tl;dr: Finnish research suggests that direct skin contact with soil, plants, and moss significantly increases microbial diversity on the skin, which supports immune regulation and may reduce risk of autoimmune and allergic conditions. Studies of children playing on forest-floor daycare yards, adults gardening with microbially-rich soil, and offices with living "green walls" all showed measurable immune benefits, including more regulatory T cells and anti-inflammatory cytokines. Researchers argue cities should be designed to encourage incidental contact with nature, since the benefits are substantial and the risks minimal.