The forest at home: in Finland, soil microbes improve human health

A Finnish study shows how introducing forest soil into domestic environments increases the microbial diversity of indoor air, with potential protective effects on the immune system of infants growing up in enclosed spaces

by Matteo Cavallito

Soil microbes may help promote human health in urban environments by rebalancing the domestic microbiome. This hypothesis is linked to a well-known correlation that may now have experimental confirmation in Northern Europe. Evidence comes from a new study conducted by the Finnish Institute for Health and Welfare and the University of Eastern Finland, published in the journal Microbiome. During the research, the authors applied forest soil to rugs in homes and observed a clear increase in bacteria associated with forest soil in the air, along with the related benefits linked to exposure.

Exposure to microbes can influence long-term health

In early childhood, exposure to microbes activates immunoregulatory pathways and can influence long-term health. However, in the early stages of life, children tend to spend most of their time indoors—which may have consequences. “Urban lifestyles are characterized by reduced encounters of environmental microbe stimuli that activate immunoregulatory pathways”, explains the study.

Moreover, “this has been linked to an increased risk of inflammatory diseases, asthma, and allergies”. For this reason, “a potential preventative solution is to modify indoor microbial exposures toward health-promoting interactions”.

The aim of the study was to verify the feasibility of transferring environmental microbiota through an experiment conducted in six homes. Forest soil was added to five of them, while one apartment was left unchanged as a control. Soil was applied to an indoor rug in the entrance area three times at four-week intervals. Household dust was sampled at the breathing height of infants and adults, as well as from floors in different locations. The microbiota in these samples was analyzed using genetic sequencing, with interesting results.

Children better protected against asthma

The intervention led to a significant increase in the relative abundance of forest soil bacteria at various measurement points inside the homes. “The magnitude of effect was influenced by building characteristics, spatiotemporal dynamics, and occupant dynamics”, explains the study, “and was greatest in a home with comparably little additional microbial influx—a home with no pets, low occupancy, and mechanical ventilation”.

Further effects were observed at the breathing height of infants near the rug. In these areas, “increases in bacterial diversity and an asthma protective microbiota index (a specific microbial composition that acts as a protective factor against the development of this desease and allergic sensitization in children, ed.), as well as decreases in the proportion of human-sourced bacteria, were also observed, but only in airborne dust close to the soil-seeded rug”, the study notes. By contrast, “effects on fungal microbiota or on the bacterial and fungal loads in house dust were inconsistent”.

An encouraging starting point for further studies

“It was encouraging to find that signs of microbial exposure associated with a lower risk of asthma can be increased in urban homes with such a simple and inexpensive intervention”, explained Pirkka Kirjavainen, researcher and co-author of the study. “The next step is to verify whether this type of intervention translates into the health benefits we expect”.

In short, the research shows how applying soil to a rug can significantly alter the bacterial microbiota present in airborne particles inside homes—opening the way for further studies. “While this approach is promising, specifically in highly urbanized settings, dosage and composition of environmental microbiota additions to reach health benefits require further study”, the authors conclude.