Getting your hands dirty while gardening, touching tree bark or playing on grass may offer more than a connection with nature, as research suggests regular exposure to natural environments can increase the diversity of microbes living on the skin.
Scientists have been studying whether contact with soil, plants and other natural materials can influence the skin microbiome and, in turn, support the immune system.
In one study by Finnish researchers, participants rubbed their hands with soil, peat and moss for 20 seconds. Researchers found a significant increase in the number and variety of microbes on their skin, with some changes remaining even after participants washed their hands with tap water.
Why skin microbes matter
The presence of bacteria on the skin is not necessarily harmful.
Experts say a diverse skin microbiome can help maintain a balance between different microorganisms and may make it more difficult for potentially harmful bacteria to become dominant.
Mira Grönroos, an environmental health researcher affiliated with the Finnish Environment Institute, said the change in microbial composition was striking even though the participants’ hands looked clean.
However, the researchers found that a brief 20-second exposure did not produce lasting changes. The microbial composition of the skin moved closer to its original state after a short period.
This suggests that repeated or prolonged contact with natural environments may be more important than a single short exposure.
Children who play in the dirt
Researchers have also been examining how regular contact with nature affects children.
At a daycare centre in central Finland, children spend several hours a day in a forest-like playground, where they climb, play with soil and wood, build sandcastles and interact with vegetation.
Researchers are collecting samples from the children’s skin, saliva and stool, along with blood and hair samples, to examine changes in their microbiome and immune systems over several years.
The daycare centre is among dozens participating in Finland’s ADELE research programme, which builds on earlier studies examining the relationship between natural environments and children’s microbial exposure.
Forest-like playgrounds linked to greater microbial diversity
In an earlier study conducted in 2016 and 2017, researchers transformed a daycare playground by adding forest-like elements, including peat and planting boxes.
After children regularly played in the environment for 28 days, researchers found greater microbial diversity on their skin and in their saliva and gut compared with children who used playgrounds with paved or gravel surfaces.
Follow-up testing a year later suggested that some of the differences in skin microbial diversity had persisted.
Researchers also found fewer potentially disease-causing microbes on the skin of children who played in forest soil.
One possible explanation is that a greater variety of microorganisms leaves less opportunity for potentially harmful bacteria to dominate the skin’s microbial community.
From healthy skin to immune balance
Scientists are also investigating whether changes in the skin microbiome can influence the immune system.
In another experiment, children played in sandboxes containing soil with either high or lower microbial diversity.
Those exposed to microbe-rich soil developed greater diversity of certain bacteria on their skin. Researchers also observed changes in blood levels of immune-related markers called cytokines.
The changes were associated with processes involved in maintaining immune balance, including the ability to respond to threats without triggering excessive immune activity.
Researchers say these findings add to growing evidence that the environments people regularly interact with may influence the microorganisms living on and in their bodies.
Does this mean we should stop worrying about dirt?
Not exactly.
The research does not suggest that all exposure to dirt is beneficial or that people should ignore basic hygiene.
Instead, scientists are examining whether regular, natural exposure to diverse microorganisms can contribute to a healthier relationship between the human body and its microbial environment.
For children in particular, outdoor play provides opportunities to interact with soil, plants, wood and other natural materials while also offering the broader physical and developmental benefits of spending time outdoors.
