The Complex Science Behind the Common Sneeze

Sneezing is a reflex that protects the nose from foreign objects, preventing access to the lungs and other organs. There are two phases:

• The first phase — the nasal or sensitive phase — begins when the chemical or physical irritants stimulate the mucosa.
• The second phase, also called the efferent or respiratory phase starts with the eyes closing, followed by a deep inhalation and a forced exhalation. The glottis, an elongated space between the vocal cords, closes. Intrapulmonary pressure increases, the glottis dilates, and the air explodes through the nose and mouth.

Sneezing is a combined effort of the respiratory, musculoskeletal, and parasympathetic nervous systems. Dust, pollen, smoke, pollution, and microscopic irritants trigger sneezes, as can viral infections, allergies, and colds. When the nerves in the mucus membranes of the nasal passage detect irritation, they send a message to the throat, chest, and abdominal muscles to contract. This contraction forces air out through the nose in an attempt to remove the irritant. The facial muscles may also react, including closing the eyes. This is likely the body’s way of preventing the forcibly removed particles from then entering the eyes. Interestingly, some people sneeze when plucking their eyebrows. Researchers say irritation in the facial nerve endings travels to the nasal nerves, which brings about the sneeze.

One of the most common reasons for sneezing is allergic rhinitis, such as hay fever and seasonal allergies. If a person breathes in substances that they are allergic to, such as animal dander, dust, or pollen, they will experience an array of symptoms, including sneezing. The allergic cells in the nasal mucous membranes release histamine, which interacts with the nerves there and causes this response.

There are more than 200 cold viruses. Unlike allergies, when a person has a cold, it is not histamine that triggers the sneezing. Excess mucus production begins when a person contracts a cold, and this stimulates the nerves in the mucous membranes of the nose. With a respiratory infection, symptoms generally occur one at a time, with sneezing being one of the first to appear.

The nerves that line the nasal passages are extremely sensitive to pain, but also to temperature changes. Because of this, moving between areas with different temperature zones can trigger a sneeze, despite a lack of irritants in the air. Being cold and shivering can prompt sneezes as well, although they are not as common as sneezes resulting from irritation of the nasal passages.

Once a person stops using a drug to which they were addicted, the body struggles to maintain homeostasis, which is vital to survival. The individual can go into withdrawal and begin to exhibit symptoms. The longer it takes to restore homeostasis, the longer the withdrawal. One of the symptoms of opioid withdrawal is sneezing, along with yawning, a runny nose, nausea, and other stomach disturbances.

Some individuals experience a series of sneezes after they finish a meal and have a full stomach. Researchers speculate that this could be an inheritable condition. Another unusual physiological cause of sneezing is sexual ideation or orgasm, a reaction recorded in medical journals as early as the end of the 19th century. Researchers say it could be a result of a fault in the autonomic nervous system that controls heart rate, digestion, and blood flow.

Before a person sneezes, they inhale, causing a pressure increase in the chest. With the forceful exhalation of air during the sneeze, the pressure in the chest decreases. Changes in blood flow resulting from these pressure changes affect the heart rate. For some people, this leads to a feeling that their heart has skipped a beat. Occasionally, sneezes cause a longer delay between heartbeats, which can make the next heartbeat feel stronger. This can manifest as a sensation in the upper chest or throat.

Physicians recommend that people don't suppress their sneezes. A halted sneeze can lead to a variety of complications, including a perforated eardrum, rupture of a brain aneurysm, or trapped air in the chest, between the lungs. Some people pinch their nose while clamping their mouth shut to prevent a forceful sneeze. Doctors report treating patients who have ruptures in the back of their throats as a result of this practice. This injury is not only painful, but it also interferes with the ability to speak and swallow.

In the 1500s, people believed that a person who sneezed ejected their soul from their body, leaving it vulnerable to evil spirits. Or, if a bad spirit entered the body, a sneeze would force it out. Saying “God bless you” after someone sneezes started in the 6th century and is still a popular custom. Pope Gregory the Great believed that sneezing was an early sign of plague. He instructed Christians to say the phrase to attract divine assistance in warding off the disease. Another common belief existing for decades is that a series of sneezes means that you will have a visitor or that gossipers are talking about you. Others say it foretells a huge financial windfall, like a lottery win.