A sneeze is generally caused by irritation in the passages of the nose. Pollens, house dust, and other particles are usually harmless, but when they irritate the nose the body responds by expelling them from the nasal passages. The nose mistakes strong odors, sudden chills, and even bright lights (see photic sneeze reflex) for parasites, and it tries to defend itself with a sneeze. A sneezer exhales with a speed of up to 46 m/s (104 miles per hour). An unimpeded sneeze sends two to five thousand bacteria-filled droplets into the air. It is almost impossible for a person to keep their eyelids open during a sneeze. The reflex of shutting the eyes serves no obvious purpose: the nerves serving the eyes and the nose are closely related, and stimuli to the one often trigger some response in the other. Orgasms amongst humans can also cause sneezes.

One of the main functions of our nose is to filter things out of the air so it's clean when it goes into our lungs. Every time we breath in through our nose, thousands of tiny pieces of dust and other particles go inside. Many of these are stopped by the hairs in our nostrils. For particles that get further inside, the mucus that lines the passages in our nose catches them. Usually, this layer of mucus holds the particles until it's swept out the back of our nose by tiny hairs called cilia. But every now and then, the mucus gets really full of particles, or something really irritating like pepper, gets in our nose. When this happens, the lining of our nose becomes irritated, causing us to sneeze. The sneeze makes us take in a big breath, and then blast it out of our nose and mouth, sometimes at over 100 mph. This causes the mucus and particles inside it to be blown out of our nose, getting rid of what was irritating our nose. Many times it takes more than one sneeze to get rid of the irritation. Sneezes are very beneficial, especially when we have colds. Sneezing helps get rid of the mucus and helps us get over the cold faster.