The instinct to close one’s mouth during moments of fear may seem logical, but when facing an explosion, the counter-intuitive advice to keep the mouth open is a matter of physics and physiology. This immediate action is a protective measure designed to mitigate the severe internal damage caused by the invisible force of a blast wave. By providing a pathway for this sudden pressure, the body can attempt to equalize forces across its most vulnerable internal membranes, offering a small but significant chance of reducing injury.
The Destructive Force of a Blast Wave
An explosion generates a supersonic pressure disturbance known as a blast wave, which travels rapidly outward from the source. This wave is characterized by a two-phase pressure profile: an instantaneous surge of overpressure followed by a period of underpressure. The initial, positive phase involves a rapid increase in atmospheric pressure that compresses the air and anything in its path.
This intense overpressure front moves faster than sound, creating a shockwave that violently pushes against surfaces. Immediately following this is the negative or suction phase, where the air rushes back toward the explosion’s epicenter, causing a temporary vacuum. The sudden, extreme change in pressure between the ambient air and the compressed wave is the mechanism responsible for primary blast injuries.
How Internal Organs React to Sudden Pressure
The human body’s gas-filled organs are particularly susceptible to the primary blast wave effect, a phenomenon known as barotrauma. The wave violently compresses the body, causing the air within internal cavities to compress and then rapidly re-expand during the suction phase. This compression-rebound cycle tears delicate internal tissues.
The lungs are highly vulnerable, and injury to them is often the most common cause of delayed death after an explosion, manifesting as a pulmonary contusion or “blast lung.” The sudden pressure difference across the alveolar membranes—the tiny air sacs responsible for gas exchange—causes them to rupture. This damage can lead to hemorrhaging, edema, and the formation of air embolisms, where air bubbles enter the bloodstream. The gastrointestinal tract is also at risk, particularly the colon, where trapped gases can cause the organ walls to tear, potentially leading to perforation and internal bleeding.
Equalizing Pressure Through the Oral Cavity
Keeping the mouth open functions as a pressure relief valve for the body’s interior air spaces. The goal is to minimize the pressure differential that forms across delicate tissues. When the sudden overpressure hits the body, it simultaneously enters the lungs through the mouth and nose, while also pressing on the outside of the chest and abdomen.
An open mouth allows the pressure wave to reach the inside of the lungs almost as quickly as it impacts the outside of the torso. This dual-pathway effect helps the internal air pressure rapidly match the external pressure, thereby reducing the force exerted across the alveolar membranes.
For the ears, which are highly sensitive to pressure changes, an open mouth creates a direct air channel to the pharynx. The pharynx connects to the middle ear via the Eustachian tubes, which can briefly allow the blast pressure to equalize on both sides of the eardrum. This action reduces the chance of eardrum rupture. Eardrum rupture occurs when the external pressure wave violently pushes on the outer membrane while the middle ear pressure remains at a lower, ambient level. By opening the mouth, a pressure pathway is created that attempts to balance the force on the inside and outside of the tympanic membrane, which can prevent the membrane from tearing.
Additional Safety Steps During an Explosion
Opening the mouth is just one component of a broader, immediate safety protocol. The primary goal is to maximize distance from the blast and minimize the body’s exposed surface area. If an explosion is imminent or has just occurred, immediately drop to the ground and lie flat on the stomach.
Lying prone reduces the profile of the body exposed to the direct force of the blast wave and potential flying debris. It is also advised to cover the ears with the hands, as opening the mouth offers only partial protection to the tympanic membranes. If a sturdy object is nearby, such as a concrete barrier or heavy furniture, seek cover immediately to shield the body from fragmentation and flying debris.