How Mud Boosts Your Immune System

Today, many parents may secretly wish their children had the chance to pick up a bit of grime. With the rise of urbanism, and the allure of video games and social media, contact with nature is much rarer than in the past. For many, there is simply no opportunity to get muddy.

What is gained in laundry bills may be lost in the child's wellbeing. According to recent research, the dirt outside is teaming with friendly microorganisms that can train the immune system and build resilience to a range of illnesses, including allergies, asthma and even depression and anxiety.

These findings show that outdoor exercise is not only beneficial because of the chance to roam free – but that certain natural materials, such as soil and mud, also contain surprisingly powerful microorganisms whose positive impact on children's health we are only beginning to fully understand.

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The new research offers a fresh take on the "hygiene hypothesis", first postulated in the late 1980s. According to this idea, the great reduction in childhood infections over the 20th Century had an undesirable side effect on people's immune systems, leading them to become overreactive to the slightest stimulation. The result was thought to be the rise in asthma, hay fever and food allergies.

Many scientists now dislike the term hygiene hypothesis, however, since it seemed to discourage important behaviours like hand-washing. And they balk at the idea that infections, per se, are beneficial for children. "It was quite problematic from a public health perspective," says Christopher Lowry, a professor of integrative physiology and the director of the behavioural neuroendocrinology laboratory at the University of Colorado, Boulder, US.

Instead, it is the non-infectious organisms that are now thought to be key – rather than the ones that actually make our children sick. These "old friends" have been around for much of our evolutionary history. They are mostly harmless, and train the immune system to moderate its activity, rather than overreacting to any potential invader.

Importantly, our bodies meet these old friends whenever we spend time in nature. With increased urbanisation, and reduced outdoor play, many children now lack that exposure – meaning that their immune systems are more sensitive to any threat, and more likely to go into overdrive.

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