a very rich microbial world. And that along with the microbes and allergens that are associated with furry,

pets and furry animals can play a foundational role in supporting immune development. In fact, children that grow up physically interacting with a dog have almost 15% reduction in the lack of developing asthma. So these are significant trajectories, you know? So what about like an indoor cat, for example? I mean, that cat's not necessarily going outside and rolling around in the dirt itself. I mean, is it beneficial for kids to have that exposure? No, weirdly, cats. are not immunostimitory in the same way as dogs. And we think the dogs are actually better than cats. And I'm a dog person.

So I'm horribly biased towards this. But yeah, I mean, I get this question all the time. And cats, no. People that grow up physically interacting with the cat don't seem to have the same kind of beneficial immune exposure as those that physically interact with the dog. Wow. Cat slander, I guess. Okay.

How does playing outside improve kids immune systems? How physically do you expose your body to these different allergens. I mean, do you have to eat llama poop to get the effect, or can you just kind of roll around in the dirt here? I did a study once. Propositioned the NIH, I think that's the right phrase, to ask them if I could have a traveling zoo or traveling farm

that I would take around and rub baby's faces into the sides of cows so they'd get that proper immune stimulation. Mostly it's through physical contacts and breathing stuff in, right? So if you plunge your hands into the dirt, your immune system is actually being stimulated by all of those microbial antigenic signals in the soil, right? And if you rub a pet with your hands or you let it lick your face, you're getting that kind of microbial exposure. And then whatever you breathe into your lungs is stimulating the antigens, antigenic receptors. They're all throughout your respiratory tract and your mouth and their skin. So these are primary ways.

physical interaction and, you know, respiratory interaction with the world. That's really important. I mean, just by touching something is fascinating to me. So in theory, could you, you know, rub your hands in a bunch of dirt and then come inside and wash your hands and still get an effect? Absolutely. And we recommend that 100%. You know, there's no problem with cleaning your hands. It can help to reduce the spread of infectious disease and reduce the chances that you're going to put something into your gastrointestinal tract, which might make you sick. So we absolutely say that. Yeah, you will get the immune benefit from interacting with the world, but hey, wash your hands afterwards before you eat. There's a study that came out recently in nature that showed even just

one month of preschool improved the diversity of kids' microbiomes. I want to ask you about the mechanisms of this at risk of maybe getting a little grossed out. But what changes can come to kids' microbiomes just by interacting with other kids. Hey, we're social creatures. We have evolved all these wonderful mechanisms to interact with each other, kissing and hugging and eating each other's boogers and licking each other's bagers. Maybe just you. I don't know about me.

But, you know, that's, I think to me that's the key piece here, right, is the kids that can socially interact with their peers will get a greater significant exposure to the world, to the microbial world, to the antigenic world, that will shape their immune system and shape their microbial diversity. In fact, you know, we worked with baboons in the Amberseli National Park with collaborators from across the US and Africa to identify how social interactions between these baboon groups drove changes in their microbial structure. And baboons that more socially interact with each other share beneficial microbes that actually help their help. So we think that these kids, when they get that, you know, a year of preschool exposure are getting much more

rigorous social interaction, which is shaping their microbial world, their immune systems, and microbial diversity, potentially for benefit. Okay, I want to ask you both, in the last decade, there's been a lot of scientific evidence that backs this idea up of dirtier kids, for lack of a better term, being healthier kids. But have we seen the culture changing. I mean, is it acceptable to have a kid rolling around in the dirt? Maybe Amber, you want to go first? Yeah, I think that there has been certainly an acceleration in interest in outdoor preschools. So the estimated number of outdoor preschools in the United States has skyrocketed. So around in 2010, they were like 25. And in 2020, there are almost 600. And this is across the

States. So to me, that means there's really been sort of a value shift. So many of these outdoor preschools fundamentally, historically, have come from European countries where being outside is, from a value system perspective, believed to be healthy. And so what we're seeing is a bit of a lag, but a sharp acceleration in the United States. So to me, that means that the people are embracing this mentality. And it's, you know, these preschools are not isolated simply to Washington. They're really distributed to Minnesota, the Carolinas, et cetera. So it's really a national distribution, and that to me is really excited. I mean, I can imagine some listeners may be skeptical of outdoor preschool just as like, you know, it could be easy to push it off as kind of like a hippie,

crunchy granola thing. But I mean, you're scientists. Is it that, you know, far out of an idea? It's absolutely not. I often talk about how we have this culture that believes that kids, for some reason, thrive in indoor spaces, in spaces that have four walls. But yet many of us know, from our own experience, as adults that taking a quick walk outside, sitting on the beach, there's an effect there. So I would also come back to COVID a little bit here, that COVID kind of pushed the margins in terms of schools and preschools thinking about what? was acceptable and what could be the norm. And a lot of schools experimented with doing educational opportunities outside and found really incredible benefits, not just for the kids, but for their teachers as well. And so I think COVID actually was a lesson and sort of pushed us in the right direction in terms of thinking about how education has, for quite frankly, decades, happened outside more. There was more recess time or kids just were allowed to be outside more. We've kind of, the pendulum shifted a little bit to being more indoor.

And now we're starting to shift back to being more outdoor. And to see that change is really exciting. And every rural farmer that I've ever spoken to says, oh, this is, I've known this for years. A peck of dirt before you die will keep you healthy, right? I think this is not something which is totally isolated to do. Hippie granolries in people. It's a fundamental precept of what it means to live in a rural environment.

After the break, we're going to keep rolling in this pig penit. of knowledge, don't go away. Okay, I want to ask about this experiment that you're both involved in that compares kids who go to traditional, mostly indoors preschools versus kids who go to an outdoor preschool like we've been talking about. Amber, what have you found so far? We're early in the project. We're just finishing year two. So we don't have results from our current project. But we use preliminary results to get funding to do this. larger project. And I think one of the more exciting things that we found is that we compared kids attending preschool-age children attending an outdoor preschool versus kids attending an indoor preschool.

And we found a difference in gut microbial diversity, which was really encouraging in that while we hypothesize that there's going to be some changes in this space, we actually did indeed see them in this sort of preliminary data. And also that it is likely, related to some cardiovascular outcomes, in particular childhood obesity and childhood overweight, so that we are hoping in some way that the project that we're doing will influence cardiovascular health, both from an adiposity framework, but also cardiovascular health long term into adulthood as well. So, Jack, let's say that you grew up as an indoor kid with extra vigilant and sanitary parents. I mean, can you make up for lost time? Can you diaper? Can you divert

diversify your microbe by, say, going hiking every weekend as an adult? I think you can definitely have an impact later on, especially on your microbiome, right? You know, you can increase the diversity of foods that you eat. You can increase the amount of time you spend outdoors. As Amber was suggesting, you know, just being outdoors makes you feel good. You know, maybe it's the fresh air, maybe it's the sunshine, or maybe it's all of those things, plus microbial stimulation. And we do see that people that spend more time out.

outdoors, have lower levels of cortisol, lower levels of systemic inflammation. And we believe that that's due to this mediating influence of the microbiome on those impacts. Okay, to wrap up, I want to play a voicemail from a VIP caller. Hey, it's Flora, long-time listener, first-time caller. Hello from Phoenix, Arizona, where I am this week. So I have little kids, and they are constantly doing gross things like little kids do, including. like last weekend we were at a bowling alley and they were eating french fries off the ground. And when I left that night, I had genuine stress where I was like, are they going to be okay? And here's my question for your guest.

What kind of gross behavior could my kids do that would actually put them in danger? Where is the line? What behaviors do I actually need to be concerned about when it comes to? you know, rolling around in the New Jersey Transit bathroom, for example. Okay. Tell me. Thank you. Have a good show.

Okay. Can you put our dear flora at ease here? I mean, is the lion llama poop? For me, for me, the lion is. And, you know, for flora's edification, they, you know, there are dangerous pathogens in your kitchen, you know, wash your hands and wash the surfaces after you've been using more chicken. Make sure your kids are not putting those potential pathogen concerning things into their body.

And the same is true for certain bathrooms. I mean, it's extremely unlikely in our modern environments in the United States that you will be exposed to enteric pathogens through those systems. But it is possible. So, hey, wash your hands. You know, I wouldn't pick up the fries if they've fallen in the toilet. Any parting words for parents or caregivers out there who are maybe rethinking a few things after this conversation? My summary, which will come as a surprise to nobody, is try to promote outdoor time for yourself and your children. Aim for frequent microdoses. It doesn't have to be a hike in the mountains.

It can be, but think about things that are pragmatic and doable. You can walk around the block, and I like the concept of exploration versus destination walk. Does your child want to walk far? Do they just want to meander around the block and find something cool? in the leaf litter or something like that. And also try and advocate for green space and outdoor time at schools. Kids are at schools. They're getting recesses. This is an important place where all kids can have access to outdoor time and beautiful outdoor spaces. Well, thank you both so much for being here. This was really fascinating. Dr. Jack Gilbert, microbiologist and professor at Scripps, Institution of Oceanography

and in the Department of Pediatrics in UC San Diego's School of Medicine and Dr. Amber Fife Johnson, Associate Professor and Pediatric Epidemiologist at the Institute for Research and Education to Advance Community Health at Washington State University, based in Spokane. Thank you both so much for being here. Thanks. It was blessed. Thank you so much. This episode was produced by Shoshana Bucksbaum. Thank you for listening. I'm Kathleen Davis. We'll see you next time.