Is Allowing Children to Play in Dirt Beneficial for Their Immune Systems?

Children planting small green plants into a bed of soil in a garden with a trowel.
As a child, you may have been told that playing outside in the dirt is beneficial because it strengthens your immune system. But is there any evidence supporting this claim?
In short, yes — several studies suggest that early exposure to dirt may decrease children’s risk of developing allergies and autoimmune conditions. In other words, it may provide protection against conditions where the immune system reacts unfavorably to allergens or to the body’s tissues.
As a child’s immune system matures in the early years of life, the protective cells within the body must learn to differentiate between the body’s own cells and foreign substances, such as bacteria and viruses, that are either harmless or disease-causing. They must recognize and attack disease-causing pathogens.
It turns out that the molecular signals driving the expansion of this regulatory arm of the immune system mainly originate from microbes in the gut, according to Graham Rook, a professor emeritus of medical microbiology at University College London. This collection of microbes, known as the “gut microbiome,” is vital to our health. Some of these microbes aid in producing essential vitamins and aid in food digestion.
The first year of life is crucial for the development of the microbiome. Babies acquire bacteria as they pass through the birth canal during vaginal birth, and from breast milk if they are breastfed. As children grow, they are progressively exposed to microbes from various sources.
The “old-friends hypothesis” proposes that the greater the variety of microbes encountered in early childhood, the more diverse our microbiomes will be, and thus the better our immune systems will be able to distinguish between friend and foe. “Old friends” refers to beneficial or “commensal” microbes that live on and within the body without causing harm.
This theory, proposed by Rook in 2003, is akin to the more widely known hygiene hypothesis, which suggests that a lack of early exposure to germs makes people more susceptible to immune conditions. Multiple studies have shown a link between growing up on a farm or in a household with pets and a reduced likelihood of children developing allergies compared to those in urban or pet-less environments.
However, the old-friends hypothesis emphasizes the importance of being exposed to commensal microbes in early life, rather than infectious pathogens. Several studies in Europe suggest that early germ exposure does not protect against allergy development. Another criticism of the hygiene hypothesis is that it diminishes the importance of good hygiene in disease prevention, suggesting that we have become “too clean,” as argued by Rook and colleagues in a 2016 review.
The old-friends hypothesis could help explain why antibiotic overuse in early life, which can deplete much of the gut microbiome, and cesarean deliveries, which do not expose newborns to vaginal bacteria, have been associated with an increased allergy risk.
A trial in Finland investigated whether urban children’s immune systems could be strengthened with grass and soil from forest floors. Within a month, children who played in the dirt had a more diverse collection of harmless bacteria on their skin and more immune-regulatory cells and signaling molecules in their blood compared to those who played on gravel playgrounds. This suggests that exposure to bacteria in dirt could aid in immune system maturation, potentially reducing the likelihood of it becoming overactive.
While the microbiome is significant, many other factors influence the risk of developing allergies, including genetics, according to Dr. Robert Wood, a professor of pediatrics at Johns Hopkins Medicine in Baltimore. Nonetheless, children should generally be encouraged to go outside and play in the dirt.
However, the current understanding of risk factors for immune conditions cannot always be translated into practical advice. For example, having a dog may slightly reduce the risk of allergies compared to not having one, but recommending getting a dog as a guaranteed way to prevent allergies is not feasible.
Dirt in heavily polluted areas can be harmful to children, as it may contain dangerous contaminants. Therefore, it is essential to prevent children from inhaling or ingesting dirt, especially in areas with pollution. Additionally, because dirt can contain potentially harmful chemicals such as lead and parasites, precautions should be taken to ensure children’s safety.

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