According to BoogieWipes.com, boogers form because the sinuses and nose secrete mucus to prevent illness and infection by keeping dirt, dust and pollen from entering the lungs. Boogers can either be sticky or dry.
When an irritant like pollen or a germ gets sucked into the nose, tiny hairs called cilia trap it. The cilia move the whole irritant toward the back of the throat while mucus forms around the cilia. According to KidsHealth, the trapped particles and the mucus become a booger when they clump together and dry out.
There are different qualities of boogers. Some are slimy due to colds, allergic reactions or exposure to harsh smells and chemicals. People who live in dry, dusty climates make drier boogers than those who in live in more humid places.
It takes a good bit of mucus to stay healthy. In fact, most swallow about a quart of mucus a day. To make sure the body has enough mucus to make boogers, it is important to stay hydrated. Mucus is mostly made up of water, and people who drink enough water make as much as 2 quarts of mucus a day. The best way to clean up boogers is with a tissue.