"Boogers" is the slang term for mucus, which is composed of water, salts, protein and other small cells. When the mucus is dry and comes out of the nose, it has already caught dust, pollen, germs and other debris.
The tissue that produces mucus lines the mouth, nose, sinuses, throat, lungs and gastrointestinal tract. Mucus prevents the tissue beneath it from drying out by keeping it moist. It also traps unwanted substances like bacteria and dust before these invasive elements can get into sensitive parts of the body. The human body produces mucus every day, but certain conditions such as an illness or eating spicy foods can trigger elevated levels of production.