What is the reason for sneezing?

When something peculiar enters nostrils or when there is swelling on its internal surface for some reason, sensory cells in the nostrils send a message of alarm to the brain. This message is couriered by a nerve named trigeminal to the brain center which controls breathing. The brain has to respond quickly to throw out the foreign substance, if any, in the nostrils before it can enter in the lungs which are made of very delicate mucous membrane. One important point to be kept in mind is that lungs do not inflate or subside. It is the diaphragm which through its action of contraction and flattening during inhalation causes the chest to expand and through action of relaxation and rising during exhalation causes the chest to subside.

At the command of the brain for a sneeze diaphragm contracts considerably allowing lungs to breathe in a greater quantity of air. Thereafter diaphragm relaxes, not as it does normally but with suddenness. Resulting exhalation is a sudden noisy burst of air and moisture through the nose and mouth expelling air at a maximum speed of 165 kilometers per hour. Foreign particles or mucous logged in nostrils or wind pipe are thrown out in that burst of air which can be compared with running a vacuum cleaner in reverse! During an average sneeze about 5,000 particles of moisture are thrown out up to 4 meters.

More reading:
Sneeze (Wikipedia)

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