Finnish scientists trucked in real forest dirt and grass and laid it over the gravel at four daycare yards. They let the kids dig around in it for a month. The blood tests came back with changes the researchers hadn’t expected to see so fast or so clear. The study ran at ten daycares in two Finnish cities with 75 kids aged three to five. Four of the yards got the forest treatment: about a tennis court worth of soil and grass laid over the gravel, plus planters and peat blocks the kids could dig and climb on. Three others stuck with their normal gravel yards. The last three were daycares where the kids were already visiting real forests every day. After one month, the variety of bacteria living on the kids’ skin shot up, and the kind that helps train the skin’s immune defenses jumped the most. Their gut bacteria started to look like the gut bacteria of the forest-visiting kids. Their blood showed more of the immune cells whose job is to keep the body from freaking out at harmless stuff like pollen and peanuts, and overall inflammation dropped. The kids on the plain gravel yards showed none of this. Childhood asthma in the US doubled between 1980 and 1995. Food allergies in kids jumped 50 percent between 1997 and 2011, then jumped another 50 percent between 2007 and 2021. And peanut allergies in one-year-olds tripled between 2001 and 2017. The Finnish researchers think one of the reasons is simple: kids today don’t get dirty enough. 37 percent of American preschoolers now spend an hour or less outside on a normal weekday. Their immune systems are getting trained in environments stripped of the bacteria humans have always lived around. Aki Sinkkonen, who led the study, put it in plain words: “It would be best if children could play in puddles and everyone could dig organic soil.” The Finnish government is now helping pay for daycares across the country to make the same changes.