Is Dirt Good for Kids? Immunology and the Microbiome in Outdoor Play

Short summary

Kathleen Davis hosts a Science Friday episode exploring whether exposure to dirt can boost kids' health. She talks with Dr. Jack Gilbert, a microbiologist at UC San Diego, and Dr. Amber Phife Johnson, a pediatric epidemiologist at Washington State University, about how outdoor play and environmental microbes shape the developing immune system and gut microbiome. The guests explain that early microbial exposure can support immune development, potentially reducing later asthma risk, while still emphasizing sensible hygiene. They discuss indoor versus outdoor settings, the role of dogs in exposing children to beneficial microbes, and the science behind preschools that operate outdoors. The episode also covers practical guidance for parents about safe outdoor time and school policies.

Overview

In this Science Friday episode, Kathleen Davis delves into a question many parents hear: does dirt help or harm kids? The discussion centers on how outdoor microbial exposure influences the developing immune system and the gut microbiome, with insights from Dr. Jack Gilbert, a microbiologist and professor at Scripps/UC San Diego, and Dr. Amber Phife Johnson, a pediatric epidemiologist at Washington State University. The show emphasizes that early microbial encounters are an integral part of healthy immune maturation and can shape immune responses across the lifespan.

Immune system development and dirt exposure

Jack Gilbert explains that humans evolved amid a microbial-rich environment and that immune systems are wired to respond to frequent microbial stimuli. The guests highlight that childhood immune development is foundational for long-term health, influencing susceptibility to infections, allergies, and chronic diseases later in life. The gut microbiome, in particular, is a central mediator, interacting with roughly 70% of immune cells and guiding the trajectory of immune education from birth through adolescence.

"The childhood immune system development is fundamentally foundational across the health of the lifespan" - Dr. Jack Gilbert

Indoor vs outdoor exposure, dogs vs cats

The conversation contrasts dirt and outdoor microbes with indoor environments. The experts discuss physical contact with soil, pets, and the inhalation of environmental signals as primary routes for immune stimulation. They note that dogs tend to be more immunostimulatory than indoor cats, with dogs contributing to a meaningful reduction in asthma risk in exposed children. They also acknowledge that hygiene practices remain important to prevent infectious disease, so washing hands after outdoor activity is encouraged.

"the soil is a very rich microbial world and microbes and animal-associated allergens can support immune development" - Dr. Jack Gilbert

Preschools, outdoor time, and health outcomes

Amber Phife Johnson discusses ongoing research comparing outdoor preschools with traditional indoor programs. Preliminary data suggest differences in gut microbial diversity tied to outdoor exposure, with potential downstream effects on cardiovascular health and childhood obesity. The researchers point to social exposure in preschool as a catalyst for microbial sharing and immune shaping, citing baboon studies that link social interaction to beneficial microbial exchange. The discussion also touches on the cultural shift toward outdoor learning, accelerated in the United States since 2010, and COVID-era experiments that broadened acceptance of outdoor education.

"the gut microbiome plays a central role" - Amber Phife Johnson

Practical guidance for families and schools

The episode closes with pragmatic advice: promote outdoor time and microdoses of exposure, aim for explorations rather than destination walks, and advocate for green spaces and outdoor recess in schools. While encouraging dirt play, the guests stress the importance of cleaning hands after outdoor activities to reduce infection risk. The conversation reinforces that outdoor time should be integrated with sound hygiene practices to maximize immune development without compromising safety.

"the childhood immune system development is fundamentally foundational across the health of the lifespan" - Dr. Jack Gilbert