Sneeze

A sneeze, also known as sternutation, is a semi-autonomous and convulsive expulsion of air from the lungs. This explosive, spasmodic action forces air out through both the nose and the mouth. The primary function of a sneeze is to cleanse the nasal cavity. It does this by expelling mucus that contains foreign particles or other irritants. During this process, mucus escapes through the nasal cavity, and saliva may escape from the oral cavity.

The sneeze reflex is a complex process involving two distinct neural pathways. The first part is the afferent pathway, which is the sensory phase. It begins when specialized sensory neurons called pruriceptors in the respiratory mucosa are stimulated. These neurons are located in the nasopharynx or the paranasal sinuses. When chemical or mechanical irritants hit these sensors, signals are sent via the trigeminal nerve to the brainstem. The second part is the efferent pathway, which is the motor phase. The medulla oblongata sends signals through motor nerves to various muscle groups to eliminate the irritant.

The motor phase of the sneeze consists of two specific steps: inhalation and exhalation. During the inhalation phase, the diaphragm and intercostal muscles contract. This action draws a large volume of air into the lungs. This is followed by the exhalation phase, where the muscles of the abdomen and chest contract powerfully. This contraction compresses the lungs and forces air out in a forceful burst. During this stage, the muscles of the eyelids also contract involuntarily, which forces the eyes to close. While people often say it is impossible to sneeze with eyes open, the eye closure is simply an involuntary reflex.

To understand how the air moves so quickly, we can look at the internal mechanics. During a sneeze, the soft palate and the palatine uvula depress. At the same time, the back of the tongue elevates to partially close the passage to the mouth. This setup creates a venturi, which is an effect where air is accelerated through a narrow space. This occurs because of Bernoulli's principle. The air ejected from the lungs is accelerated through the mouth, creating a low pressure point at the back of the nose. This low pressure forces air in through the front of the nose, launching mucus and contaminants out of the mouth. It is important not to sneeze with the mouth closed. Doing so creates very high pressure in the head that can be potentially harmful.

Various triggers can cause a person to sneeze. Beyond simple irritants, some people experience the photic sneeze reflex, or PSR. This is a condition where sudden exposure to bright light triggers a sneeze. This is an autosomal dominant trait, meaning it is passed through genes, and it affects 18% to 35% of the human population. Another rare trigger is snatiation, which is sneezing caused by a particularly full stomach after a large meal. This is also a medical disorder passed along as an autosomal dominant trait. Additionally, sudden drops in temperature or a cold breeze can trigger the reflex.

Sneezes have a significant impact on the air around us. A single sneeze can produce as many as 40,000 droplets. These particles are often very small, ranging from 0.5 to 5 micrometers in size. While scientists once thought sneeze plumes traveled only a short distance, 2020 research showed they can reach up to 7 or 8 meters.

Because these droplets can spread infectious diseases like the flu, hygiene is vital. It is recommended to cover the mouth and nose with a forearm, the inside of the elbow, or a tissue. Using hands is discouraged because it spreads germs to objects like doorknobs. One should also avoid holding in a sneeze by pinching the nose. A computer simulation suggests that holding a sneeze in creates a burst of air pressure of 39 kPa, which is about 24 times the pressure of a normal sneeze.

Humanity has a long history of interpreting sneezes through culture and superstition. In ancient Greece, sneezes were viewed as prophetic signs from the gods. For instance, in 401 BC, a soldier's sneeze was seen as a favorable sign during a speech by the general Xenophon. In parts of East Asia, such as China and Japan, a sneeze was often thought to mean someone was talking about you. In these cultures, the number of sneezes could even signal if the person was thinking of you or if someone was in love with you. In the Middle Ages in Europe, people believed life was tied to breath, which may be why the phrase "God bless you" became common.