A sneeze is a powerful, involuntary expulsion of air from the lungs, designed to clear irritants from the nasal passages. When this explosive event occurs while the head is submerged, the mechanics are instantly transformed by the surrounding fluid and the body’s defense systems. The sudden, high-velocity burst of air interacts with the water, creating a unique and momentarily disorienting physical event.
The Body’s Protective Reflexes During Submersion
The human body possesses built-in reflexes that activate to prevent water from entering the lungs when the face is submerged. The most immediate defense is the laryngospasm, which causes the vocal cords to involuntarily clamp shut, closing off the glottis. This muscular contraction is triggered by water touching the sensitive laryngeal area, effectively blocking the airway to the trachea and lungs.
This reflex provides a temporary barrier against aspiration, containing the initial shock of the sneeze. Another element is the mammalian dive reflex, a broader physiological response to facial immersion, especially in cold water. It triggers a reduction in heart rate (bradycardia) and peripheral vasoconstriction, diverting oxygenated blood to the brain and heart. These coordinated defenses conserve oxygen and override the respiratory urge, reinforcing the body’s attempt to stop the sneeze from becoming a dangerous intake of water.
The Physical Outcome of Sneezing Underwater
When a sneeze is triggered underwater, the body attempts to expel air with force against the resistance of the water. A typical sneeze generates significant pressure, reaching approximately 7000 Pascals (Pa) in the trachea, focused on the nasal and oral passages. This powerful blast of air is immediately released into the surrounding water, resulting in a sudden, voluminous cloud of bubbles.
The expelled air and water mixture creates a distinct, visible plume that rapidly displaces the surrounding fluid. The mean velocity of the air and droplets being expelled can range from 2 to 5.4 meters per second, forcefully interacting with the water. This high-pressure air expulsion causes a momentary sensation of water being forcefully shoved into the nasal cavities. This uncomfortable experience can result in water rushing further into the sinuses and nasopharynx.
Safety Concerns and How to Mitigate Risk
The primary medical concern when sneezing underwater, particularly while diving, is barotrauma, a pressure-related injury to air-filled spaces. If the sneeze is suppressed by holding the nose and mouth, the internal pressure can increase by 5 to 24 times the normal sneeze pressure. This extreme pressure differential can damage the delicate tissues lining the sinuses and middle ear, potentially leading to sinus or ear barotrauma.
A forceful sneeze can cause sudden, sharp pain in the face or ears. In rare cases, the pressure can lead to serious complications like pneumocephalus, where air enters the cranial cavity. If the sneeze is unavoidable, the safest action is to allow the air to escape naturally through the nose and mouth, rather than attempting to pinch the nostrils shut. Divers should ensure their mask is held firmly and the regulator is secured, allowing excess air to vent out the sides of the mask or regulator.