Apr 4, 2022 7:00 AM

How Explosions Actually Kill

Wars often spark misinformation about the nature of blast trauma. Russia's unprovoked bombardment of Ukraine is no different.

A wounded Ukrainian soldier rests at a military hospital

This Story Is adapted from In the Waves: My Quest to Solve the Mystery of a Civil War Submarine, by Rachel Lance.

The war in Ukraine is new. The patterns of injuries in that war are anything but. Since the 1867 invention of the world’s first high explosive, TNT, people have been inflicting these same patterns of blast trauma on each other with regularity. Sometimes it seems like we even do it with eagerness. Every few decades, we concoct a new delivery vehicle to enhance the mayhem, such as cluster bombs or thermobarics, but the underlying physics of an explosion, and the vulnerable anatomies of our softest body parts, has not changed.

At the start of each new war, false claims about blast trauma start to fly as quickly as the shrapnel. A month into this one, we already have leading public figures making inaccurate declarations about how thermobarics “suck” the air out of your lungs. (They don’t, but more on that further down.) Regardless of the level and prevalence of the many misunderstandings about blasts, one thing is inarguably, eternally true: People near explosions might die. Here’s how that really works.

Medically speaking, the injuries from an explosion are neatly categorized into one of four tidy bins, which are labeled by numbers: primary, secondary, tertiary, and quaternary. A blast victim might receive only one type, or they can receive a grab bag of trauma containing any painful mixture of the four. Quaternary trauma is a sort of “other” pile of things that can, but do not always, occur as the result of a blast, such as burns, chemical agents, or radiation exposure. Tertiary trauma is the injury type that most people expect—think an action hero injuring his back after getting blown across the room. Notably, tertiary trauma almost never happens in the real world. Secondary injuries are unfortunately an overwhelmingly common injury type. They are the result of objects, like shrapnel, or even fragments of the bomb casing, getting thrown and hitting a person because of the explosion. Secondary injuries are dark, and visually horrifying, as they frequently take the forms of trauma to the limbs, cuts deep enough to reach the skeleton, and amputations.

These three injury types— secondary, tertiary, and quaternary— make obvious sense as the expected possibilities. Primary blast injuries, on the other hand, are an impressive, sometimes invisible, horrifying fluke of nature. They are the byproduct of the bizarre physics of explosions mixed together with human frailty. Primary injuries result from solely the pressures produced by an explosion, usually because of a shock wave.

To understand how a shock wave maims, first it is crucial to understand how a shock wave is born. Normally, sound moves like billiard balls on a massive, smooth felt table. First, a noisy event occurs, like an impact. A gas molecule in close proximity to the action gets pushed away: This is the cue hitting the cue ball. The cue ball travels outward until it hits the 4-ball, another gas molecule. Clunk. They impact, and the cue ball transfers some of its energy to the 4. Both balls now move, slightly slower and in an outward direction, until they impact other balls, hitting their next closest neighbors. The overall wave front of the motion moves forward, but each individual ball travels only slightly across the table. The motion gets passed outward, expanding and slowing just a bit with each collision as the leading edge of movement travels across the table.

Sound travels outward, each molecule of material transferring energy to the next, growing in reach but decaying in strength as it moves. Eventually it hits an ear and gets heard, or a wall and echoes back toward the source. It moves the same way in water as it does in gas, except faster, because the molecules start out closer together in the denser liquid.

A shock wave occurs when the pool cue is placed in the hands of the most furious, most irate patron in the hall. He is a high explosive. Apoplectic and red-faced, the explosive burns quickly. In fact, the burn front moves through the entirety of the explosive much faster than normal sound. Therefore, the entire reaction happens too fast for the gaseous products created by the burn to expand outward in a normal way. The material is burned and gone before the balls can travel outward on their own, too quickly for them to thwack their neighbors at their natural speed. The whole charge has reacted, is consumed, becomes a tiny, superheated ball of hyper-pressurized gas before the 4-ball ever gets the message. The resulting gases expand all at once, together, suddenly, violently, and the pool cue is shoved, hurtled, rammed down the length of the table, picking up ball after ball and adding them to the front of the wall of molecules moving forward, picking them up faster than they can move on their own.

This is how a shock wave develops. The molecules accumulated at the wave front are densely packed together by the gas urgently expanding behind them. They are so densely packed that each molecule can reach its neighbor more quickly than it could in a normal situation, and so this unique wave moves faster than the speed of normal sound.

The molecules downstream get hit without warning. In its purest form, the shock wave goes straight from zero to its maximum pressure in an instant; on a graph it is a vertical line followed by a sloping decay back down. If it were a car it would go from 0 to 60 in exactly zero seconds.

When they reach high enough pressures, these waves can disintegrate everything in their paths. The substantive fabric of objects gets jerked into motion by the instantaneous rise of the shock, and they break apart into chaos like a porcelain teacup thrown onto a concrete floor.

Most of the human body handles mild to moderate levels of shock surprisingly well. Severe pressures will cause tissue disruption, which is a polite phrase that describes a horrifying concept. However, the lower-pressure shock waves can travel through most of our anatomy without harm. These waves can move straight through water without much chaos and disruption, and human bodies are, after all, mostly water. It’s the gas pockets inside certain organs that cause the real drama.

In the chest wall, which is mostly water, sound moves at roughly 1,540 meters per second. In a gas pocket, which is basically air, it moves at roughly 343 meters per second. Therefore, waves moving through the body that hit any gas pocket are forced to slow down at the interface by about 80 percent. In the lungs, they are forced to slow down to a measly 30 meters per second, a 98 percent drop in speed. And as they are forced to slow, that energy must get transferred somewhere. It gets transferred into the delicate tissues that form the walls of the lungs. They rupture and shred, and blood sprays into the alveoli, filling the precious gas pockets needed for breathing. This process is called spalling.

Gas pockets in the intestines can cause a similar problem, leading to bruising and tearing of the intestinal tract. The same is true of some of the smaller bones in the skull, particularly the ones that form the fragile archways around the sinus cavities. These bones will on occasion show spider webs of fracture from primary blast, but they are sufficiently difficult to injure that these patterns are typically only seen in autopsy reports.

If a shock wave is strong enough to throw a person, then it is strong enough to kill that person through the damage to their lungs. Some blast victims report feeling as if they were thrown because the rapid pressure changes of a shock wave will manipulate the parts of the ears that control balance and orientation. However, in general, if a victim has been thrown, then that victim has not survived. That is why real-world explosions leave no tastefully battered action heroes, and shock waves deal few tertiary injuries to the living.

The goal of a thermobaric explosive is to lengthen the duration of the shock wave. They achieve this goal by mixing in other types of fuels, like aluminum, that burn slower than the main explosive, drawing out the reaction, and often producing a spectacular fireball as a result. If a regular blast is like a person touching an electrified fence and receiving one painful but brief zap, then thermobarics are like wrapping a hand firmly around the wires and not letting go. The violence gets delivered for a longer time period and wreaks more havoc because of the substantially lengthened time period during which it can stampede through the frail human body. Similarly, the elongated shock wave of a thermobaric explosion smashes against the human lungs for a longer period of time. An explosion can feel like a blow to the chest, a sharp, strong hit that leaves a victim gasping for breath afterward. But there is no evidence that thermobarics pull the air out of the lungs.

Even though thermobarics often explode with lower pressures than conventional high explosives, their shock waves are such dramatic master-works of prolonged force that they can cause more damage overall, especially in enclosed spaces or densely constructed cities. The Russians fine-tuned these bombs in the 1980s to fire into caves in Afghanistan. As the shock waves bounce off the walls of caves or other firm structures like tall buildings, they add onto themselves. When they add, they increase the total pressure level of the blast exposure. Inside an enclosed space, the long shock wave of a thermobaric explosion can build itself up to reach the extreme pressure levels of a much larger blast.

Each shock wave in air has a brief time period where the pressure dips down into the negative levels, creating a slight vacuum that sucks some materials back in toward the direction of the blast. Since the early 1900s, people have blamed this negative period for injury and trauma, and of course, with enough of a vacuum it is theoretically possible to damage the frail human lungs. However, the blast cases of World War II and the brilliant blast researchers of the same time period determined that it was not this negative phase that caused the damage. Blasts that occur underwater do not always have a negative, or suction, phase, but even still they always kill more easily than comparable blasts in air.

The idea of thermobarics vacuuming out the air from the lungs is one of blast trauma’s most resilient myths because the horrific, dizzying, feeling of a blast impact seems to reinforce the idea that some sort of massive trauma has been perpetrated against the body. It has. But, unfortunately, an explosion has many ways to kill.

This excerpt is adapted from In the Waves: My Quest to Solve the Mystery of a Civil War Submarine, by Rachel Lance. Copyright © 2020 Rachel Lance. Published by arrangement with Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC.