The limits of human survival

The human body can be tricky to judge when it comes to the limits of what we would be able to survive. Take the case of Vesna Vulović for example, she currently holds the Guinness world record for the longest survived fall after the plane she was working on exploded mid-flight, sending her falling 30,000 feet (9.14 km) to the ground where she sustained serious injuries, but ultimately survived. This is quite a contrast from the 650,000 estimated global deaths from falling each year, most of which happen at heights under 20 feet.

(Vesna Vulović, the current record holder for furthest survived fall)

There seem to be two main factors when it comes to the limits of human survival, or surviving an accident of some kind, and these are luck and personal resilience. There are people who have been shot over a dozen times in places that should kill them but managed to survive, compared with the story about a man in the US who was shot in the toe with an air rifle, only for the wound to cause a blood clot which traveled to his brain and killed him. This is a case of luck that is out of anyone’s control, but personal resilience is more controllable and plays a big part in how well someone survives, with the single best example being cold resistance.​

How cold is too cold to survive?

Normal body temperature ranges from 36.5–37.5 °C (97.7–99.5 °F), with the average being somewhere in the middle. A difference of only a single degree can cause someone to experience an intense fever and feel very hot, or losing as little as 2 degrees can cause hypothermia to set in. The problem is that there are no standard numbers to determine how quickly someone will get cold and how long they can hold on at low temperatures. This is where the personal resilience factor plays its part, which makes a much bigger difference than you’d think. A person native to a cold country would be able to stay alive longer than someone from a hot country if they both found themselves floating in cold waters, but ultimately both would eventually die. The average human will die from the cold when their body temperature reaches 21 °C, but they would be incapable of doing anything long before it hits this temperature, with the average passing out temperature being around 28 °C.

(A windchill chart that shows how quickly you can expect to get cold in various conditions. What it doesn’t mention is that clothing, current activity, body type, and a whole range of other factors can greatly change the time length)

How hot is too hot to survive?

If someone’s body temperature was raised by a single degree they would experience symptoms similar to a fever. Dizziness, fatigue, headaches, and a whole range of other nasty symptoms would start to affect the victim, and when their internal temperature reached 43 °C death would occur. There have been people who have survived an internal temperature as high as 46 °C, but reaching this temperature would almost certainly cause other effects like convulsions, shock, respiratory failure, and even brain damage. Unfortunately like with the cold, there is no set figure as to how long it would take to get this hot as clothing, environmental factors, the person’s current activity, and a whole range of other factors would play into the overall time figure.

A chart from Americas NOAA Weather Radio network that shows the dangers of humidity in high temperatures.

How long can a person survive without food?

The general estimate is that a person can survive without food for about 3 weeks, but this can vary greatly depending on many things. If someone is in very good shape, and they spend all their time just sitting there not using up any energy, then it would be possible to survive for 30 days or more, with out-of-shape people sometimes lasting for as little as two weeks. The single biggest factor is what a person’s body is used to eating, as a complete lack of food would be a greater shock to the system than for someone with a smaller diet.

After 24 hours with no food, a person will start to experience bouts of nausea which will slowly get worse, and after 48 hours a person will be completely drained of energy and have great difficulty moving around. For the last third of the time frame before a person starves to death, they will be so weak that walking will be close to impossible, and the lack of food may cause other painful symptoms like intense stomach cramps and muscle pains.

How long can a person survive without water?

The average time is 70 to 80 hours, but this can vary greatly from person to person. Thirst affects someone’s ability to continue much worse than a lack of food does, and even though the average time is about 3 days, a person will be in no state to do anything long before the point they actually die. After 24 hours with no water, a person’s kidneys will begin to struggle and intense headaches will set in, after 48 hours the body will begin to rapidly degrade and it would be close to impossible to eat anything at this point. Every single organ in the human body relies on blood to feed it oxygen, and with a lack of water, a person’s blood will begin to thicken and circulation will slow to the point that organs will begin to suffocate and shut down. Before people die of thirst they usually go blind due to the lack of moisture available in their body to feed their eyes. A good example of how a lack of water affects the body would be the story of Aron Ralston, who cut off his own arm after more than 4 days with only a tiny amount of water. His blood had thickened so much that he barely bled from a severed arm, and was even able to walk a few miles without bleeding to death.

How long can a person survive without sleep?

The longest someone has ever survived without sleep is 264 hours, a record set by a high school student in the US named Randy Gardener in 1964. By the end of the experiment, he was completely unresponsive and had no ability to do anything, including moving. After 24 hours of being awake, a person’s ability to function is the same as after drinking several glasses of wine, and after 48 hours someone’s puzzle-solving skills will only be a fraction of their normal level. Because it’s not ethical to see how long it will take for someone to die from a lack of sleep, there haven’t been any official studies conducted, but claims from various places like prison camps and stories of torture put the time length of how long it would take from a person to die of sleep deprivation at between 8 and 13 days.