Children who played in formerly gravel-covered urban daycare center yards renovated with natural forest floor, sod, and vegetation developed more diverse microbiomes and signs of a better-regulated immune system within one month, according to a new study with 75 children between 3 and 5 years old. The findings suggest it may be possible to improve immune systems in urban communities simply by granting young children daily access to green spaces. Previous studies have shown that urban populations are more susceptible to immune-mediated diseases, with the prominent "biodiversity hypothesis" blaming a lack of exposure to diverse microbiota. To test the biodiversity hypothesis, Marja Roslund and colleagues performed an intervention in the yards of 4 standard Finnish daycare centers, transferring natural forest floor rich with dwarf shrubs, blueberries, crowberry, and mosses to the bare spaces. Children spent an average of 1.5 hours per day over 28 days planting, crafting natural materials, and playing games in the renovated green spaces. Roslund et al. found that children in the 4 centers that received the intervention maintained a high diversity of commensal skin microbiota over the study period and developed a higher ratio of the anti-inflammatory cytokine IL-10 to the pro-inflammatory cytokine IL-17A, suggesting that playing in the forest dirt stimulated their immunoregulatory pathways. These results agreed with findings from 3 nature-oriented daycare centers that served as a positive control. In contrast, microbiotic diversity declined in children at 3 standard centers without the intervention, and they did not experience a similar immune boost. The researchers also found that a high abundance of the bacteria Faecalibacterium prausnitzii was associated with decreased expression of IL-17A in healthy children, noting that previous studies reported a decreased abundance associated with immune disorders.