How Big An Explosion Could You Realistically Survive?

Everyone knows you can't survive a nuclear blast in a refrigerator (unless you're Indiana Jones). But what can the human body withstand? We take a look at the damage explosions cause and how humans survive it.

Most of the time, we're ready to forgive movies for the crimes against science they regularly perpetrate. We don't pay 20 bucks to see realism. We pay to see someone on a skateboard jump off the seat of an exploding motorcycle, which has just jumped from the top of an exploding bus. And if there's a way to make the skateboard also explode - get on that. But occasionally movies push credibility just a little too far. People outrun fireballs. People survive explosions by hopping away from them. People sometimes just walk away from explosions without even looking back, because they're that cool. The reality is a lot grislier.

The Basics of How Explosions Kill

Many movie explosions are pretty flashy, with smouldering slow-motion fireballs and flying pieces of sharp metal twisting through the air - often in three dimensions. Fire and fast-moving objects are two visual ways to show the destruction that many explosions wreak, which is why they're so lovingly and carefully rendered on screen. But there are invisible ways that explosions kill. When a hero ducks out of the way of a 20m long fireball, or dodges a car door being thrown through the air like a tinkertoy, movies make it look like they would manage to survive the blast.

Although fire and shrapnel do cause many injuries during explosions, perhaps the most major destructive force in an explosion is simply the blast wave. When an explosion goes off, it pushes a great deal of air outward in a small amount of time. Although people are generally unaware of the air they move through, it has a way of making itself known when it's propelled outwards by TNT. This wave of sudden pressure causes a huge amount of damage.

The PSI of Staying Alive

The air around us already puts pressure on us to the tune of 15kg per square inch. This is the kind of pressure the human body has evolved to deal with, though. We have a great deal of trouble getting by without it, in fact. A sudden variance causes a lot of problems for the body. Explosions exacerbate this problem by varying the pressure two ways. The blast wave is comprised of compressed air being shoved out of the way. Since it is doing this in an even sphere around the explosive, there's no way for new air to fill the void. What's left is a partial vacuum - the body goes from a hit of overpressure to a near vacuum in a split second.

Surprisingly, the human body is pretty tough. If the pressure is stacked up slowly, it can survive as 180kg per square inch, if the pressure is gradually increased and decreased to allow the body to adjust. A sudden change of pressure causes damage as far lower levels. Anywhere from 10 to 20 kilograms per square inch can be fatal, depending on the time it takes to wash over the body.

At the high pressure shifts, the body just comes apart. There's only so much trauma that flesh and bone can take. Although lower-level explosions could be survivable, they do cause seriuos internal injuries. The ears are the first organs to feel the change in pressure. That's what they're designed for, after all. Ears are meant to allow people to sense minute changes in air pressure - sound - over short amounts of time. As such, they're extremely sensitive to pressure, and especially to pressure over time. A wave of pressure that lasts less than .3 milliseconds leaves the eardrum no time to adjust to changes in pressure, and simply tears it. This can happen with pressure change as small as 5 psi. Lungs are the next thing to feel the change. Filled with tiny air sacs, they rupture and bleed when too much pressure is applied too quickly. The bowels are also destroyed. Although they're not the first thing most people think of being injured in an explosion, they are sensitive to changes in pressure. Filled as they are with liquid and gas, a change in pressure can cause them to expand or shrink suddenly and simply tear themselves open.

The Hidden Dangers of Changes in Pressure

Although it seems like the primary danger to the human body would be the changes in pressure itself, there are other ways that pressure waves kill. The human body can survive blasts of sudden pressure of 20-40 psi, but it's not the only thing receiving that pressure. The pressure radiates outward from the blast in all directions. When it leaves a vacuum behind it, air from the surrounding atmosphere moves in to fill that vacuum. This means wind. A lot of it.

Pressure changes of 5 psi can cause 260km/h winds. Changes of 20 psi can cause winds of 760km/h. This kind of wind doesn't knock people over, it lifts them through the air. It drags them over the ground like they were caught behind train, or blows them out of windows to the ground below. It slams them into cars and buildings hard enough to kill. Even if someone were to survive a blast - they would most likely be killed by being knocked into their surroundings.

Jumping Behind an Object Doesn't Work

Some films show heroes escaping by ducking behind a wall, or into a tunnel, or sometimes (I'm looking at you, Independence Day.) around a corner. Unfortunately, pressure isn't something that a person can hide from. Most people have felt a building or a car shake when a wave of wind hits it. Although the wave of air is stopped, the force moves on, kicking the structure forward.

As for running down a hallway, or into an enclave, that could actually be worse than facing the onslaught. Many people here have seen a hose dribble water until they put their thumb over the nozzle. Instead of dripping out, the water begins to spray, moving farther and faster than it did before. The water is coming out of the hose with a certain forcer per square inch. Reducing the number of inches it can leave the hose by doesn't change the force the water is coming with - it just concentrates it. Ducking into smaller and smaller corners - depending on their position, could concentrate the blast wave. (To be fair, though, ducking into a sheltered area is a good way to hide from shrapnel.)

When getting to shelter, it's also important to consider the durability of the shelter itself. This is probably one thing that movies get right. When a building is blowing up - get out of it. Apparently most buildings are build to withstand snow and wind. Those can be pretty heavy, and buildings are meant to be built solid, but roofs generally support snow loads of 345kg/m walls withstand winds of 160km/h - which works out to .2 pounds per square inch. Structures can crumble under pressure anywhere between 5 and 20 psi. Downright puny compared to what people can live through. The building might provide more danger than the explosion.

So How Does One Survive An Explosion?

Generally, one doesn't. At least not any movie explosion. Films that show people using missiles or building-destroying dynamite as a 'diversion' or a way to propel themselves or their vehicles or their chairs (Looking at you, Long Kiss Goodnight.) are doing the equivalent of running someone over with a car as a way of patting them on the back. Military-grade explosives unleash millions of pounds per square inch of pressure. Anything near it is getting destroyed.

For more modest explosives, the best defence is distance. Since force is applied over area, it decreases by the square of the distance it travels. Run like hell. A good hundred meter dash will put you in the safe range of one kilogram of TNT. A thousand metres will keep you safe from a thousand kilograms of it. Keep moving directly away from the explosive and keep doing it as far as you can. If you can run while covering your head - especially your ears - you'll decrease incidental injuries but don't let anyone distract you from distance. Just get away.

Via Gexcon, How Stuff Works, Fire Engineering, CDC, Global Security, and JEPHC, Aussie Survivalist.