What forest floor playgrounds teach us about children and germs

When dusk fell In the Finnish city of Lahti, on a cool day in May 2016, a crew of workers settled in the courtyard of an empty daycare center. Under the swings and climbing frames, they installed squares of forest floor – unkempt shrubs, shiny, tall berry bushes, fluffy meadow grasses and velvety moss mounds – harvested from the forest somewhere in a less developed part of the country. At the edges they put in soft green turf. In the morning when the children arrived, they found their playground – previously a cloudy patchwork quilt of asphalt, gravel and sand – transformed into micro-oases of wilderness overnight.

That scenario played out three more times this month in day care centers in Lahti and 500 miles west in the city of Tampere. It wasn’t the work of a few nature-loving guerrilla artists, but the start of an ambitious scientific experiment to find out if the shortage of microbes in paved urban settings could turn people’s immune systems against them. “There is this ‘biodiversity hypothesis’ that people without different environmental microbiota are more likely to develop immune-mediated diseases,” says Aki Sinkonnen, evolutionary ecologist at the University of Helsinki. “But nobody had really tested this with children.”

You probably know the “hygiene hypothesis” better. First described by a British epidemiologist named David Strachan in the early 1990s, it believes the rise in chronic disorders related to overreactive immune systems such as asthma, diabetes and allergies is caused by children growing up in increasingly sterile bladders. Immune systems are basically object classifiers. Your job is to recognize what yourself and what others are. Microbes encountered early in life are the first tutors in the process, helping the developing immune system recognize what is dangerous and what is not. The more families have used antibacterial soaps and gels, sealed themselves up in high-rise apartments, and driven cars through concrete jungles, the less living space bacteria, protozoa, fungi, and viruses have – and the less likely children are to develop immune systems to walk into. And less exposure means fewer exercise opportunities. A poorly trained immune system can fail when it comes to distinguishing between the body’s own cells and food allergens, gut microbes or pollen in the air.

Laboratory experiments on rodents in the early 2000s supported this idea: wild rats had a well-balanced immune system to fight dangerous pathogens but no minor irritants, while their laboratory-bred counterparts went into overdrive at the slightest stimulus. Epidemiological studies in humans also provided evidence – allergy and asthma rates tend to be higher in more industrialized areas than in rural areas. To counteract these perceived negative effects of the urban, modern lifestyle, dozens of companies have decided to sell immune-boosting probiotics – pills, drinks, and creams filled with cocktails made from live bacterial cultures. In the Covid-19 era, thousands of posts with the tag #immuneboost appear every week promoting these and other home remedies. So far there is little evidence that anything has worked.

That is why scientists like Sinkkonen have taken this idea one step further in recent years. They observed that people are increasingly living in deserts of microbiodiversity and are not exposed to harmful insects. “The immune system does not recognize microbes by type, but by type,” says Sinkonnen. “Probiotics usually contain only one or two types of bacteria, so it is unlikely that the entire immune system will be activated. We wanted to see what would happen if we created a diverse microbial environment. “Hence the forest floors in the playgrounds – the first randomized controlled trial to test the biodiversity hypothesis in children. Biohacking, but make it cute.