Our skin is hostile to incoming microbes. The pH is too low for most of them to grow and the salt levels are high from sweat. Both skin cells and the microbes that live on skin defend against newcomers and attack them with Anti-Microbial Peptides (AMPs). The sebum (oil) produced by skin is crafted to feed specific beneficial microbes, while discouraging the growth of unfamiliar inhabitants.
These factors make it difficult for the microbes that we encounter in our daily lives to establish themselves as residents on our skin. However, this doesn’t mean that interaction with the microbes in our environment is not impactful.
Because we evolved as part of natural ecologies, our skin and its residents are used to interacting with the vast array of microbes present in these environments. When we spend time in nature, we expose ourselves to the microbiomes of the plants and the soil around us. The transient microbes that interact with us and our microbiomes in these settings are important. They provide a kind of “microbial chatter”, to which our immune systems and our resident microbes are accustomed. The skin uses this communication to educate its immune responses, so that it doesn’t overreact to unimportant stimuli.
In contrast, when we spend time indoors in a city, this vast array of microbes isn’t present, and we’re exposed to a much lower diversity – consisting mostly of microbes of human origin. With the absence of natural transient microbes and their communication, our immune systems can become hyperreactive. This can lead to a state of frequent inflammatory responses – and consistent inflammation poses many problems, as we know.