Finnish researchers replaced hard, urban daycare surfaces with forest floor, turf, planter boxes and peat blocks and tracked 75 children aged 3–5 across 10 centres. After 28 days, children using the greened yards showed increased skin microbiota diversity, higher regulatory T cell counts and elevated plasma TGF-β1 — biomarkers linked to improved immune regulation. The findings support the "biodiversity hypothesis" and helped prompt roughly $966,000 in funding to rewild 43 daycare grounds in Finland.
A Month in the Mud: Finnish Daycare 'Rewilding' Improved Young Kids' Immune Systems
Two little kids play in the mud.
Finnish researchers tested a simple question: what happens to young children’s immune systems if urban daycare yards made of concrete, gravel and plastic are replaced with natural materials?
To answer it, teams coordinated by the Natural Resources Institute Finland transplanted patches of forest floor into city daycare play areas, laid turf, installed planter boxes for growing plants, and added peat blocks for climbing and digging. The trial followed 75 children aged three to five across 10 daycare centres in Lahti and Tampere. Some sites remained unchanged, some continued regular nature outings, and four daycares had their outdoor spaces transformed with forest undergrowth and added greenery.
What Researchers Measured
Scientists sampled children’s skin and gut microbiota, drew blood, and monitored immune markers before and after the intervention. The results — published in Science Advances — were measurable after just 28 days.
Key Findings
Children who spent time in the greened play areas showed several biological changes linked to improved immune regulation: a more diverse skin microbiota, increased counts of regulatory T cells (which help prevent the immune system from attacking the body), higher plasma TGF-β1 concentrations, and shifts in other immune markers associated with better immune control.
Two young girls splashing and playing in water. Photo credit:Canva
"The results of this study support the biodiversity hypothesis and the concept that low biodiversity in the modern living environment may lead to an un-educated immune system," the study authors wrote.
Marja Roslund, a scientist at the Natural Resources Institute Finland whose thesis included this work, framed the implications practically: "Immune diseases are expensive. Even a small reduction in the burden of these diseases is good for national health and the economy."
Policy and Practical Impact
The findings helped spur funding to scale the idea: according to The Guardian, Finland allocated about $966,000 to rewild 43 daycare centres, replacing asphalt, plastic and rubber with trees, flowers, sandpits, rocks and grass. Some centres, such as Poutapilvi-Puimuri daycare, are already being remodeled to move more activity outdoors. Delegations from Iceland, Denmark and Norway have visited Finnish sites with interest in replicating the approach.
What this does — and doesn’t — show: The study provides short-term biomarker evidence that increased contact with natural soil and vegetation can alter microbiota and immune markers in young children. It does not directly measure long-term clinical outcomes (for example, lower rates of allergies or autoimmune disease), but the biomarker changes are consistent with the "biodiversity hypothesis" that early microbial exposures help educate the immune system.
Many parents worry about children getting dirty, but this research suggests that contact with soil and natural materials may offer meaningful immune benefits — an argument for thoughtful, supervised outdoor play in biodiverse settings.
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