Sneezing or sternutation, that quick blast of air out the nose and mouth, has many causes, but generally, it's the body's attempt to get rid of some material in the nasal passages. Often, people know what the source is: they may have runny noses from colds or flus, for example, which cause the mucous membranes in the sinus passages to become irritated. The body responds by attempting to clear these passages, causing an individual to sneeze.
At other times, allergies to pollens, dust, or molds cause sneezing. The nose, which is acting as a filtration device, works to remove these allergens from the body by expelling them. Since allergies entering the nose are often made up of tiny particles, it may take multiple attempts to do the trick of removing irritants. Furthermore, provided that the person remains in an environment with the offending allergens, she is likely to keep sneezing periodically.
Sometimes, cures for sneezing from allergies bring on larger ones. Using corticosteroid nasal sprays, for instance, can bring on a bout of sneezing. Again, the nose reacts at the introduction of a foreign agent, even if it is beneficial, and attempts to get rid of it. The nasal passages will also note the presence of irritating foreign agents that get in the nose, like pepper and certain other spices. Some people also sneeze after swimming for a while.
One unusual cause is drug withdrawal from opiates. Opiates make up many different legal pain relievers, as well as several illegal drugs, like heroin. Anyone who has been taking opiates for a long period of time can have withdrawal symptoms once she stops taking them. These can include runny nose and sneezing that may be caused by nasal congestion. Some people appear to suffer from chronic runny nose as a result, and the presence of extra mucus in the sinuses is more likely to provoke the reaction.
There is one type of sneeze that has little to do with nasal irritants: the photic sneeze is an inherited reflex that affects about 20-35% of humans. What happens is that looking at bright light stimulates the optic nerve to send a wrong signal to the trigeminal nerve, which is thought to be the nerve responsible for sending a signal to create a sneeze. Some scientists consider photic sternutation as a congenital defect, but given the high number of people who experience this reaction when looking at light, it may simply be a natural variant.
Scientists do know that the issue is autosomal dominant in nature. In other words, if a child's parents sneeze when they get sudden exposure to sunlight, chances are that the child will too, and he has a good chance of passing this trait onto his children. Fortunately, a sneeze is seldom dangerous unless a person is pursuing an activity that requires constant vigilance. Someone who find himself having several in succession while driving, for instance, might want to pull to the side of the road until they have ended.
