The difference is, the inside of the body is lined with a barrier called the mucosa, or mucous membrane. Everything from your nose, mouth, lungs, stomach, and intestines are covered with this protective lining.
The problem is that the mucosa takes on the brunt of the fight against infection, bacteria, everyday toxins and chemicals, processed foods, pollution, and more. It's no wonder then that, without reinforcements, the mucosa starts to break down over time leading to greater intestinal permeability (and gut and health issues), a vulnerable immune system, and less efficient bodily functions overall.