A sneeze seems simple enough; your nose itches, you breathe in, and you violently (and quickly) breathe out your nose! It turns out, however, that a sneeze is a rather complex reaction of your body to something (dust, pollen, viruses, bacteria) that has entered your respiratory system.
There are actually two phases of sneezing. In the “sensitive” phase, something is climbing into your nose, triggering nerve cell receptors. These receptors then transmit chemical messages through a nerve cell network to the “sneeze center” in the medulla, in your brain (note that the cortex and higher brain centers aren’t involved in this). When enough messages have reached the sneeze center, it’s time for the second “respiratory” phase to kick in. Here, other parts of the nervous system direct the body’s preparation of a sneeze. Your eyes close, your diaphragm and lungs take in a large breath of air. Then, your glottis closes. At that point, your diaphragm and chest pushes your lung, which force air out. Achoo!
Sneezes are very, very fast. Researchers have measured sneezing speeds between 150 km per hour and nearly 85% percent of the speed of sound! They’re also very efficient at expelling invaders. The mucus lining your nasal passages is expelled during a sneeze, and that mucus can contain up to 40,000 particles.
There have been claims that the pressure behind a sneeze is strong enough to pop a blood vessel in the head or brain (known as an aneurysm). Other problems caused by a high-pressure sneeze (or suppressing a sneeze) have included loss of hearing, glaucoma, and blood clots. However, these appear to be very rare.
Of course, this speed and efficiency is not only good at expelling a foreign object. It’s also quite effective at spreading a disease around. So cover your mouth!