Researchers added forest floor, sod, plants, and peat blocks to parts of four urban daycare yards. After 28 days, 3- to 5-year-old children showed changes in skin and gut microbiota, along with shifts in IL-10, IL-17A, TGF-β1, and regulatory T-cell measures.

A later placebo-controlled study found similar immune-related changes after children played with microbially diverse soil. The evidence is promising, but it does not yet prove that dirt prevents allergies or autoimmune disease. Nor does it show that recycled-rubber playgrounds are poisoning children.

Primary studies:
https://doi.org/10.1126/sciadv.aba2578
https://doi.org/10.1016/j.ecoenv.2022.113900

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