pathogenic microorganism growth.
* An acidic pH (around 5.0) and low amounts of water make the epidermis hostile to many microorganic pathogens.
* Non-pathogenic microorganisms on the surface of the epidermis help defend against pathogens by competing for food, limiting its availability, and producing chemical secretions that inhibit the growth of pathogenic microbiota.

;Permeability
* Psychological