People Share 65 Photos Of The Unusual Things That Happened To Their Bodies

Many of the boundaries within which a typical human can survive have been fully established; the well-known "rule of threes" dictates how long we can forgo air, water, and food (roughly three minutes, three days, and three weeks, respectively). Other limits are more speculative, because people have seldom tested them.

For example, how long can we stay awake? Air Force pilots have been known to become so delirious after three or four days of sleep deprivation that they crash their planes (having fallen asleep).

Even a single all-nighter impairs driving abilities as much as being drunk. The absolute longest anyone has voluntarily stayed awake before nodding off is 264 hours (about 11 days)—a record set by 17-year-old Randy Gardner for a high-school science fair project in 1965. Before falling asleep on day 11, he was essentially a vegetable with his eyes open.

Radiation poses a long-term danger to us because it mutates DNA, rewriting the genetic code in ways that can lead to cancerous growth of cells. But even then, it would take a lot to take us down immediately.

According to Peter Caracappa, a nuclear engineer and radiation safety specialist at Rensselaer Polytechnic Institute, 5 to 6 Sieverts (Sv) over the course of a few minutes will shred too many cells for your body to repair at once.

And the longer the time period over which the dose is accumulated, the higher that range would be, since the body works to repair itself over that time as well.

For comparison, some workers at Japan's Fukushima nuclear plant absorbed 0.4 to 1 Sv of radiation per hour while contending with the nuclear disaster.

When it comes to acceleration, NASA and military researchers have made strides in testing our limits (you don't want astronauts blacking out during liftoff).

Lateral acceleration—jerking to the side—does a number on our insides because of the asymmetry of the forces. 14 Gs of lateral acceleration can supposedly tear your organs loose from one another, but even fewer can be fatal.

Head-to-foot motion, meanwhile, plunges all the blood to the feet, and between 4 and 8 longitudinal Gs should knock you out.

(A force of 1 G is the normal force of gravity we feel here on Earth.)

Forward or backward acceleration appears to be easiest on the body because it allows the head and heart to accelerate together.

Military experiments in the 1940s and 1950s with a "human decelerator," which was essentially a rocket sled that zipped back and forth across Edwards Air Force Base in California, suggest we can slow down at a rate of 45 Gs, or the equivalent of the gravity of 45 Earths, and still live. At that rate, you slow from 630 miles per hour to 0 mph in fractions of a second over a few hundred feet.

Researchers estimate we probably turn into a bag of spare parts this way around 50 Gs.

People vary greatly in how well they tolerate differences in atmospheric conditions, whether in temperature, pressure, or the oxygen content of the air.

The bounds of survival also depend on how slowly those changes occur, because the body can, to some extent, gradually adjust to external conditions.

However, most humans will suffer hyperthermia after 10 minutes in extremely humid, 140°F (60°C) heat.

Our limit of cold is harder to define. A person usually expires when their body temperature drops to 70°F (21°C), but how long it takes depends on how “used to the cold” a person is and whether a mysterious, latent form of hibernation sets in, which has been known to happen.

As attested to by any sci-fi movie in which an astronaut’s helmet pops during a spacewalk, we don’t fare too well with abnormal oxygen or pressure levels. At atmospheric pressure, air contains 21% oxygen. We are in serious danger when that concentration drops below 11%. Too much oxygen is also fatal, as it gradually causes lung inflammation.

When you put things into perspective, we have evolved to adapt to a surprising range of challenges!