A sneeze (sternutation) is an involuntary, powerful reflex action that serves as the body’s rapid defense mechanism. It is triggered when foreign particles or irritants stimulate the delicate lining of the nasal passages. This sudden, convulsive expulsion of air from the lungs through the nose and mouth is meant to forcefully clear the nasal cavity of unwanted matter. The sheer power and speed of this reflex often prompt curiosity, leading to wildly varying estimates regarding its actual velocity, ranging from ten miles per hour to over one hundred miles per hour.
Measuring the True Speed of a Sneeze
The widely circulated figure of a sneeze traveling at 100 miles per hour is a historical overestimate, often traced back to indirect calculations made decades ago. This figure resulted from an inference about the speed needed to create droplets of a certain size, not from a direct measurement of the air leaving the nose. Modern scientific investigations have employed sophisticated methods to measure the actual velocity of the air and expelled particles.
Researchers now use high-speed imaging techniques, such as Schlieren photography or particle image velocimetry, to visualize and track the movement of the sneeze plume in real-time. These methods capture thousands of frames per second, allowing for the precise calculation of air speed. Studies using these techniques typically find the maximum speed of a sneeze to be between 10 and 40 miles per hour at the point of exit, significantly lower than historical claims. Some experiments record velocities closer to 10 miles per hour for the air jet itself. However, analyses tracking the fastest-moving individual droplets may record speeds up to 50 miles per hour, depending on the individual’s physiology and the force of the expulsion.
The Physiological Process Behind the Force
The force behind a sneeze is generated by a complex, coordinated physiological sequence initiated in the brainstem. The reflex begins when irritants like dust or pollen stimulate sensory nerve endings in the nasal lining, sending a signal via the trigeminal nerve to the medulla oblongata, the brain’s sneeze center. This center then orchestrates the rapid, multi-stage motor response that creates the explosive expulsion.
The first phase is a deep, involuntary inhalation, where the diaphragm and intercostal muscles draw a large volume of air into the lungs. This is immediately followed by a compression phase, which builds high internal pressure. During compression, the glottis—the opening between the vocal cords—shuts tightly, while the muscles of the chest and abdomen contract powerfully.
This simultaneous action traps the air and dramatically increases the pressure within the chest and respiratory system. The final phase, expulsion, occurs when the glottis suddenly opens, releasing the compressed air through the nasal and oral cavities. This rapid decompression, driven by the contracted muscles, is what propels the air and mucus at such high velocities, effectively scouring the nasal passages.
The Role of Speed in Expulsion and Transmission
The high velocity of a sneeze serves the primary purpose of clearing foreign substances from the nasal passages. The force is necessary to overcome the viscosity of the mucus and eject irritants from the upper respiratory tract. This mechanism maintains respiratory health by preventing foreign matter from traveling deeper into the lungs.
However, this high velocity also has a significant consequence for the spread of infectious disease. The speed of the expulsion causes the rapid breakup and aerosolization of respiratory fluid. The sneeze stream transforms the fluid into a turbulent, multiphase cloud composed of droplets of various sizes.
Smaller droplets, or aerosols, are suspended in the turbulent air current created by the sneeze’s velocity, allowing them to travel much farther than they would under gravity alone. This sneeze cloud can carry pathogens a considerable distance from the source and remain airborne for some time, making the sneeze a highly effective, albeit unintended, vector for the transmission of viruses and bacteria to others.
Risks Associated with Stopping a Sneeze
The powerful pressure generated during the compression phase makes suppressing a sneeze a potentially hazardous action. When a person attempts to stifle a sneeze by holding their nose and clamping their mouth shut, the high-pressure air that would normally be released is redirected internally. This redirection can increase the internal pressure by five to twenty times the force of a natural sneeze.
This redirected pressure seeks a path of least resistance, which can lead to damage in delicate areas of the head and neck. Complications include a ruptured eardrum, as the air is forced up the eustachian tube into the middle ear. Although rare, the extreme pressure can also cause air to be forced into the chest cavity, a condition known as pneumomediastinum, leading to chest pain and breathing difficulties.
In extreme, though infrequent, cases, the force can cause blood vessels in the eyes to rupture, resulting in visible red spots, or damage the fragile blood vessels in the brain. The internal pressure can also damage the larynx or throat tissues. It is generally recommended to allow a sneeze to exit naturally, perhaps by directing it into the crook of the elbow to minimize droplet spread.