Posted by Claire Warden on

Playing in mud helps our bodies function because we are in fact, not individuals. We are ecosystems with microbial partners (microbiota) that are involved in the development (particularly in early life) and function of essentially every organ, including the brain. This blog seeks to focus on the research on the long-term health benefits of being exposed to dirt not just in playing with mud, but through our changing habits in society.


The Hygiene Hypothesis, first put forth in the 1980s, holds that when children are too clean and their exposure to parasites, bacteria, and viruses is limited early in life, they face a greater chance of having allergies, asthma, (Yazdanbakhsh, 2002) and other autoimmune diseases such as multiple sclerosis and type-one diabetes during adulthood (Platts-Mills, 2005).

Immune circuits

Dr. Joel Weinstock, director of gastroenterology and hepatology at Tufts Medical Centre in Boston suggests that children raised in an ultraclean environment are not being exposed to organisms that help them develop appropriate immune regulatory circuits. For their own benefit, children need to play with dirt and generally being outside benefits the heart, skin, and immune system.

Cardiovascular health

A Northwestern University study (Channick 2010), suggests that dirt is good for childrens’ cardiovascular health. Analysing data collected from thousands of children over two decades, researchers have concluded that when children are exposed to germs and pathogens during infancy their risk of cardiovascular inflammation in adulthood, a precursor to heart attacks and strokes, is reduced. The study found that children who had early exposure to animal faeces and infectious microbes like those found outside in soil resulted in lower levels of CRP, or C-Reactive Protein, a biomarker for cardiovascular problems, later in life.


Researchers at the University of California School of Medicine, San Diego, found that a common bacterial species that lives on skin, Staphylococci, triggers a pathway that helps prevent inflammation, improving skin’s ability to heal (BBC News, 2009). Lowry and University of Colorado-Boulder microbiologist Noah Fierer, have undertaken an unusual experiment: they are collecting water from shower heads in the US and Europe to determine how many mycobacteria, including Mycobacterium vaccae, live there.


Rook, a University College London microbiologist, developed the “old friends” hypothesis to explain how modern human lifestyles—indoor living, pasteurized food and obsessive cleanliness—and medical practices, particularly the overuse of antibiotics, have upset the gut’s natural diversity. Ruebush suggests that the typical human probably harbours some 90 trillion microbes. The very fact that you have so many microbes of so many different kinds is what keeps you healthy most of the time. When we destroy some of the microbes we upset the balance in our bodies.


According to a four-year study that examined approximately two million children under the age of 18, antidepressant use is on the rise in kids, with the fastest growing segment found to be preschool children aged 0-5 years (Delate, 2004). While not a substitute for medication, an increasing number of experts are recognizing the role of nature in enhancing mental health.


Children’s stress levels fall within minutes of seeing green spaces, making outside play a simple, no-cost, and time efficient antidote for an overstressed child (Kuo et al., 2004). So, although mud isn’t listed specifically in this research, it comes in as a part of being part of a nature-based space. White et al (2019) suggest that 120 minutes in nature is enough to make a difference to stress levels. The current findings offer valuable support to health practitioners in making recommendations about spending time in nature to promote basic health and wellbeing, similar to guidelines for weekly physical activity.


In a study by Bristol University, Mycobacterium vaccae, or M. vaccae, a “friendly” bacteria found in soil, was shown to activate a group of neurons that produce the brain chemical serotonin, enhancing feelings of well-being, much in the same manner as antidepressant drugs and exercise. Interest in the study arose when patients treated with M. vaccae for another health issue reported increases in their quality of life (Lowry, 2007).

The Brain

Mycobacterium vaccae supplements called psychobiotics, are probiotics that proport to have mental health benefits and boost brain power and suppress depression following the research by Matthews and Jenks (2010).

The overriding message is that our bodies are eco-systems that need a wide variety of microbes to function effectively. Playing outside generally, but in nature specifically, exposes us to the range of microbes that our bodies need to be healthy.

Do you want to incorporate more play and learning with mud within your setting? Claire Warden has developed an online course that will take you through the creation of a mud kitchen that truly engages children and provides a wealth of fun and learning. Find out more:

Our portable learning cards give you a pocket full of ideas for mud play


Brody, Jane E. “Babies Know—A Little Dirt is Good for You.” The New York Times. January, 27, 2009: Pages D7.

Channick, Robert. “NU Study: dirt’s good for kids.” March 24, 2010. February 15, 2012.

Delate T, Gelenberg AJ, Simmons VA, Motheral BR. (2004) “Trends in the use of antidepressant medications in a nationwide sample of commercially insured pediatric patients, 1998-2002.”

Kuo, PhD, F., E., and Taylor, A., PhD. “A Potential Natural Treatment for Attention-Deficit/Hyperactivity Disorder: Evidence From a National Study.” American Journal of Public Health 94.9.Sept. 2004.

Lowry, C, et al., “Identification of an immune-responsive mesolimbocortical serotonergic system: Potential role in regulation of emotional behavior,” Neuroscience (2007), doi: 10.1016/j.neuroscience.2007.01.067

Matthews. D., & Jenks, S., in American Society for Microbiology. (2010, May 25). Can bacteria make you smarter? ScienceDaily. Retrieved June 20, 2019 from

Platts-Mills TA, Erwin E, Heymann P, Woodfolk J (2005). “Is the hygiene hypothesis still a viable explanation for the increased prevalence of asthma?”. Allergy 60 Suppl 79: 25–31. doi:10.1111/j.1398-9995.2005.00854.x. PMID 15842230.

Ruebush. M., Why Dirt is Good: 5 Ways to Make Germs Your friends.

Science Daily, “Can Bacteria Make You Smarter?” May 24, 2010. February 15, 2012.

Yazdanbakhsh M, Kremsner PG, van Ree R (2002). “Allergy, parasites, and the hygiene hypothesis”. Science 296 (5567): 490–4. doi:10.1126/science.296.5567.490. PMID 11964470.

White, P., Alcock, I., Grellier, J., Wheeler, B., Hartig, T., Warber, S., Bone, A., Depledge, M., Fleming, L. (2019) Spending at least 120 minutes a week in nature is associated with good health and wellbeing. Scientific Reports, 2019; 9 (1) DOI: 10.1038/s41598-019-44097-3

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