Overpressure is any air or gas pressure above the normal atmospheric level. In everyday conditions at sea level, the atmosphere pushes on everything at about 14.7 pounds per square inch (psi). When an explosion, industrial malfunction, or rapid pressure change pushes that number higher, the difference between the spike and normal atmospheric pressure is the overpressure. Even small increases, measured in fractions of a psi, can shatter windows. Larger surges can collapse buildings, rupture eardrums, and damage internal organs.
How a Blast Wave Creates Overpressure
When a bomb, gas leak, or chemical reaction detonates, it creates a single pulse of compressed air that radiates outward in all directions. This initial surge is the positive pressure phase, and it lasts only a few milliseconds. The front of this wave is the point of highest overpressure: it hits fast, compresses everything in its path, and then drops off sharply. Immediately after the positive phase, a negative pressure phase follows, creating a brief moment of suction that can pull debris and structures back toward the blast center.
The destructive power of a blast depends on several factors: the peak overpressure of that initial wave, how long the overpressure lasts, the surrounding medium (open air versus an enclosed room), and the distance from the detonation point. Confined spaces like hallways, tunnels, or rooms amplify overpressure because the wave bounces off walls and focuses its energy instead of dissipating freely.
Units of Measurement
Overpressure is most commonly expressed in pounds per square inch (psi) in the United States and in pascals (Pa) or kilopascals (kPa) in scientific and international contexts. One psi equals 6,894.76 pascals. You may also see values in bar, where 1 bar is roughly equal to standard atmospheric pressure (14.5 psi). In blast injury research and emergency planning, psi is the most frequently cited unit because it maps directly to human injury thresholds and structural damage levels.
Structural Damage at Different Pressure Levels
NOAA maintains a reference scale that maps overpressure values to expected damage, and the numbers are surprisingly low. At just 0.04 psi, a blast produces a noise equivalent to 143 decibels, enough to cause sonic boom-style glass failure. By 0.15 psi, typical window glass shatters. At 0.5 to 1.0 psi, most windows break along with their frames.
As the pressure climbs, the destruction escalates quickly:
- 1.0 psi: Houses are partially demolished and rendered uninhabitable.
- 2.0 psi: Walls and roofs of houses partially collapse.
- 2.0 to 3.0 psi: Unreinforced concrete or cinder block walls shatter.
- 2.5 psi: Half of a home’s brickwork is destroyed.
- 7.0 psi: Loaded train cars overturn.
- 10.0 psi: Total building destruction is probable.
- 14.5 to 29.0 psi: The range for 1% to 99% fatalities from direct blast effects in exposed populations.
For emergency planning, NOAA’s default alert thresholds are 1.0 psi (glass shatters), 3.5 psi (serious injury likely), and 8.0 psi (buildings destroyed).
How Overpressure Injures the Human Body
The human body handles steady pressure well, which is why you can dive underwater or fly in a pressurized cabin without harm. Rapid pressure spikes are a different story. A blast wave passes through the body in milliseconds, and tissues respond differently depending on their density. Air-filled organs like the lungs, ears, and intestines are the most vulnerable because the compressed wave hits the boundary between air and tissue with maximum force.
Ear Damage
The eardrum is the body’s most pressure-sensitive structure and often the first thing injured. Research from the Lovelace Foundation estimates the threshold for eardrum rupture at about 5 psi. At 15 psi, roughly 50% of exposed eardrums rupture. Between 14.7 and 29.4 psi (one to two atmospheres of overpressure), studies on cadavers found that 66% of eardrums ruptured. Above two atmospheres, virtually all remaining eardrums failed.
Lung Injury
The lungs are the organ most likely to cause death from a blast wave. Animal studies have identified approximately 8.5 psi as the threshold where lung hemorrhage and contusion begin to appear. At 13 psi and above, significant increases in the area of lung injury become evident, and the damage worsens with repeated exposures. At 16 and 19 psi, pronounced inflammation sets in. An overpressure of 60 to 80 psi (414 to 552 kPa) is considered potentially lethal.
Brain Injury
Blast-related traumatic brain injury is one of the most studied consequences of overpressure exposure, particularly in military contexts. The pressure wave can damage the brain through at least three pathways: it can accelerate the head rapidly enough to injure tissue through rotational and translational forces; it can pass directly through the skull and compress brain tissue; or it can travel upward through the chest and blood vessels into the brain. These mechanisms likely work together, and researchers are still working to pin down the exact pressure thresholds where each one becomes significant.
Overpressure in Diving
Overpressure injuries aren’t limited to explosions. Scuba divers face a version of the same physics every time they ascend. Water pressure increases by one full atmosphere for every 10 meters (about 33 feet) of depth. At 10 meters down, the pressure on your body is double what it is at the surface, and your lung volume is compressed to half its normal size.
The danger comes on the way up. If a diver breathes compressed air at depth, filling their lungs to full capacity, and then rises without exhaling, Boyle’s law dictates that the gas will expand as the surrounding pressure drops. A diver with 6 liters of air in their lungs at 10 meters would see that volume try to expand to 12 liters at the surface. The lungs cannot stretch that far. When the pressure difference across the lung wall exceeds about 60 to 70 mmHg, the tiny air sacs rupture along with their surrounding blood vessels. This is pulmonary barotrauma, and it can happen in as little as a few meters of ascent if a diver holds their breath with full lungs.
Overpressure in Industrial Settings
In chemical plants, refineries, and pressurized piping systems, overpressure refers to any situation where the internal pressure of a vessel or pipeline exceeds its design limits. The causes are varied and sometimes overlap. A blocked outlet on a positive-displacement pump will keep forcing fluid into a fixed volume until the pressure exceeds the mechanical limits of the system. Liquids are especially unforgiving in this scenario because, unlike gases, they barely compress at all, so pressure builds almost instantly.
Other common triggers include cooling system or reflux failures that throw off the heat balance inside a vessel, unintended chemical reactions that produce gas faster than the system can vent it, check valve failures that allow backflow, control valve malfunctions where valves open fully when they shouldn’t, and external fires that heat the liquid contents of a vessel until they boil. Pressure relief valves and rupture disks are the primary safeguards. These devices are sized to vent enough fluid to keep the system below its maximum allowable pressure. Designing them correctly requires identifying every plausible overpressure scenario, because an overlooked one means the relief device may be too small to handle the actual emergency.