05/03/2026
Did you know a Finnish study found that adding forest floor to urban daycare yards changed children's immune systems in just 28 days?
In 2020, researchers at the University of Helsinki and Finland's Natural Resources Institute ran a study that has been widely cited ever since. They trucked forest floor — soil, moss, leaf litter, dwarf heather, and other low vegetation — into the playgrounds of 10 urban daycare centers in the cities of Lahti and Tampere. For 28 days, 75 children between the ages of three and five played in these "rewilded" yards.
The results, published in Science Advances, were striking. Compared to children at standard urban daycares with rubber mats and gravel, the children playing in the enriched yards developed more diverse skin and gut microbiomes and showed measurable shifts in immune regulation, including higher levels of regulatory T-cells and the anti-inflammatory protein TGF-β1. Lead researcher Marja Roslund noted that the gut microbiomes of intervention children began to resemble those of children who visit forests every day.
The findings supported the "biodiversity hypothesis" — the idea that sterile urban environments may contribute to rising rates of allergies and immune-mediated illnesses. The study was small and has not yet been replicated at scale, but the Finnish government has since funded biodiversity-based outdoor programs in dozens of additional daycares, and the model is being studied by educators in other European countries.
